**3. Results and discussion**

Although the primary emphasis in this chapter is the discussion on controlling Palmer ama‐ ranth and, to some extent, common waterhemp which have become troublesome weeds in corn and cotton, other weed species will be discussed since they are/can become problematic weeds as well.

#### **3.1. Corn PRE studies**

Since not all treatments were included in each year of the study, no attempt was made to com‐ bine results over years or locations. Also, rainfall amounts varied from site to site and year to year affecting herbicide response (**Table 1**). Rainfall during the 7 days after the application of PRE herbicide treatments occurred at all locations with the exception of Ganado in 2013 and 2014 when no rainfall occurred. Rainfall between 8 and 14 days after the PRE application var‐ ied from no rainfall at Ganado in 2013 to 78.2 mm at Ganado in 2015 (**Table 1**). Rainfall 15–21 days after the PRE application was low at Taylor in 2013 and Ganado in 2015, and no rainfall occurred at the other sites.

With respect to annual grasses, browntop panicum and Texas millet were present in 2013 and 2014 at the Taylor and Ganado sites, respectively. Common barnyardgrass was present at Taylor only in 2015. Broadleaf weeds were present at the Taylor and Ganado locations. Palmer amaranth was present in 2013 and 2015, while hophornbeam copperleaf and common sun‐ flower were present in 2013 and 2014. Although this chapter discusses the control of herbicide‐ resistant weeds, the control of other weeds will also be discussed since they are also a large part of the problem when providing effective weed control under normal growing conditions.

#### *3.1.1. Annual grass control*

Atrazine alone controlled common barnyardgrass 33%, while acetochlor (74.8%) or pendi‐ methalin alone, acetochlor plus atrazine, *S*‐metolachlor plus mesotrione, or *S*‐metolachlor plus


**Table 1.** Rainfall amounts at test locations for 21 days following application of PRE herbicides.

atrazine plus mesotrione provided 90–97% control (**Table 2**). The dinitroaniline herbicides, such as pendimethalin, are registered for use in over 40 crops [72]. These herbicides usually provide excellent control of annual grasses [73–75].

The center two rows of all plots were mechanically harvested, and lint cotton yields were recorded. Means were compared with Fisher's Protected LSD test at the 5% probability

Although the primary emphasis in this chapter is the discussion on controlling Palmer ama‐ ranth and, to some extent, common waterhemp which have become troublesome weeds in corn and cotton, other weed species will be discussed since they are/can become problematic

Since not all treatments were included in each year of the study, no attempt was made to com‐ bine results over years or locations. Also, rainfall amounts varied from site to site and year to year affecting herbicide response (**Table 1**). Rainfall during the 7 days after the application of PRE herbicide treatments occurred at all locations with the exception of Ganado in 2013 and 2014 when no rainfall occurred. Rainfall between 8 and 14 days after the PRE application var‐ ied from no rainfall at Ganado in 2013 to 78.2 mm at Ganado in 2015 (**Table 1**). Rainfall 15–21 days after the PRE application was low at Taylor in 2013 and Ganado in 2015, and no rainfall

With respect to annual grasses, browntop panicum and Texas millet were present in 2013 and 2014 at the Taylor and Ganado sites, respectively. Common barnyardgrass was present at Taylor only in 2015. Broadleaf weeds were present at the Taylor and Ganado locations. Palmer amaranth was present in 2013 and 2015, while hophornbeam copperleaf and common sun‐ flower were present in 2013 and 2014. Although this chapter discusses the control of herbicide‐ resistant weeds, the control of other weeds will also be discussed since they are also a large part of the problem when providing effective weed control under normal growing conditions.

Atrazine alone controlled common barnyardgrass 33%, while acetochlor (74.8%) or pendi‐ methalin alone, acetochlor plus atrazine, *S*‐metolachlor plus mesotrione, or *S*‐metolachlor plus

**Taylor Ganado Taylor Ganado Taylor Ganado**

**2013 2014 2015**

1–7 29.5 0 2.8 0 7.4 3.3 8–14 6.6 0 0.5 18.6 65.6 78.2 15–21 7.3 0 0 0 0 3.3

**Table 1.** Rainfall amounts at test locations for 21 days following application of PRE herbicides.

level [71].

weeds as well.

**3.1. Corn PRE studies**

occurred at the other sites.

*3.1.1. Annual grass control*

**Day Mm**

**3. Results and discussion**

72 Herbicide Resistance in Weeds and Crops

In 2013, pendimethalin alone, alachlor plus atrazine, *S*‐metolachlor plus mesotrione, or *S*‐ metolachlor plus atrazine plus mesotrione provided 96% or better browntop panicum control, while isoxaflutole, *S*‐metolachlor, and pyroxasulfone alone, and *S*‐metolachlor plus atrazine controlled this weed 80–88% (**Table 2**). In 2014, only the dose of acetochlor (74.8%) provided acceptable control (83%). The lack of effective control in 2014 can be attributed to greater plant populations at the test site in 2014 compared to 2013 and also the low rain‐ fall amounts after the PRE application in 2014 (**Table 1**). Since many of the PRE herbicides can volatilize and photodecompose on the soil surface over time, these herbicides need to be mechanically incorporated or need rainfall or irrigation to move these herbicides into the weed seed zone [76–78], which explains the erratic control noted with these herbicides under the droughty conditions observed at Taylor in 2014.



**Table 2.** Annual grass control in corn with PRE herbicides.

In 2013, isoxaflutole alone, thiencarbazone‐methyl plus isoxaflutole, acetochlor plus atra‐ zine, or saflufenacil plus dimethenamid‐P controlled Texas millet at least 92% (**Table 2**). Pendimethalin or saflufenacil alone, atrazine plus either *S*‐metolachlor, alachlor, or dime‐ thenamid‐P, and the three‐way combination of *S*‐metolachlor plus atrazine plus mesotrione provided 81–89% control. In 2014, acetochlor, pendimethalin, or pyroxasulfone alone or *S*‐ metolachlor plus mesotrione controlled this weed at least 95%, while isoxaflutole or mesotri‐ one alone and atrazine plus either acetochlor or *S*‐metolachlor controlled 83–89% (**Table 2**). In the two years, *S*‐metolachlor alone provided 75–78% Texas millet control compared with 75–99% control with pyroxasulfone. Typically, *S*‐metolachlor alone provides poor control of this weed [79, 80]. With high populations of Texas millet, Grichar et al. [79] reported less than 70% control with 1.7 and 3.4 kg ha−1 of metolachlor in dryland peanut (*Arachis hypogaea* L.) and 25–76% control under irrigated conditions. Steele et al. [80] reported that pyroxasulfone, at a 10‐fold lower use rate than *S*‐metolachlor, controlled Texas millet 84–96%, while *S*‐meto‐ lachlor provided 75–85% control when rated 9 weeks after treatment. They attributed the results to the longer residual activity of pyroxasulfone [81].

#### *3.1.2. Broadleaf weed control*

At Taylor in 2013, under moderate weed pressure (4 plants m<sup>2</sup> ), all herbicides, with the excep‐ tion of atrazine (73%), provided at least 97% Palmer amaranth control, while in 2015 under increased populations (8 plants m<sup>2</sup> ), atrazine controlled Palmer amaranth 79%, while isoxa‐ flutole, mesotrione, or saflufenacil provided no better than 71% control (**Table 3**). All other herbicide treatments provided at least 96% control. At the Ganado location, in 2013 and 2015, control was more erratic than at the Taylor location. This may be due to the greater weed pop‐ ulations noted in 2013 (10 plants m<sup>2</sup> ) and variable populations in 2015. In 2013, either atrazine or isoxaflutole alone, acetochlor, alachlor, *S*‐metolachlor, or dimethenamid‐P plus atrazine, or the three‐way combination of *S*‐metolachlor plus atrazine plus mesotrione provided 97–100% control, while mesotrione, dimethenamid‐P, or acetochlor (33%) alone and rimsulfuron plus mesotrione controlled this weed 61% or less (**Table 3**). In 2015, acetochlor (74.8%) alone, dimethenamid‐P plus atrazine, fluthiacet‐methyl plus pyroxasulfone, and saflufenacil plus dimethenamid‐P controlled Palmer amaranth at least 95%, while isoxaflutole, mesotrione, *S*‐metolachlor, and pendimethalin alone and rimsulfuron plus thifensulfuron‐methyl con‐ trolled this weed less than 70%.

In previous research, mesotrione applied PRE controlled smooth pigweed (*Amaranthus hybridus* L.), but control of morningglory species (*Ipomoea* spp.) and common lambsquar‐ ter (*Chenopodium album* L.) was inconsistent and dependent upon a timely rainfall follow‐ ing application [38, 82]. Armel et al. [38] reported improved weed control with mixtures of mesotrione plus acetochlor or atrazine over that of mesotrione alone. As seen in this study, the combination of mesotrione with metolachlor plus atrazine has enhanced weed control in other studies [38].

In 2013, isoxaflutole alone, thiencarbazone‐methyl plus isoxaflutole, acetochlor plus atra‐ zine, or saflufenacil plus dimethenamid‐P controlled Texas millet at least 92% (**Table 2**). Pendimethalin or saflufenacil alone, atrazine plus either *S*‐metolachlor, alachlor, or dime‐ thenamid‐P, and the three‐way combination of *S*‐metolachlor plus atrazine plus mesotrione provided 81–89% control. In 2014, acetochlor, pendimethalin, or pyroxasulfone alone or *S*‐ metolachlor plus mesotrione controlled this weed at least 95%, while isoxaflutole or mesotri‐ one alone and atrazine plus either acetochlor or *S*‐metolachlor controlled 83–89% (**Table 2**). In the two years, *S*‐metolachlor alone provided 75–78% Texas millet control compared with 75–99% control with pyroxasulfone. Typically, *S*‐metolachlor alone provides poor control of this weed [79, 80]. With high populations of Texas millet, Grichar et al. [79] reported less than 70% control with 1.7 and 3.4 kg ha−1 of metolachlor in dryland peanut (*Arachis hypogaea* L.) and 25–76% control under irrigated conditions. Steele et al. [80] reported that pyroxasulfone, at a 10‐fold lower use rate than *S*‐metolachlor, controlled Texas millet 84–96%, while *S*‐meto‐ lachlor provided 75–85% control when rated 9 weeks after treatment. They attributed the

tion of atrazine (73%), provided at least 97% Palmer amaranth control, while in 2015 under

flutole, mesotrione, or saflufenacil provided no better than 71% control (**Table 3**). All other herbicide treatments provided at least 96% control. At the Ganado location, in 2013 and 2015, control was more erratic than at the Taylor location. This may be due to the greater weed pop‐

or isoxaflutole alone, acetochlor, alachlor, *S*‐metolachlor, or dimethenamid‐P plus atrazine, or

), all herbicides, with the excep‐

), atrazine controlled Palmer amaranth 79%, while isoxa‐

**Browntop panicum Texas millet Barnyardgrass**

**Dose 2013 2014 2013 2014 2015**

**Treatment Kg ai ha−1 Taylor Ganado Taylor** Dimethenamid‐P 0.8 78 53 55 63 73 Pendimethalin 1.6 96 52 86 99 97 Saflufenacil 0.05 69 7 81 23 33 Saflufenacil+dimethenamid‐P 0.08+0.7 61 28 92 78 57 Acetochlor (33%) 1.7 75 10 78 96 63 (S)+(M) 2.8+0.3 98 65 67 95 96 (P) 0.1 88 37 75 99 42 Untreated ‐ 0 0 0 0 0 LSD (0.05) 33 33 22 48 29

) and variable populations in 2015. In 2013, either atrazine

results to the longer residual activity of pyroxasulfone [81].

**Table 2.** Annual grass control in corn with PRE herbicides.

74 Herbicide Resistance in Weeds and Crops

At Taylor in 2013, under moderate weed pressure (4 plants m<sup>2</sup>

*3.1.2. Broadleaf weed control*

increased populations (8 plants m<sup>2</sup>

ulations noted in 2013 (10 plants m<sup>2</sup>



**Table 3.** Palmer amaranth control in corn with PRE herbicides.

In 2013, thiencarbazone‐methyl plus isoxaflutole provided perfect control (100%) of hopho‐ rnbeam copperleaf while acetochlor (33%), saflufenacil or pyroxasulfone alone, alachlor plus atrazine, rimsulfuron plus thifensulfuron‐methyl, *S*‐metolachlor plus atrazine plus mesotri‐ one, saflufenacil plus dimethenamid‐P, and *S*‐metolachlor plus mesotrione controlled this weed at least 92% (**Table 4**). Atrazine and mesotrione alone and rimsulfuron plus mesotrione provided unacceptable control (<60%). In 2014, either acetochlor (74.8%), isoxaflutole, safluf‐ enacil, or pyroxasulfone alone controlled hophornbeam copperleaf at least 93% (**Table 4**). The combinations of *S*‐metolachlor plus either atrazine or mesotrione and saflufenacil plus dimethenamid‐P controlled this weed 90–98%, while rimsulfuron plus either mesotrione or thifensulfuron‐methyl and acetochlor (33%) provided 67–70% control.

In 2013, under low common sunflower pressure (2–3 plants m<sup>2</sup> ), all herbicides, with the excep‐ tion of atrazine alone (73%) and rimsulfuron plus thifensulfuron‐methyl (87%), controlled this weed at least 95% (**Table 4**). In 2014, under slightly greater common sunflower populations (4–6 plants m<sup>2</sup> ), control was more variable. Acetochlor (74.8%) alone, thiencarbazone‐methyl plus isoxaflutole, rimsulfuron plus thifensulfuron‐methyl, saflufenacil plus dimethena‐ mid‐P, and *S*‐metolachlor plus mesotrione controlled this weed at least 97%. Mesotrione, pendimethalin, or pyroxasulfone alone provided unacceptable control (<60%). The develop‐ ment of ALS‐resistant common sunflower has limited the options for growers having to con‐ trol common sunflower with POST herbicides [83, 84]. Results from this study are consistent with previous findings which found that common sunflower control with herbicide systems containing isoxaflutole was at least 85% in most instances [84, 85].

#### *3.1.3. Corn injury and yield*

Grain yields were obtained only in 2013 at both locations and in 2015 at Taylor. Early season crop injury consisted of stunting and was never more than 3% with any herbicide treatment (data not shown). Corn recovered from the slight early season stunting and typically by harvest Using Herbicide Programs to Control Weeds in Corn (*Zea mays* L.) and Cotton (*Gossypium hirsutum* L.) http://dx.doi.org/10.5772/intechopen.68226 77


In 2013, thiencarbazone‐methyl plus isoxaflutole provided perfect control (100%) of hopho‐ rnbeam copperleaf while acetochlor (33%), saflufenacil or pyroxasulfone alone, alachlor plus atrazine, rimsulfuron plus thifensulfuron‐methyl, *S*‐metolachlor plus atrazine plus mesotri‐ one, saflufenacil plus dimethenamid‐P, and *S*‐metolachlor plus mesotrione controlled this weed at least 92% (**Table 4**). Atrazine and mesotrione alone and rimsulfuron plus mesotrione provided unacceptable control (<60%). In 2014, either acetochlor (74.8%), isoxaflutole, safluf‐ enacil, or pyroxasulfone alone controlled hophornbeam copperleaf at least 93% (**Table 4**). The combinations of *S*‐metolachlor plus either atrazine or mesotrione and saflufenacil plus dimethenamid‐P controlled this weed 90–98%, while rimsulfuron plus either mesotrione or

**Treatment Dose Taylor Ganado Taylor Ganado** Dimethenamid‐P 0.8 98 53 96 92 Pendimethalin 1.6 97 83 98 47 Saflufenacil 0.05 99 72 70 73 Saflufenacil+dimethenamid‐P 0.08 + 0.7 100 95 99 100 Acetochlor (33%) 1.7 100 50 99 88 (S)+(M) 2.8 + 0.3 100 91 100 94 (P) 0.12 100 91 99 84 Untreated ‐ 0 0 0 0 LSD (0.05) 17 27 22 24

**2013 2015**

tion of atrazine alone (73%) and rimsulfuron plus thifensulfuron‐methyl (87%), controlled this weed at least 95% (**Table 4**). In 2014, under slightly greater common sunflower populations

plus isoxaflutole, rimsulfuron plus thifensulfuron‐methyl, saflufenacil plus dimethena‐ mid‐P, and *S*‐metolachlor plus mesotrione controlled this weed at least 97%. Mesotrione, pendimethalin, or pyroxasulfone alone provided unacceptable control (<60%). The develop‐ ment of ALS‐resistant common sunflower has limited the options for growers having to con‐ trol common sunflower with POST herbicides [83, 84]. Results from this study are consistent with previous findings which found that common sunflower control with herbicide systems

Grain yields were obtained only in 2013 at both locations and in 2015 at Taylor. Early season crop injury consisted of stunting and was never more than 3% with any herbicide treatment (data not shown). Corn recovered from the slight early season stunting and typically by harvest

), control was more variable. Acetochlor (74.8%) alone, thiencarbazone‐methyl

), all herbicides, with the excep‐

thifensulfuron‐methyl and acetochlor (33%) provided 67–70% control.

In 2013, under low common sunflower pressure (2–3 plants m<sup>2</sup>

**Table 3.** Palmer amaranth control in corn with PRE herbicides.

76 Herbicide Resistance in Weeds and Crops

containing isoxaflutole was at least 85% in most instances [84, 85].

(4–6 plants m<sup>2</sup>

*3.1.3. Corn injury and yield*

**Table 4.** Hophornbeam copperleaf and common sunflower control in corn with PRE herbicides.

no differences in corn plant growth between the untreated check and any herbicide treat‐ ments were noted (data not shown). Although no appreciable crop injury was noted in these studies, this is not always true. Instances of isoxaflutole phytotoxicity in corn have been docu‐ mented [85, 86] and attributed to several factors, including application timing [87], increased use dose [37], and varied susceptibility of corn hybrids to isoxaflutole [88]. Environmental factors (cool and wet) and soil characteristics [89] can also lead to corn injury by isoxaflutole. Johnson et al. [85] reported that PPI herbicide applications resulted in greater injury than PRE applications, and this was probably due to increased amount of precipitation. Armel et al. [38] reported that acetochlor, atrazine, or mesotrione combinations did cause 11–18% corn stunt‐ ing when followed by 32 mm of rainfall, but that the corn recovered quickly and by 4 weeks after treatment injury did not exceed 2%.

In 2013 at the Taylor location, atrazine, isoxaflutole, and pyroxasulfone alone, *S*‐metola‐ chlor plus atrazine and/or mesotrione produced grain yields that were greater than the untreated check (**Table 5**). Although not significant, all herbicide treatments resulted in a numerical increase in grain yield over the untreated check. At the Ganado location, grain yields from the herbicide treatments were not significantly different from the untreated check; however, all yields from the herbicide treatments were numerically greater than the untreated check with the exception of *S*‐metolachlor plus mesotrione which pro‐ duced a 10% decrease in yield from the untreated check. No reason for this reduction can be determined.

In 2014, no significant differences between the untreated check and any herbicide treatments were noted, although several herbicide treatments produced numerically greater yields than the untreated check (**Table 5**). Dimethenamid‐P and pyroxasulfone alone, fluthiacet‐methyl plus pyroxasulfone, thiencarbazone‐methyl plus isoxaflutole, dimethenamid‐P plus atrazine, *S*‐metolachlor plus atrazine plus mesotrione, and saflufenacil plus dimethenamid‐P produced grain yields that were 14–21% greater than the untreated check.


Using Herbicide Programs to Control Weeds in Corn (*Zea mays* L.) and Cotton (*Gossypium hirsutum* L.) http://dx.doi.org/10.5772/intechopen.68226 79


**Table 5.** Corn yield as influenced by PRE herbicides.

With glyphosate‐resistant pigweed becoming more widespread throughout the state, the use of soil‐applied herbicides can not only control resistant weed species in glyphosate‐resistant corn production systems but can also reduce the risk of new herbicide‐resistant weed species occurring. In general, many treatments with two or three herbicide modes of action provided better weed control than one herbicide alone, and the chance of corn injury appears to be min‐ imal with any herbicide combinations under normal growing conditions. Our results indicate that in a year with little or no rainfall within 7–14 days after PRE herbicide application, any combination of PRE herbicides may need to be followed by POST herbicides for control of escaped weeds.

#### **3.2. Corn POST Studies**

applications, and this was probably due to increased amount of precipitation. Armel et al. [38] reported that acetochlor, atrazine, or mesotrione combinations did cause 11–18% corn stunt‐ ing when followed by 32 mm of rainfall, but that the corn recovered quickly and by 4 weeks

In 2013 at the Taylor location, atrazine, isoxaflutole, and pyroxasulfone alone, *S*‐metola‐ chlor plus atrazine and/or mesotrione produced grain yields that were greater than the untreated check (**Table 5**). Although not significant, all herbicide treatments resulted in a numerical increase in grain yield over the untreated check. At the Ganado location, grain yields from the herbicide treatments were not significantly different from the untreated check; however, all yields from the herbicide treatments were numerically greater than the untreated check with the exception of *S*‐metolachlor plus mesotrione which pro‐ duced a 10% decrease in yield from the untreated check. No reason for this reduction can

In 2014, no significant differences between the untreated check and any herbicide treatments were noted, although several herbicide treatments produced numerically greater yields than the untreated check (**Table 5**). Dimethenamid‐P and pyroxasulfone alone, fluthiacet‐methyl plus pyroxasulfone, thiencarbazone‐methyl plus isoxaflutole, dimethenamid‐P plus atrazine, *S*‐metolachlor plus atrazine plus mesotrione, and saflufenacil plus dimethenamid‐P produced

**Herbicide treatment Dose Taylor Ganado Taylor Kg ai ha−1 Kg ha−1**

Atrazine (A) 1.1 5586 7695 7556

(F)+(P)+(A) 0.004+0.2+1.3 ‐ ‐ 8092 *S*‐metolachlor (S) 1.3 5143 7082 8806 Isoxaflutole (I) 0.05 5434 6980 7669 (S)+(A) 1.4+1.8 5396 7627 8582 Alachlor+(A) 2.5+1.5 4940 7466 ‐ Mesotrione (M) 0.1 4851 7727 8970 Thiencarbazone‐methyl+(I) 0.02+0.06 5256 7318 9494 Acetochlor+(A) 2.1+1.3 4915 7031 8899 Dimethenamid‐P (D) 0.8 5275 7172 9447 (D)+(A) 1.6+3.2 5294 8350 9611 Acetochlor (74.8%) 6.9 ‐ ‐ 8738 Rimsulfuron (R)+(M) 0.02+0.2 4972 8295 ‐ (R)+thifensulfuron‐methyl 0.02+0.02 5168 7991 7934

**2013 2015**

0.006+0.2 ‐ ‐ 9342

grain yields that were 14–21% greater than the untreated check.

after treatment injury did not exceed 2%.

78 Herbicide Resistance in Weeds and Crops

be determined.

Fluthiacet‐methyl (FM)+pyroxasulfone

(P)

#### *3.2.1. Annual grass control*

Limited control of browntop panicum was noted when using POST herbicides. Glyphosate and tembotrione alone provided 99% browntop panicum control, while the combinations of atrazine plus *S*‐metolachlor plus glyphosate, mesotrione plus *S*‐metolachlor plus glyphosate, and thiencarbazone‐methyl plus tembotrione provided 96–98% control (**Table 6**). Mesotrione and topramezone alone and the combination of primisulfuron‐methyl plus pyroxasulfone controlled this weed 77–83%; however, no other herbicides provide better than 68% control. Stephenson et al. [13] noted that thiencarbazone plus tembotrione controlled browntop mil‐ let (*Urochloa ramose* L.) 93% which was greater than tembotrione, atrazine, or glufosinate alone. They also noted that the co‐application of atrazine, glufosinate, or glyphosate with thiencarbazone plus tembotrione did not increase browntop millet control.

In 2014 at Beasley only nicosulfuron, primisulfuron‐methyl, and topramezone alone or the com‐ bination of pyroxasulfone plus glyphosate provided acceptable Texas millet control (>84%), while at Beyersville only the combinations of mesotrione plus *S*‐metolachlor plus glyphosate and fluthi‐ acet‐methyl plus pyroxasulfone plus atrazine controlled this weed at least 81% (**Table 6**). Prostko et al. [88] found that glyphosate applied sequentially was more effective at controlling Texas millet than either nicosulfuron or foramsulfuron. Again, the added control noted with pyroxasulfone can be attributed to the extended residual activity of this herbicide [81].

The combinations of atrazine plus *S*‐metolachlor plus mesotrione plus bicyclopyrone, atra‐ zine plus *S*‐metolachlor plus glyphosate, dimethenamid plus glyphosate, fluthiacet‐methyl plus pyroxasulfone plus glyphosate, mesotrione plus *S*‐metolachlor plus glyphosate, pyroxa‐ sulfone plus glyphosate, and thiencarbazone‐methyl plus tembotrione controlled barnyard‐ grass at least 93% (**Table 6**). Lamore et al. [89] reported that tembotrione at 92 g ha−1 provided greater than 90% control, which is similar to the results in this study. Stephenson et al. [13] reported that thiencarbazone plus tembotrione or tembotrione alone provided equivalent control of barnyardgrass to atrazine plus either glufosinate or glyphosate.


Using Herbicide Programs to Control Weeds in Corn (*Zea mays* L.) and Cotton (*Gossypium hirsutum* L.) http://dx.doi.org/10.5772/intechopen.68226 81


a AMS (ammonium sulfate) at 3.86 kg/378.4 L.

at Beyersville only the combinations of mesotrione plus *S*‐metolachlor plus glyphosate and fluthi‐ acet‐methyl plus pyroxasulfone plus atrazine controlled this weed at least 81% (**Table 6**). Prostko et al. [88] found that glyphosate applied sequentially was more effective at controlling Texas millet than either nicosulfuron or foramsulfuron. Again, the added control noted with pyroxasulfone

The combinations of atrazine plus *S*‐metolachlor plus mesotrione plus bicyclopyrone, atra‐ zine plus *S*‐metolachlor plus glyphosate, dimethenamid plus glyphosate, fluthiacet‐methyl plus pyroxasulfone plus glyphosate, mesotrione plus *S*‐metolachlor plus glyphosate, pyroxa‐ sulfone plus glyphosate, and thiencarbazone‐methyl plus tembotrione controlled barnyard‐ grass at least 93% (**Table 6**). Lamore et al. [89] reported that tembotrione at 92 g ha−1 provided greater than 90% control, which is similar to the results in this study. Stephenson et al. [13] reported that thiencarbazone plus tembotrione or tembotrione alone provided equivalent

**2013 2014 2015**

98 69 93 53

**Dose Browntop panicumTexas milletg Barnyardgrass**

can be attributed to the extended residual activity of this herbicide [81].

80 Herbicide Resistance in Weeds and Crops

control of barnyardgrass to atrazine plus either glufosinate or glyphosate.

**Herbicide treatment Kg ai or ae ha−1 Taylor Bea Beyersh Taylor**

%

Atrazine (A)<sup>c</sup> 1.1 45 ‐ 58 70 Carfentrazone‐ethyl<sup>a</sup>, <sup>d</sup> 0.02 66 ‐ ‐ 60 Fluroxypyr<sup>a</sup>, <sup>c</sup> 0.3 64 ‐ 68 40 Fluthiacet‐methyl (FM)<sup>a</sup>, <sup>c</sup> 0.07 43 ‐ 53 39 Glufosinate ammoniuma 0.7 ‐ ‐ ‐ 85 Glyphosate (G) 1.5 ae 99 25 63 85 Halosulfuron‐methyl (HM) <sup>a</sup>, <sup>c</sup> 0.07 49 ‐ 72 20 Mesotrione (M)<sup>a</sup>, <sup>c</sup> 0.1 79 ‐ 57 58 Nicosulfuron<sup>b</sup>, <sup>d</sup> 0.04 ‐ 84 ‐ ‐ Primisulfuron‐methyl (PM)<sup>b</sup>, <sup>d</sup> 0.04 68 89 ‐ ‐ Prosulfuron<sup>c</sup> 0.04 65 ‐ 40 20 Tembotrione<sup>a</sup>, <sup>e</sup> 0.09 99 37 73 87 Topramezone<sup>a</sup>, <sup>c</sup> 0.15 77 88 77 ‐

(A)+(S)+(G)<sup>a</sup>, <sup>c</sup> 1.8+1.5+0.8 ae 98 56 73 99 Diflufenzopyr+dicamba (D)<sup>a</sup>, <sup>c</sup> 0.06+0.1 ae 59 ‐ ‐ 50 Dimethenamid+(G) 0.8+1.54 ae ‐ ‐ 73 99

(A)+*S*‐metolachlor (S)+(M)+

bicyclopyrone<sup>a</sup>, <sup>c</sup>

Days after treatment

0.7+1.5+ 0.17+0.04 ‐ ‐ ‐ 93

b UAN (urea‐ammonium nitrate) added at 2.2 L.

c Crop oil concentrate (Agridex) added at 1.0% v/v.

<sup>d</sup> Non‐ionic surfactant (Induce) added at 0.25% v/v.

e Methylated seed oil (Phase) added at 1.1 L.

f Grass height at application: Taylor, ≤ 15 cm; Besley, ≤ 10 cm; Coupland, ≤ 5 cm; Taylor, ≤ 15 cm.

g Texas millet locations: Bea, Beasley; Beyers, Beyersville.

<sup>h</sup> Glyphosate at 1.54 kg ae ha−1 added to all treatments with the exception of glufosinate ammonium and glyphosate alone.

**Table 6.** Annual grass control in corn with POST herbicidesf .

#### *3.2.2. Broadleaf weed control*

In 2014 at Yoakum, under dense Palmer amaranth populations, atrazine, prosulfuron, and topramezone alone or the combinations of atrazine plus *S*‐metolachlor plus glyphosate, diflufenzopyr plus dicamba, dimethenamid plus glyphosate, halosulfuron‐methyl plus dicamba, mesotrione plus *S*‐metolachlor plus glyphosate, pyroxasulfone plus glyphosate, and thiencarbazone‐methyl plus tembotrione provided at least 91% control (**Table 7**). Armel et al. [38] reported improved weed control with mixtures of mesotrione plus acetochlor or atrazine.

At the Taylor location in 2015, under low populations, only carfentrazone‐ethyl, fluroxypyr, fluthiacet‐methyl, and primisulfuron‐methyl plus pyroxasulfone failed to provide at least 85% Palmer amaranth control. At the Ganado location, only pyroxasulfone plus glyphosate con‐ trolled this weed at least 80%, and this general lack of control was probably due to weed height (40–60 cm) at the time of herbicide application. Herbicide application to weeds 10–15 cm tall can result in corn grain yields equal to those in weed‐free plots [90], but POST applications when weeds are greater than 15 cm tall provided inconsistent season‐long weed control when compared with applications when weeds are less than 15 cm tall [91]. Stephenson et al. [13] reported that atrazine alone provided 96% control of this weed, while thiencarbazone plus tembotrione or tembotrione, glufosinate, and glyphosate alone provided 92% or less control.

Glyphosate alone provided 100% pitted morningglory control, while mesotrione plus *S*‐ meto‐lachlor plus glyphosate controlled this weed 82% (**Table 8**). Typically, glyphosate pro‐ vides inadequate control of pitted morningglory when applied alone at normal label use doses [92–94]. However, greater than 90% late season control of tall morningglory (*Ipomoea purpurea* L.), ivyleaf morningglory (*I. hederacea* L.), and entireleaf morningglory (*I. hederacea* var. *integriuscula* Gray) in the field has been documented with 1.12 kg ha−1 of glyphosate applied to plants with six true leaves or less [95]. However, sequential in‐season glyphosate applications are often required to provide similar levels of pitted morningglory control [96, 97]. No other herbicides provided better than 68% control. Bararpour et al. [98] observed 90–100% control of entireleaf and pitted morningglory with the combination of thiencarbazone plus tembotrione plus either atrazine, glufosinate, or glyphosate, while Stephenson et al. [13] observed 85–88% control with thiencar‐ bazone plus tembotrione alone.


Using Herbicide Programs to Control Weeds in Corn (*Zea mays* L.) and Cotton (*Gossypium hirsutum* L.) http://dx.doi.org/10.5772/intechopen.68226 83


a AMS (ammonium sulfate) added at 3.86 kg/378.4 L.

b UAN (urea‐ammonium nitrate) added at 2.2 L.

compared with applications when weeds are less than 15 cm tall [91]. Stephenson et al. [13] reported that atrazine alone provided 96% control of this weed, while thiencarbazone plus tembotrione or tembotrione, glufosinate, and glyphosate alone provided 92% or less control. Glyphosate alone provided 100% pitted morningglory control, while mesotrione plus *S*‐ meto‐lachlor plus glyphosate controlled this weed 82% (**Table 8**). Typically, glyphosate pro‐ vides inadequate control of pitted morningglory when applied alone at normal label use doses [92–94]. However, greater than 90% late season control of tall morningglory (*Ipomoea purpurea* L.), ivyleaf morningglory (*I. hederacea* L.), and entireleaf morningglory (*I. hederacea* var. *integriuscula* Gray) in the field has been documented with 1.12 kg ha−1 of glyphosate applied to plants with six true leaves or less [95]. However, sequential in‐season glyphosate applications are often required to provide similar levels of pitted morningglory control [96, 97]. No other herbicides provided better than 68% control. Bararpour et al. [98] observed 90–100% control of entireleaf and pitted morningglory with the combination of thiencarbazone plus tembotrione plus either atrazine, glufosinate, or glyphosate, while Stephenson et al. [13] observed 85–88% control with thiencar‐

**Dose 2014 2015**

%

Days after treatment

0.8+1.5+0.2+0.04 ‐ 99 75

40 53 32

**Herbicide treatment Kg ai or ae ha−1 Yoakum Taylor Ganadog**

Atrazine (A)<sup>c</sup> 1.1 100 100 43 Carfentrazone‐ethyl<sup>a</sup>, <sup>d</sup> 0.02 67 20 25 Fluroxypyr<sup>a</sup>, <sup>c</sup> 0.3 48 70 10 Fluthiacet‐methyl (FM)<sup>a</sup>, <sup>c</sup> 0.07 58 54 33 Glufosinate ammoniuma 0.7 63 100 58 Glyphosate (G) 1.5 ae 68 100 59 Halosulfuron‐methyl (HM)<sup>a</sup>, <sup>c</sup> 0.07 53 85 13 Mesotrione (M)<sup>a</sup>, <sup>c</sup> 0.1 83 100 55 Prosulfuron<sup>c</sup> 0.04 91 98 33 Tembotrione<sup>a</sup>, <sup>e</sup> 0.1 83 100 63 Topremazone<sup>a</sup>, <sup>c</sup> 0.15 97 100 63

(A)+(S)+(G)<sup>a</sup>, <sup>c</sup> 1.8+1.5+0.8 ae 100 99 73 Diflufenzopyr+dicamba (D)<sup>a</sup>, <sup>c</sup> 0.06+0.14 ae 91 99 65 Dimethenamid‐P+(G) 0.8+1.5 ae 100 100 67 (FM)+(M)<sup>a</sup>, <sup>c</sup> 0.09+0.09 ‐ 100 47

bazone plus tembotrione alone.

82 Herbicide Resistance in Weeds and Crops

(A)+*S*‐metolachlor(S)+(M) +bicyclopyrone<sup>a</sup>, <sup>c</sup>

c Crop oil concentrate (Agridex) added at 1.0% v/v.

<sup>d</sup> Non‐ionic surfactant (Induce) added at 0.25% v/v.

e Methylated seed oil (Phase) added at 1.1 L.

<sup>f</sup> *A. palmeri* height at application: Yoakum, ≤ 7.6 cm; Taylor, ≤ 10 cm; Ganado, ≤ 61 cm.

g Glyphosate at 1.54 kg ae ha−1 added to all treatments with the exception of glyphosate and glufosinate ammonium alone.

**Table 7.** Palmer amaranth control in corn with POST herbicides<sup>f</sup> .



a AMS (ammonium sulfate) added at 3.86 kg/378.4 L.

b UAN (urea‐ammonium nitrate) added at 2.2 L.

c Crop oil concentrate (Agridex) added at 1.0% v/v.

<sup>d</sup> Non‐ionic surfactant (Induce) added at 0.25% v/v.

e Methylated seed oil (Phase) added at 1.1 L.

f Pitted morningglory height at application, ≤ 20 cm; hophornbeam copperleaf, ≤ 15 cm;Asiatic dayflower ≤ 7.6 cm.

**Table 8.** Broadleaf weed control in corn with POST herbicidesf .

Fluthiacet‐methyl and tembotrione alone and the combinations of atrazine plus *S*‐metolachlor plus glyphosate and mesotrione plus *S*‐metolachlor plus glyphosate provided at least 90% hophornbeam copperleaf control, while atrazine, carfentrazone‐ethyl, and prosulfuron alone and the combination of thiencarbazone‐methyl plus tembotrione provided 81–86% control (**Table 8**).

Glufosinate ammonium alone controlled Asiatic dayflower 90%, while the combinations of atra‐ zine plus *S*‐metolachlor plus mesotrione plus bicyclopyrone and atrazine plus *S*‐metolachlor plus glyphosate provided 93% control (**Table 8**).

#### *3.2.3. Corn injury and yield*

Crop injury consisted of stunting with some leaf chlorosis and necrosis and was never more than 8% with any herbicide treatment (data not shown). Corn recovered from the slight early season stunting and typically by harvest no differences in corn plant growth between the untreated check and any herbicide treatments were noted (data not shown). Although no appreciable crop injury was noted in these studies this is not always true. Other stud‐ ies have reported corn injury more than 50% with isoxaflutole, imazethapyr, imazapic, and prosulfuron in field or sweet corn [99–101]. In addition, herbicides such as halosulfuron and dicamba plus diflufenzopyr have been reported to cause as much as 25 and 15% injury, respectively [102, 103]. Corn phytotoxicity has been attributed to several factors, including application timing [104], increased use doses [93], and varied susceptibility of corn hybrids to different herbicides [105].

Corn yield was combined over locations due to a lack of treatment by location interaction. Yields were likely affected more by weed control than any other factor (rainfall, etc.) in any year. Pyroxasulfone plus glyphosate produced the greatest yield while halosulfuron alone and the untreated check produced the least yield (**Table 9**). Treatments that contained the combination of atrazine plus glyphosate resulted in yields that were greater than 5200 kg ha−1.

Some research suggests that timely POST control can be an effective alternative to soil‐applied herbicides in corn [45, 105, 106]. The use of POST herbicides only is generally considered a greater risk and requires careful management [45, 105, 106]. Also, weed density and applica‐ tion timing are factors in weed efficacy with POST herbicides. Halford et al. [107] reported a reduction in yield when weeds remained beyond V6 corn. In addition, Gower et al. [106] found that subsequent emergence and competition after early glyphosate applications was likely responsible for corn yield reductions. Also, late POST applications can reduce corn grain yields, although weed control was nearly perfect [105, 108].


Fluthiacet‐methyl and tembotrione alone and the combinations of atrazine plus *S*‐metolachlor plus glyphosate and mesotrione plus *S*‐metolachlor plus glyphosate provided at least 90% hophornbeam copperleaf control, while atrazine, carfentrazone‐ethyl, and prosulfuron alone and the combination of thiencarbazone‐methyl plus tembotrione provided 81–86% control (**Table 8**). Glufosinate ammonium alone controlled Asiatic dayflower 90%, while the combinations of atra‐ zine plus *S*‐metolachlor plus mesotrione plus bicyclopyrone and atrazine plus *S*‐metolachlor

.

Pitted morningglory height at application, ≤ 20 cm; hophornbeam copperleaf, ≤ 15 cm;Asiatic dayflower ≤ 7.6 cm.

**2013 2015**

**copperleaf**

**Asiatic dayflower**

**Herbicide treatment Dose Pitted morningglory Hophornbeam** 

Primisulfuron‐methyl (PM)<sup>b</sup>, <sup>d</sup> 0.04 50 65 ‐ Prosulfuron<sup>c</sup> 0.04 25 86 ‐ Tembotrione<sup>a</sup>, <sup>e</sup> 0.1 53 90 65 Topramezone<sup>a</sup>, <sup>c</sup> 0.15 36 74 82

(A)+(S)+(G)<sup>a</sup>, <sup>c</sup> 1.8+1.5+0.8 ae 63 99 93 Diflufenzopyr+(D)<sup>a</sup>, <sup>c</sup> 0.06+0.14 ae 30 62 79 (FM)+(M)<sup>a</sup>, <sup>c</sup> 0.09+0.09 ‐ ‐ 78 (FM)+pyroxasulfone (P)+ (A)<sup>d</sup> 0.004+0.15+1.3 ‐ ‐ 70 (HM)+(D)<sup>a</sup>, <sup>c</sup> 0.07+0.3 ae 20 40 83 (M)+(S)+(G)<sup>a</sup>, <sup>c</sup> 0.1+1.1+1.1 ae 82 92 81 Primisulfuron‐methyl+(P)<sup>b</sup>, <sup>c</sup> 0.03+0.01 3 76 ‐

Untreated ‐ 0 0 0 LSD (0.05) 30 25 18

**Kg ai or ae ha−1 Days after treatment**

0.8+1.5+0.2+0.04 ‐ ‐ 93

0.02+0.07 68 81 72

Crop injury consisted of stunting with some leaf chlorosis and necrosis and was never more than 8% with any herbicide treatment (data not shown). Corn recovered from the slight

plus glyphosate provided 93% control (**Table 8**).

**Table 8.** Broadleaf weed control in corn with POST herbicidesf

AMS (ammonium sulfate) added at 3.86 kg/378.4 L.

UAN (urea‐ammonium nitrate) added at 2.2 L.

Methylated seed oil (Phase) added at 1.1 L.

 Crop oil concentrate (Agridex) added at 1.0% v/v. <sup>d</sup> Non‐ionic surfactant (Induce) added at 0.25% v/v.

*3.2.3. Corn injury and yield*

A + *S*‐metolachlor (S)+(M)+

84 Herbicide Resistance in Weeds and Crops

bicyclopyrone<sup>a</sup>, <sup>c</sup>

Thiencarbazone‐ methyl+tembotrione<sup>a</sup>, <sup>c</sup>

a

b

c

e

f


a AMS (ammonium sulfate) added at 3.86 kg/378.4 L.

b UAN (urea‐ammonium nitrate) added at 2.2 L.

c Crop oil concentrate (Agridex) added at 1.0% v/v.

<sup>d</sup> Non‐ionic surfactant (Induce) added at 0.25% v/v.

e Methylated seed oil (Phase) added at 1.1 L.

**Table 9.** Corn yield as influenced by POST herbicides.

With glyphosate‐resistant pigweed becoming more widespread throughout the state, the use of POST herbicide combinations, which may or may not contain glyphosate, can not only control resistant weed species in glyphosate‐resistant corn production systems but can also reduce the risk of new herbicide‐resistant weed species occurring. In general, many treatments with two or three herbicides with different modes of action provided better weed control than one herbicide alone, and the chance of corn injury appears to be minimal with any herbicide combination under normal growing conditions.

#### **3.3. Cotton studies**

#### *3.3.1. South‐central Texas*

A significant year‐by‐treatment interaction existed for all weed control and cotton yield data, thus data were analyzed separately by year. Weed control data required arcsine transforma‐ tion in order to meet the assumption of homogeneity of variances for ANOVA; however, the non‐transformed means are reported in the (**Table 10–13**).

In 2012, control of Palmer amaranth ranged from 29 to 97% while common waterhemp control ranged from 55 to 100% prior to EPOST applications (**Table 10**). At that timing, control of Palmer amaranth was lowest with pyrithiobac applied PRE. Similar results were seen for com‐ mon waterhemp control, where pyrithiobac applied PRE provided only 55% control. After EPOST and MPOST applications, no differences in Palmer amaranth control were detected among treatments, with means ranging from 93 to 100%. After the EPOST application timing, control of common waterhemp with *S*‐metolachlor plus fomesafen applied PRE was lower than control provided by trifluralin applied PPI followed by either glyphosate plus dicamba‐ or glufosinate plus dicamba applied EPOST, and all treatments that included glyphosate plus dicamba plus acetochlor applied EPOST. After MPOST applications, no differences in com‐ mon waterhemp control among herbicide treatments were observed. Seed cotton yields of treated plots ranged from 3581 to 4002 kg ha−1, which were all greater than the non‐treated check (1823 kg ha−1).

In 2013, Palmer amaranth control prior to EPOST application ranged from 63 to 100%, while control of common waterhemp ranged from 60 to 100% (**Table 11**). Similar to 2012, pyrithiobac applied PRE resulted in the lowest control of both Palmer amaranth and common waterhemp (63 and 60%, respectively). Treatments that included pendimethalin‐applied PRE provided reduced control of Palmer amaranth (88–90%) when compared with many other treatments at the early rating. A similar pattern was observed with common waterhemp, where control was numerically lower from treatments of pendimethalin‐applied PRE than many other treatments,


a Herbicide abbreviations, product name and doses: acetochlor, Warrant (Ace) at 1.26 kg ai ha−1; dicamba, Clarity (D) at 0.56 kg ae ha−1; glufosinate, Liberty (Gluf) at 0.59 kg ai ha−1; fomesafen, Reflex (F) at 0.28 kg ai ha−1;glyphosate (A), Roundup PowerMAX (Glyp [A]) at 1.26 kg ae ha−1; glyphosate (B), Touchdown Total (Glyp [B]) at 0.88 kg ae ha−1; pendimethalin, Prowl H<sup>2</sup> 0 (P) at 1.6 kg ai ha−1; prometryn, Caparol (Pr) at 0.56 kg ai ha−1; pyrithiobac, Staple LX (Pyr) at 58.84 g ai ha−1 PRE, 72.86 g ai ha−1 POST; *S‐*metolachlor, Dual Magnum (*S*) at 1.07 kg ai ha−1; trifloxysulfuron, Envoke (Trif) at 5.25 g ai ha−1; and trifluralin, Treflan (T) at 1.12 kg ai ha−1.

**Table 10.** Palmer amaranth and common waterhemp control and seed cotton yield in 2012.

With glyphosate‐resistant pigweed becoming more widespread throughout the state, the use of POST herbicide combinations, which may or may not contain glyphosate, can not only control resistant weed species in glyphosate‐resistant corn production systems but can also reduce the risk of new herbicide‐resistant weed species occurring. In general, many treatments with two or three herbicides with different modes of action provided better weed control than one herbicide alone, and the chance of corn injury appears to be minimal with any herbicide

**Herbicide treatment Dose Yield**

(FM) + (M)a, <sup>c</sup> 0.09+0.09 4909 (FM) + pyroxasulfone (P) + (A)<sup>d</sup> 0.004+0.15+1.3 2178 (HM) + (D)a, <sup>c</sup> 0.07+0.3 ae 3490 (M) + (S) + (G)a, <sup>c</sup> 0.1+1.1+1.1 ae 4149 (PM) + (P)a, <sup>c</sup> 0.03 + 0.01 1506 (P) + (G) 0.1+1.5 ae 5781 Thiencarbazone‐methyl + tembotrionea, <sup>c</sup> 0.02+0.07 4281 Untreated ‐ 395 LSD (0.05) 2988

**Kg ai or ae ha−1 Kg ha−1**

A significant year‐by‐treatment interaction existed for all weed control and cotton yield data, thus data were analyzed separately by year. Weed control data required arcsine transforma‐ tion in order to meet the assumption of homogeneity of variances for ANOVA; however, the

In 2012, control of Palmer amaranth ranged from 29 to 97% while common waterhemp control ranged from 55 to 100% prior to EPOST applications (**Table 10**). At that timing, control of Palmer amaranth was lowest with pyrithiobac applied PRE. Similar results were seen for com‐ mon waterhemp control, where pyrithiobac applied PRE provided only 55% control. After EPOST and MPOST applications, no differences in Palmer amaranth control were detected among treatments, with means ranging from 93 to 100%. After the EPOST application timing,

combination under normal growing conditions.

AMS (ammonium sulfate) added at 3.86 kg/378.4 L.

UAN (urea‐ammonium nitrate) added at 2.2 L.

86 Herbicide Resistance in Weeds and Crops

Methylated seed oil (Phase) added at 1.1 L.

 Crop oil concentrate (Agridex) added at 1.0% v/v. <sup>d</sup> Non‐ionic surfactant (Induce) added at 0.25% v/v.

**Table 9.** Corn yield as influenced by POST herbicides.

non‐transformed means are reported in the (**Table 10–13**).

**3.3. Cotton studies**

a

b

c

e

*3.3.1. South‐central Texas*


a Herbicide abbreviations, product name and doses: acetochlor, Warrant (Ace) at 1.26 kg ai ha−1; dicamba, Clarity (D) at 0.56 kg ae ha−1; fomesafen, Reflex (F) at 0.28 kg ai ha−1; glufosinate, Liberty (Gluf) at 0.59 kg ai ha−1; glyphosate (A), Roundup PowerMAX (Glyp [A]) at 1.26 kg ae ha−1; glyphosate (B), Touchdown Total (Glyp [B]) at 0.88 kg ae ha−1; pendimethalin, Prowl H<sup>2</sup> 0 (P) at 1.6 kg ai ha−1; prometryn, Caparol (Pr) at 0.56 kg ai ha−1; pyrithiobac, Staple LX (Pyr) at 58.84 g ai ha−1 PRE, 72.86 g ai ha−1 POST; S‐metolachlor, Dual Magnum (*S*) at 1.07 kg ai ha−1; trifloxysulfuron, Envoke (Trif) at 5.25 g ai ha−1; and trifluralin, Treflan (T) at 1.12 kg ai ha−1.

**Table 11.** Palmer amaranth and common waterhemp control and seed cotton yield in 2013.

though this was not always significant. Prior to the MPOST application, control of Palmer ama‐ ranth with *S*‐metolachlor applied PRE was less than that of all other treatments except for pendimethalin applied PRE followed by pyrithiobac applied EPOST, which itself was lower than treatments other than *S*‐metolachlor plus prometryn applied PRE. Control of common waterhemp prior to the MPOST application was lowest with *S*‐metolachlor applied PRE (81%) and pendimethalin applied PRE followed by pyrithiobac applied EPOST (86%). At the last rat‐ ing, control of Palmer amaranth and common waterhemp was reduced with pendimethalin applied PRE followed by pyrithiobac applied EPOST followed by glufosinate applied MPOST (92 and 93%, respectively) when compared to all other herbicide treatments. Mean yield of the non‐treated control was 254 kg ha−1, which was lower than that of all herbicide treatments (3526–4209 kg ha−1).

Pyrithiobac applied PRE has been shown to provide satisfactory control of Amaranthus weeds [109, 110]; however, the opposite was observed in this experiment, where pyrithiobac applied PRE resulted in decreased levels of control of both Palmer amaranth and common waterhemp. The reasons for this lack of control are unknown, as the treatment was applied at the recommended rate and timing [111]. Pendimethalin applied PRE provided varied levels of control of common waterhemp, particularly in 2012. This may be due to the utilization of furrow irrigation for herbicide incorporation rather than overhead irrigation, which is recom‐ mended on the product label [112]. Trifluralin applied PPI consistently provided the best levels of control of both species. This is likely due in large part to the thorough mechanical incorporation of herbicide into the soil, which has been observed to affect the efficacy of trifluralin [113, 114].

In 2013, a decreased level of control of both Palmer amaranth and common waterhemp later in the season was observed with pendimethalin applied PRE followed by pyrithiobac‐ applied EPOST followed by glufosinate applied MPOST. This is attributed to a failure of glufosinate to control weeds that survived pyrithiobac EPOST and grew to a size larger than that recommended for control with glufosinate [115]. This weed size effect on glufosinate performance was observed by Craigmyle et al. [116], where control of common waterhemp was found to decrease with increasing plant height. Pendimethalin applied PRE followed by glyphosate EPOST provided excellent control of both species; however, in the presence of a glyphosate‐resistant population, this treatment would likely not provide acceptable levels of control. In addition, this reliance on glyphosate as the single POST herbicide mecha‐ nism of action is not recommended due to the potential for selection of glyphosate‐resistant plants [117]. Excellent control of both Palmer amaranth and common waterhemp was achieved in both years in treatments that included glyphosate plus dicamba plus acetochlor‐applied EPOST. In addition to providing successful levels of weed control, this tank‐mix would likely be very resilient against selecting resistant biotypes as suggested by Evans et al. [118], who found that the presence of glyphosate‐resistant biotypes of common waterhemp was much less common in fields that received applications of mixed herbicide mechanisms of action.

#### *3.3.2. High Plains studies*

though this was not always significant. Prior to the MPOST application, control of Palmer ama‐ ranth with *S*‐metolachlor applied PRE was less than that of all other treatments except for pendimethalin applied PRE followed by pyrithiobac applied EPOST, which itself was lower than treatments other than *S*‐metolachlor plus prometryn applied PRE. Control of common waterhemp prior to the MPOST application was lowest with *S*‐metolachlor applied PRE (81%) and pendimethalin applied PRE followed by pyrithiobac applied EPOST (86%). At the last rat‐ ing, control of Palmer amaranth and common waterhemp was reduced with pendimethalin applied PRE followed by pyrithiobac applied EPOST followed by glufosinate applied MPOST (92 and 93%, respectively) when compared to all other herbicide treatments. Mean yield of the non‐treated control was 254 kg ha−1, which was lower than that of all herbicide treatments

Herbicide abbreviations, product name and doses: acetochlor, Warrant (Ace) at 1.26 kg ai ha−1; dicamba, Clarity (D) at 0.56 kg ae ha−1; fomesafen, Reflex (F) at 0.28 kg ai ha−1; glufosinate, Liberty (Gluf) at 0.59 kg ai ha−1; glyphosate (A), Roundup PowerMAX (Glyp [A]) at 1.26 kg ae ha−1; glyphosate (B), Touchdown Total (Glyp [B]) at 0.88 kg ae ha−1;

at 58.84 g ai ha−1 PRE, 72.86 g ai ha−1 POST; S‐metolachlor, Dual Magnum (*S*) at 1.07 kg ai ha−1; trifloxysulfuron, Envoke

0 (P) at 1.6 kg ai ha−1; prometryn, Caparol (Pr) at 0.56 kg ai ha−1; pyrithiobac, Staple LX (Pyr)

**Herbicide and application timinga Palmer amaranth Common waterhemp Seed cotton PPI PRE EPOST MPOST Early Mid Late Early Mid Late Yield**

> P [A] 88 100 100 88 100 99 4084 P [A]+D+Ace 88 99 100 79 100 100 3838 P Pyr Gluf 90 86 92 92 86 93 3526 Pyr [A] + D+Ace 63 99 100 60 99 100 4034 *S* [B]+Trif 93 82 99 91 81 98 3773 F [B]+Trif 100 98 99 100 100 100 3986 *S*+F [B]+Trif 100 100 100 100 100 100 4003 *S*+Pr [B]+Trif 99 93 100 97 96 99 3881

T Gluf Gluf 99 99 100 99 100 100 3983 T [A]+D [A] 99 99 99 99 100 100 4122 T Gluf + D [A]+D 99 100 100 98 100 100 4207 T [A]+D+Ace 98 100 100 96 100 100 4209 ‐ ‐ ‐ ‐ 0 0 0 0 0 0 254 LSD (0.05) 8 4 2 8 6 5 530

**% Kg ha−1**

Pyrithiobac applied PRE has been shown to provide satisfactory control of Amaranthus weeds [109, 110]; however, the opposite was observed in this experiment, where pyrithiobac

(3526–4209 kg ha−1).

pendimethalin, Prowl H<sup>2</sup>

88 Herbicide Resistance in Weeds and Crops

(Trif) at 5.25 g ai ha−1; and trifluralin, Treflan (T) at 1.12 kg ai ha−1.

**Table 11.** Palmer amaranth and common waterhemp control and seed cotton yield in 2013.

a

In the glyphosate plus 2,4‐D choline study, trifluralin alone failed to control Palmer amaranth with only 20% control early season and no control late season, while systems which include POST applications of either glyphosate or glufosinate alone controlled this weed 23–53% (**Table 12**). Systems which included an EPOST and MPOST application of glyphosate plus 2,4‐D choline provided at least 94% season‐long control of this weed. Chahal and Johnson [119] reported that the addition of 2,4‐D to glyphosate provided 99% control of glyphosate‐ resistant horseweed [*Conyza canadensis* (L.) Cronq.] compared to only 12% with glyphosate alone. In a similar study, 2,4‐D added to glufosinate provided an increased level of com‐ mon waterhemp control compared to herbicide treatments consisting of glufosinate only [116]. Miller and Norsworthy [120] reported that the addition of a residual herbicide, such as trifluralin, would provide an additional effective herbicide mode of action for managing resistant Palmer amaranth. Applications of 2,4‐D, glyphosate, and glufosinate alone or tank‐ mixed represent broad‐spectrum POST herbicides that have the potential to control 9 of the 10 most problematic weeds in the southern cotton and soybean production [121].


a Herbicides abbreviations and doses: Trif, trifluralin at 1.12 kg ha−1; Gly, glyphosate at 1.36 kg ae ha−1; Glu, glufosinate at 0.59 kg ai ha−1; Gly + D, glyphosate at 0.48 kg ha−1 + 2,4‐D choline at 0.45 kg ae ha−1; S, *S*‐metolachlor at 1.08 kg ai ha−1; D, 2,4‐D choline at 1.06 kg ae ha−1.

**Table 12.** Palmer amaranth control with herbicide systems using glyphosate plus 2,4‐D choline.

Cotton injury was greatest (19%) with trifluralin‐applied PPI followed by *S*‐metochlor plus glufosinate‐applied EPOST followed by glyphosate plus 2,4‐D choline applied MPOST (**Table 12**). Cotton lint yields were greatest with herbicide treatments which provided greater than 90% Palmer amaranth control with the exception of trifluralin applied PPI followed by glyphosate plus 2,4‐D choline applied EPOST and MPOST.

In the glyphosate herbicide systems study, a late season rating suggests that the herbicide system which included glyphosate plus the pre‐mix of rimsulfuron plus thifensulfuron‐methyl applied preplant controlled Palmer amaranth less than 70%, while all other systems which included glyphosate plus either flumioxazin, fomesafen, or diruron applied preplant pro‐ vided 89–99% control (**Table 13**). Diverse herbicide programs for controlling resistant Palmer amaranth and common waterhemp is an important herbicide‐resistant management strategy [122]. Additionally, full labeled‐use doses should always be used to achieve the greatest level


a Herbicide abbreviations and doses: Ace, acetochlor at 1.27 kg ai ha−1; Dim, dimethenamid‐P at 0.63 kg ai ha−1; D, direx at 1.12 kg ai ha−1; Flumi, flumioxazin at 0.07 kg ai ha−1; Flume, flumeturon at 1.12 kg ai ha−1; Fome, fomesafen at 0.28 kg ai ha‐ 1; Gly, glyphosate at 1.3 kg ae ha−1; Gly + S, a premix of glyphosate at 0.95 kg ae ha−1 + *S*‐metochlor at 1.26 kg ha−1; MSMA at 2.11 kg ai ha−1; Par, paraquat at 0.56 kg ai ha−1; Pyr, pyrithiobac at 0.06 kg ai ha−1; Rim + Thi, a premix of rimsulfuron at 0.02 kg ha−1 + thifensulfuron‐methyl at 0.04 kg ha−1; MSMA, MSMA at 2.11 kg ai ha−1.

b Palmer amaranth control and cotton yield combined over years (2015, 2016) due to lack of year by treatment interaction.

**Table 13.** Palmer amaranth control and cotton response to herbicide systems.

Cotton injury was greatest (19%) with trifluralin‐applied PPI followed by *S*‐metochlor plus glufosinate‐applied EPOST followed by glyphosate plus 2,4‐D choline applied MPOST (**Table 12**). Cotton lint yields were greatest with herbicide treatments which provided greater than 90% Palmer amaranth control with the exception of trifluralin applied PPI followed by

**Table 12.** Palmer amaranth control with herbicide systems using glyphosate plus 2,4‐D choline.

Herbicides abbreviations and doses: Trif, trifluralin at 1.12 kg ha−1; Gly, glyphosate at 1.36 kg ae ha−1; Glu, glufosinate at 0.59 kg ai ha−1; Gly + D, glyphosate at 0.48 kg ha−1 + 2,4‐D choline at 0.45 kg ae ha−1; S, *S*‐metolachlor at 1.08 kg ai ha−1;

**Herbicide and applicationa Palmer amaranth control Cotton injury Lint yield**

Trif None None 20 0 0 0 Trif Gly Gly 46 23 0 0 Trif Glu Glu 53 24 5 0 Trif Gly+D Gly+D 96 98 9 959 Trif Gly+D+S Gly+D 97 99 8 947 Trif Gly+D Glu 73 68 9 0 Trif Gly+D Glu+D 94 94 3 728 Trif Gly+D Glu+D+S 96 97 3 1086 Trif S Glu+D 99 100 0 953 Trif Glu+D Gly+D 97 99 4 976 Trif Glu+D+S Gly+D 98 100 0 830 Trif Glu+D Glu+D 96 95 3 850 Trif S+Glu Gly+Glu 64 35 3 196 Trif S+Glu Gly+D 83 89 19 776 None None None 0 0 0 0 LSD (0.05) 7 10 7 325

**% Kg ha−1**

**PPIa EPOST MPOST Early Late Early**

In the glyphosate herbicide systems study, a late season rating suggests that the herbicide system which included glyphosate plus the pre‐mix of rimsulfuron plus thifensulfuron‐methyl applied preplant controlled Palmer amaranth less than 70%, while all other systems which included glyphosate plus either flumioxazin, fomesafen, or diruron applied preplant pro‐ vided 89–99% control (**Table 13**). Diverse herbicide programs for controlling resistant Palmer amaranth and common waterhemp is an important herbicide‐resistant management strategy [122]. Additionally, full labeled‐use doses should always be used to achieve the greatest level

glyphosate plus 2,4‐D choline applied EPOST and MPOST.

a

D, 2,4‐D choline at 1.06 kg ae ha−1.

90 Herbicide Resistance in Weeds and Crops

of possible control and reduce the likelihood for the evolution of resistance. These results further displayed the high level of weed control this new technology is capable of providing. Also, emphasis should be placed on a zero‐tolerance weed threshold [17], herbicides should also be applied at or less than the recommended weed height, and programs should not begin with an EPOST or MPOST application but rather start prior to planting with the application of residual herbicides.

While a few late emerging Palmer amaranth plants may be considered as being harmless, previous research has reported that late season Palmer amaranth seedlings are capable of seed production within 30 days after emergence [123]. Previous research also has shown that weeds left in the field at the time of harvest have the potential to enter harvesting machinery and be distributed across the field [124]. Thus, leaving weeds in the field prior to harvest can result in spreading viable weed seeds across the field. This practice will not only lead to increasing weed populations in that field but will also negatively impact sustainable weed management [125].

Also, a major challenge in managing weeds is minimizing the return of weed seed to the soil seed bank [126]. Menges [127] reported that maintaining fields weed free for 6 years reduced the soil seed bank of Palmer amaranth by 98%; however, 18 million seeds ha−1 remained in the soil. Palmer amaranth seed viability decreased when buried below the depth of optimal germination for at least 36 months [128]. Given that Palmer amaranth has become the most challenging weed to manage in corn and cotton [129], understanding population dynamics of this weed may help lead to strategies that more effectively manage this weed.
