**4. Research**

While auxins, PS IIs, and SUs are effective in wheat production, they have traditionally been POST applied for weeds that have already emerged. By applying after the crops emergence, the potential for weed infestation increases leading to a greater production costs, resulting in yield loss and potential quality issues. By utilizing herbicides either as PRE or soon after emergence, weed control could be enhanced in soft red winter wheat. Pyroxasulfone is labeled for delayed PRE or early POST application. The registrations for application of pyroxasulfone differ by company. One company defines that applications of pyroxasulfone must be delayed PRE as to when wheat has 80% germinated seed with a 1.2 cm long shoot, as well as having an early POST from spike to the fourth tiller timing [44]. Other companies have regional and state requirements that wheat must be planted 2.5–3 cm deep for PRE application (Pacific north western region of the United States) [45] but can also have POST applications for specific states [46]. Therefore, this chapter will emphasize pyroxasulfone, pendimethalin ME, and other herbicides for PRE and POST weed control and wheat response.

### **4.1. Field studies**

**3.5. Soil residual herbicides**

198 Wheat Improvement, Management and Utilization

as a Group 15 (WSSA)/Group K<sup>3</sup>

that was previously unavailable.

**4. Research**

New herbicide chemistries and new formulations of older compounds are available for weed control in soft red winter wheat. These include options for grass and broadleaf weed species. Pendimethalin [*N*-(1-ethylpropyl)-3,4-dimethyl-2,6-dintrobenzenamine] formulated as a microencapsulated (ME) aqueous capsule suspension contains 38.7% (0.47 kg L−1) active ingredient and can be applied after wheat has the first true leaf. This will provide residual weed control to later emerging weeds, but does not overcome the issue of weeds emerging right after wheat planting. Pyroxasulfone (3-[5-(difluoromethoxy)-1methyl-3-(trifluoromethyl)pyrazol-4-ylmethylsulfonyl-4,5-dihydro-5,5-dimethy-1,2-oxazole) is an isoxazoline PRE soil residual herbicide registered for soft red winter wheat since 2014 in the United States [41]. It has been researched and registered in multiple wheat production regions of the world including Australia [42], Japan, Canada, Saudi Arabia, South Africa, and the United States [43]. Pyroxasulfone inhibits the biosynthesis of very-long-chain fatty acids (VLCFAs) leading to the buildup of fatty acid precursors, specifically inhibiting many elongation steps catalyzed by VLCFA elongases,

the herbicide benthiocarb (*S*-[(4-chlorophenyl)methyl]diethylcarbamothioate) was used as the basis for research development of pyroxasulfone by developing a novel chemical structure by using various substitutions. This resulted in a compound with low water solubility (3.49 mg L−1), no pKa, and hydrolytically stable at all pH values at 25 C, allowing less susceptibility to decomposition and thus providing extended soil residual activity [39, 43]. Dissipation rates (DT50) for pyroxasulfone have ranged from 8 to 71 days in the top 8 cm of Tennessee soils [45] and 54 to 94 days in the top 7.5 cm of Colorado soils [46]. Pyroxasulfone's soil residual activity and utility have allowed it to be registered for multiple uses including corn (field, sweet, and pop) (*Zea mays* L.), soybean, cotton, fallow land, and non-crop areas [47–49]. Winter wheat tolerance has been well documented with only minor injury in the form of stunting with no negative effects on yield [50–52]. With PRE soil activity on broadleaf and grass species including ALS- [52], ACCase- [41], and glyphosate- [1] resistant Italian ryegrass biotypes, pyroxasulfone use in wheat will afford growers an early season weed control option

While auxins, PS IIs, and SUs are effective in wheat production, they have traditionally been POST applied for weeds that have already emerged. By applying after the crops emergence, the potential for weed infestation increases leading to a greater production costs, resulting in yield loss and potential quality issues. By utilizing herbicides either as PRE or soon after emergence, weed control could be enhanced in soft red winter wheat. Pyroxasulfone is labeled for delayed PRE or early POST application. The registrations for application of pyroxasulfone differ by company. One company defines that applications of pyroxasulfone must be delayed PRE as to when wheat has 80% germinated seed with a 1.2 cm long shoot, as well as having an early POST from spike to the fourth tiller timing [44]. Other companies have regional and state requirements that wheat must be planted 2.5–3 cm deep for PRE application (Pacific

(HRAC) herbicide [39, 44]. Nakatani et al. [43] noted that

Field studies were conducted to evaluate herbicides used for soft red winter wheat production focusing on residual and contact active ingredients, as well as timing when applied either PRE or POST with respect to crop emergence. All studies were conducted as described in **Table 1** for soil nomenclature, soil texture, soil pH, organic matter content, wheat cultivar, and dates associated with seeding, herbicide application timings, and harvest. Experiments were conducted from autumn to spring in 2009–2010 (**Table 2**), 2010–2011 (**Tables 2** and **3**), 2011–2012 (**Tables 2** and **3**), 2012–2013 (**Tables 4** and **5**), and 2013–2014 (**Table 4**). Experiments were conducted on the University of Georgia property at the Bledsoe Research Farm near Williamson, at the Southwest Georgia Branch Experiment Station located near Plains, or at the Ponder and Lang Research Farms near Tifton. Treated plots included eight rows of wheat on 19 cm spacing (1.8 m wide), in plots 7.6 or 9.1 m long, with wheat seeding rates of 90 kg ha−1. A randomized complete block design with four replications was used for all experiments. Herbicides were applied with a CO2 -pressurized sprayer calibrated to deliver 187 L ha−1 at 210 kPa for all experiments. PRE applications were made prior to wheat emergence; at emergence (AE) applications were made at Feekes 0.9 [53] when the coleoptile was soil emerged. POST applications were applied between Feekes 1.0 and 1.9. Fertilizer and liming requirements were based on the University of Georgia Extension recommendations for wheat. Insects and plant diseases were monitored and sprayed when necessary. Wheat stand counts were made multiple times during the season on 1 m of length of row. Wheat injury and natural infestations of weeds were evaluated for each location at multiple times during the growing season. Wheat injury and weed control were visually estimated on a scale of 0 (no injury) to 100% (death). Data for experiments that were identical were combined for analysis. Weed control, wheat stand counts, wheat injury, and wheat yield were subjected to mixed model analysis of variance (ANOVA) in SAS 9.2 [54]. Complete treatment description for all 15 experiments is listed in **Table 1**.



**Table 1.** Location information by table for soft red winter wheat herbicide trials and weed control evaluations in Georgia.


a Site-year locations: Griffin, Plains, and Tifton, Georgia

**Griffin Plains Tifton Plains Tifton 2009–2010 2010–2011 2009–2010 2010–2011 2009–2010 2010–2011 2010–2011 2011–2012 2010–2011**

> 5 Nov 2009

> 6 Nov 2009

> 12 Jan 2010

28 May 2010

**Griffin Plains Tifton Griffin Plains 2012–2013 2013–2014 2012–2013 2012–2013 2012–2013 2012–2013**

12 12 12 12 12 12 14 14 14

2 Nov 2010

8 Nov 2010

17 Jan 2010

2 June 2010

Fine-loamy, kaolinitic, thermic Plinthic Kandiudult

Tifton loamy sand

19 Nov 2010

8 Nov 2010

2010

2011

12 May 2011

> Clayey, kaolinitic, thermic, Typic Hapludult

Cecil sandy clay loam

26 Nov 2010

22 Nov 2011

9 Nov 2011

22 Nov 2011

1 Dec 2011

14 Dec 2011

1 May 2012

23 Nov 2010

12 Nov 2010

23 Nov 2010

3 Dec 2010

10 Jan 2011

6 June 2011

Clayey, kaolinitic, thermic, Typic Kandiudults

Faceville sandy loam

Seeding date

PRE application(s)

POST application(s)

Harvest date

Number of treatments

PRE application

POST application(s)

Number of treatments

5 Nov 2009

7 Nov 2009

14 Jan 2010

4 June 2010

Soil name Clayey, kaolinitic, thermic, Typic Hapludult

2 Nov 2010

200 Wheat Improvement, Management and Utilization

8 Nov 2010

17 Jan 2011

18 Nov 2009

18 Nov 2009

13 Jan 2010

NYa 27 May 2010

Soil texture Cecil sandy clay loam Faceville

19 Nov 2010

19 Nov 2010

\_\_\_\_\_\_\_ \_\_\_\_\_\_\_ \_\_\_\_\_\_\_ \_\_\_\_\_\_\_ \_\_\_\_\_\_\_ \_\_\_\_\_\_\_ 19 Nov

24 Jan 2011

\_\_\_\_\_\_\_ \_\_\_\_\_\_\_ \_\_\_\_\_\_\_ \_\_\_\_\_\_\_ \_\_\_\_\_\_\_ \_\_\_\_\_\_\_ 24 Jan

6 June 2011

> Clayey, kaolinitic, thermic, Typic Kandiudults

sandy loam

15 15 15 15 10 10

Harvest date NYa NY 28 May 2013 29 May 2013 NY 28 May 2013

**Table 1.** Location information by table for soft red winter wheat herbicide trials and weed control evaluations in Georgia.

<sup>a</sup>No yield (NY) taken as natural infestations of ryegrass populations made harvest impossible.

2 Nov 2012 4 Nov 2013 20 Nov 2012 14 Nov 2012 2 Nov 2012 20 Nov 2012

13 Nov 2012 9 Nov 2013 3 Dec 2012 26 Nov 2012 13 Nov 2012 3 Dec 2012

6 Dec 2012 20 Nov 2013 14 Dec 2012 18 Dec 2012 6 Dec 2012 14 Dec 2012 \_\_\_\_\_\_\_ \_\_\_\_\_\_\_ \_\_\_\_\_\_\_ \_\_\_\_\_\_\_ 15 Jan 2013 11 Jan 2013

Soil pH 6.0 6.3 6.5 5.9 6.0 6.5 Organic matter 1.0 1.5 2.0 1.1 1.0 2.0 Wheat cultivar SS 8641 SS8461 AGS 3035 AGS 2020 SS 8641 AGS 3035 Seeding date 1 Nov 2012 31 Oct 2013 20 Nov 2012 14 Nov 2012 1 Nov 2012 20 Nov 2012 b Abbreviations: a.i., active ingredient; DAP, days after planting; ME, microencapsulated; PRE, preemergence; POST, postemergence applied 65–70 DAP at Feekes scale 1.5–1.9

**Table 2.** Herbicide, rates, and timing of applications for evaluating weed control and soft red winter wheat growth response in Georgia, 2010–2011 and 2011–2012: data represents six site-year locations<sup>a</sup> .



a Site-year locations: Griffin, Plains, and Tifton, Georgia

b Abbreviations: a.i., active ingredient; DAP, days after planting; ME, microencapsulated; 12 DPRE, 12 days before planting; PRE, preemergence; AE, at wheat emergence; POST, postemergence applied 65–70 DAP at Feekes scale 1.5–1.9 <sup>c</sup>Henbit at Plains location 2010–2011 and 2011–2012

**Table 3.** Herbicide, rates, and timing of applications for evaluating weed control and soft red winter wheat growth response in Georgia, 2010–2011 and 2011–2012: data represents three site-year locations<sup>a</sup> .


a Site-year locations: Tifton and Griffin, Georgia.

b Abbreviations: a.i., active ingredient; DAP, days after planting; ME, microencapsulated; 12 DPRE, 12 days before planting; PRE, preemergence; AE, at wheat emergence; POST, postemergence applied 65–70 DAP at Feekes scale 1.5–1.9. c Henbit at Griffin location 2012–2013 and 2013–2014.

**Table 4.** Herbicide, rates, and timing of applications for evaluating weed control and soft red winter wheat growth response in Georgia, 2012–2013 and 2013–2014: data represents four site-year locations<sup>a</sup> .


a Site year locations: Plains and Griffin, Georgia.

**Herbicide Timing Rate** 

**Herbicide Timing Rate** 

202 Wheat Improvement, Management and Utilization

a Site-year locations: Griffin, Plains, and Tifton, Georgia

<sup>c</sup>Henbit at Plains location 2010–2011 and 2011–2012

Pyroxasulfone + saflufenacil

Pendimethalin

ME

Pyroxasulfone + saflufenacil

Pyroxasulfone + pendimethalin ME

Pyroxasulfone + pendimethalin ME

a Site-year locations: Tifton and Griffin, Georgia.

c Henbit at Griffin location 2012–2013 and 2013–2014.

Metribuzin + pendimethalin ME **(g a.i. ha−1) b**

response in Georgia, 2010–2011 and 2011–2012: data represents three site-year locations<sup>a</sup>

**(g a.i.  ha−1)b**

**Injury (%) Italian ryegrass (%) Henbitc Yield (kg ha−1)**

.

**Injury (%) Italian ryegrass (%) Henbitc Yield (kg ha−1)**

**14 DAP 28 DAP 30 DAP 175 DAP 30 DAP**

AE 1064 0 b 1 cd 48 c 27 g 91 a 4650 a

**30 DAP 75 DAP 175 DAP 30 DAP**

PRE 60+119 0 b 95 a 92 ab 95 ab 6080 a

PRE 80+119 3 b 97 a 96 a 90 abcd 6310 a

AE 60+1064 0 b 74 cde 69 ed 98 a 6160 a

AE 80+1064 0 76 bcd 67 edf 98 a 6440 a

AE 476+1064 17 a 87 ab 75 bcd 98 a 5350 b

Nontreated control 0 b 0 f 0 h 0 f 5120 b

Pinoxaden POST 119 0 b 3 bcd 68 b 40 efg 0 b 4450 a

b Abbreviations: a.i., active ingredient; DAP, days after planting; ME, microencapsulated; 12 DPRE, 12 days before planting; PRE, preemergence; AE, at wheat emergence; POST, postemergence applied 65–70 DAP at Feekes scale 1.5–1.9

**Table 3.** Herbicide, rates, and timing of applications for evaluating weed control and soft red winter wheat growth

Pyroxasulfone PRE 60 0 b 95 a 93 a 75 e 5680 ab Pyroxasulfone PRE 80 1 b 95 a 95 a 69 e 5762 ab

Pyroxasulfone AE 60 0 a 74 bcd 54 efg 75 e 6070 a Pyroxasulfone AE 80 0 b 78 bc 72 cde 81 bcde 6320 a Metribuzin AE 476 18 a 85 abc 73 cde 97 a 5670 ab

Pyroxasulfone POST 60 0 b 60 de 50 fg 76 de 6260 a Pyroxasulfone POST 80 0 b 70 cde 62 ed 78 cde 6020 a Diclofop POST 840 6 b 58 e 42 g 0 f 5750 ab Pyroxsulam POST 18 0 b 62 de 49 fg 92 abc 6330 a

b Abbreviations: a.i., active ingredient; DAP, days after planting; ME, microencapsulated; 12 DPRE, 12 days before planting; PRE, preemergence; AE, at wheat emergence; POST, postemergence applied 65–70 DAP at Feekes scale 1.5–1.9.

**Table 4.** Herbicide, rates, and timing of applications for evaluating weed control and soft red winter wheat growth

.

response in Georgia, 2012–2013 and 2013–2014: data represents four site-year locations<sup>a</sup>

b Abbreviations: a.i., active ingredient; DAP, days after planting; PRE, preemergence; AE, at wheat emergence; POST, postemergence applied 65–70 DAP at Feekes scale 1.5–1.9.

**Table 5.** Herbicide, rates, and timing of applications for evaluating weed control and soft red winter wheat growth response in Georgia, 2012–2013: data represents two site-year locations<sup>a</sup> .
