**3. Background information on wheat herbicides**

As the records for the US wheat production indicated in **Figure 1**, yield and hectares increased from the 1870s to the 1950s due to improved agronomic practices. Herbicides were introduced in small grain production in the 1940s for broadleaf weed control [20] and marked the beginning for the trend of reduced hectares producing greater yields. These two facts are born out in regression of the data over this era, with a negative regression for hectares planted beginning in the 1960s. In contrast, yield in kg per ha has maintained a positive slope, with slight declines in production during the 1930's Dust Bowl. With the introduction of improved farming techniques, pesticides, fertility, and improved cultivars, wheat production after World War II began to increase significantly as herbicides were incorporated into production practices.

## **3.1. Herbicides**

Herbicides are used for PRE and POST control of grass and broadleaf weed species in wheat. However, control with POST herbicide applications is often the most commonly used as noted by **Figure 4**. Herbicide-applied POST can have less than desired weed control. Reduced efficacy has been associated with variables such as delayed application, suboptimum rates, not including a suitable adjuvant, including in tank mixture with other herbicides that are antagonistic, or during environmentally induced plant stress. The second factor that contributes to control failure is herbicide-resistant weeds. Herbicides that inhibit ACCase include the aryloxyphenoxypropionates and cyclohexanediones. Within the United States, there has been a rapid increase in ACCase-resistant Italian ryegrass biotypes since 1990 [21]. For example, Italian ryegrass resistant to diclofop was first reported in 1987 in Oregon [22, 23]. It has subsequently been reported in the Southeastern United States [21] and throughout the world [22, 24–27]. The widespread development of herbicide resistance in Italian ryegrass will reduce control options in wheat. While wild radish herbicide resistance has been reported in multiple wheat production regions including Australia, Brazil, and South Africa [4], no reports have occurred in North America.

### **3.2. Synthetic auxin herbicides**

Wild radish (*Raphanus raphanistrum* L.) is another common and troublesome winter annual weed in soft red winter wheat production regions of the Southeastern United States [6]. Cruciferous species compete vigorously with wheat, and data indicate that significant yield losses can occur if these weeds are not controlled soon after crop emergence [15]. Seeds of cruciferous species are high in erucic acid and glucosinolates that can pose quality problems in harvested wheat [16]. Once wild radish is established in wheat, it can be controlled with POST-applied herbicides, but these herbicides are not always used for economic, management, or even herbicide-resistant reasons [17–19]. Other winter weeds in soft red winter wheat production include henbit (*Lamium amplexicaule* L.), swine cress [*Coronopus didymus*

**Figure 3.** Italian ryegrass [*Lolium multiflorum* L. ssp. multiflorum (Lam.) Husnot] in seedling, tillering, and reproductive

**Figure 2.** Italian ryegrass [*Lolium multiflorum* L. ssp. multiflorum (Lam.) Husnot] in soft red winter wheat field, spikelet,

and single seed, respectively (photos by Sidney Cromer).

194 Wheat Improvement, Management and Utilization

(L.) Sm.], and cutleaf evening primrose (*Oenothera laciniata* Hill.) [6].

phases, respectively [photos by Timothy Grey (center) and Sidney Cromer (left and right)].

The first herbicide to be introduced for chemical weed control in any crop was 2,4-(dichlorophenoxy)acetic acid (2,4-D). Reports of the plant growth regulatory effects were first noted by Marth and Mitchell [25] in the journal *Botanical Gazette*. They reported via a personal communication that 2,4-D could potentially be used for weed control. Marth and Mitchell [28] reported on the delivery of 2,4-D specifically via POST aqueous spray solutions at 500 and 1000 ppm, with efficacy on several broadleaf weed species including dandelion (*Taraxacum officinale* F.H. Wigg.) and plantain (*Plantago lanceolata* L.) that were controlled in Kentucky bluegrass (*Poa pratensis* L.). Klingman [29] experimented with wheat and noted 2,4-D tolerance when applied with postemergence to the crop. After decades of further research on wheat evaluating rate and timing of applications, 2,4-D became a standard herbicide used for broadleaf weed control and is still currently used as a POST treatment. Other auxin herbicides which used POST in wheat for broadleaf weed control include (4-chloro-2-methylphenoxy) acetic acid (MCPA) and 3,6-dichloro-2-methoxybenzoic acid (dicamba). These herbicides have had consistent use patterns for the past 25 years in winter wheat with 2,4-D averaging over 1,000,000 kg applied in the United States annually (**Figure 4**). Dicamba and MCPA have averaged between 200,000 and 400,000 kg annually since 2006 (**Figure 4**) in winter wheat [30].

**Figure 4.** Herbicide use in winter wheat from 1990 to 2012 in the United States for multiple mechanisms of action. Data available at http://www.nass.usda.gov/Statistics\_by\_Subject/Environmental/index.asp.

#### **3.3. Photosystem II herbicides**

had consistent use patterns for the past 25 years in winter wheat with 2,4-D averaging over 1,000,000 kg applied in the United States annually (**Figure 4**). Dicamba and MCPA have averaged between 200,000 and 400,000 kg annually since 2006 (**Figure 4**) in winter wheat [30].

196 Wheat Improvement, Management and Utilization

**Figure 4.** Herbicide use in winter wheat from 1990 to 2012 in the United States for multiple mechanisms of action. Data

available at http://www.nass.usda.gov/Statistics\_by\_Subject/Environmental/index.asp.

Photosystem II (PS II) herbicides used in winter wheat include metribuzin and bromoxynil and are utilized in the Southeastern United States [16]. Metribuzin can be POST applied to winter wheat for control of annual grasses and dicot weeds including Italian ryegrass and wild radish [16] just as the coleoptile is emerging from soil. While metribuzin can control Italian ryegrass effectively, careful management, including cultivar selection and timely application, is required to achieve acceptable crop tolerance and weed control. Many agronomically desirable, high-yielding wheat cultivars are sensitive to metribuzin and cannot be planted if metribuzin is to be applied and some cultivars are extremely sensitive [31–33]. Bromoxynil in wheat will control wild radish but is ineffective on Italian ryegrass. Bromoxynil use in soft red winter wheat has averaged over 200,000 kg in the United States since 2006 (**Figure 4**).

#### **3.4. Acetolactate synthase (ALS) herbicides**

Sulfonylurea (SU) herbicides were first synthesized by E.I. DuPont Corp. in the mid-1950s and screened for pesticide properties, but first attempts revealed no significant biological activity [34]. It was not until the 1970s that the analogs of SUs began to be synthesized and their herbicidal activity evaluated. Prior to this there was no precedence for high potency and extremely low use rates in the g ha−1 range for weed control. One example described by Bhardwaj [34] was that university researchers would move the decimal two places as they could not believe that herbicides could be effectively applied at g ha−1, rather than kg ha−1. The result was that weeds would not grow in treated test plots after 2 years, despite halflives of 6–8 weeks. Thus, the potency of the SUs was recognized, and their use in plant production systems, including wheat, was quickly established. The key components to SUs are two moieties (R1 and R2) on either side of a sulfonylurea bridge. Generally, the moieties are composed of an aryl group, a pyrimidine ring, or a triazine ring [35, 36]. Variation in herbicidal activity occurs by substitutions made to branches on these rings. Chlorsulfuron was the first SU herbicide released by E.I. DuPont for weed control in small grains [37]. LaRossa and Schloss [38] reported that sulfometuron methyl was a potent acetolactate synthase (ALS) isozyme II inhibitor by testing of *Salmonella typhimurium*. Since then, all SUs have been identified as ALS inhibitors [39]. There are currently several SUs used in wheat weed control including chlorsulfuron, metsulfuron, sulfosulfuron, mesosulfuron, thifensulfuron, and tribenuron. Use rates vary but fall primarily within a range of 4–280 g ha−1. These use patterns are reflected in the masses of herbicides used when comparing the auxin and PS II inhibitors combined to average over 2,450,000 kg in 2012, versus the ALS herbicides at 53,000 kg (**Figure 4**): a 46 times greater application mass. This comparison reflects the potency and reduces environmental impact aspect of the ALS herbicides. Another POST ALS wheat herbicide is the triazolopyrimidine pyroxsulam that is specifically preferred in the Southeastern United States because it controls Italian ryegrass and wild radish. However, there are multiple reports of ALS resistance in Italian ryegrass that make these herbicides less viable options and essentially render those useless [40].

#### **3.5. Soil residual herbicides**

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, as a Group 15 (WSSA)/Group K<sup>3</sup> (HRAC) herbicide [39, 44]. Nakatani et al. [43] noted that 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 that was previously unavailable.
