Wheat (Triticum aestivum L.) is an important cereal grain crop well suited for dryland cropping systems of the northwestern United States, including Montana. In 2015, Montana ranked third among the wheat producing states, accounting for 9% of the total US wheat production [1] . About 75% of the total wheat grown in the United States is winter wheat [2] , which is primarily grown in rotation with chemical fallow, pulse crops (dry pea, chickpea, or lentil), or canola in this region. Weed management is a major obstacle in successful winter wheat production [3] , and a season-long weed infestation can cause severe reductions in yield and grain quality of winter wheat [4] [5] .

Downy brome (commonly known as cheatgrass) is an invasive, self-pollinated, winter annual grass weed, widespread in rangeland and winter wheat production areas of this region [6] . It is a native of Mediterranean region of Europe, and was first identified in the United States in 1861 in New York and Pennsylvania [7] . Currently, downy brome infests 23 million hectares in 17 states in the northwestern United States [8] . The increased downy brome abundance in the semi-arid wheat production systems is attributed to the adoption of no-till practice, mainly for soil moisture conservation and soil erosion prevention, and limited selective herbicides for downy brome management [6] . Downy brome seeds are short-lived (1 to 2 years), best germinating at 20˚C, and seedlings emerge from shallow soil depths (1 to 5 cm) [9] [10] . Fall-emerging downy brome seedlings overwinter in a semi-dormant state and resume growth in the spring after winter vernalization. Those plants attain physiological maturity by mid-June in this region [11] . Downy brome is a prolific seed producer (up to 2.6 billion seeds ha⁻¹) [12] .

Downy brome interference in winter wheat has been studied extensively, and moderate to severe grain yield reductions have been reported [9] [13] [14] [15] . Downy brome at densities of 24, 40, and 65 plants∙m⁻² reduced winter wheat yields by 10%, 15%, and 20%, respectively [15] . In a study conducted in Washington, a moderate infestation of downy brome at a density of 54 plants∙m⁻² reduced winter wheat yield by 28% [14] . High levels of downy brome infestation (>500 plants∙m⁻²) can cause near-total (>92%) yield loss in winter wheat [14] .

The timing of downy brome emergence has a greater influence on wheat yield reductions than the plant density [8] [15] . The early-emerging cohorts of downy brome (that emerged between planting and 1 to 2 weeks after winter wheat emergence) were found to be more competitive than the late-emerging cohorts (that emerged after 3 weeks of winter wheat emergence) [13] [15] . For instance, downy brome at densities of 200 to 400 plants∙m⁻² that emerged in the fall with winter wheat, reduced winter wheat yields up to 68% [13] . Up to 40% winter wheat yield reductions were observed when fall-emerging downy brome at 132 plants∙m⁻² were not controlled by early spring; however, the yield loss was only 6% when those plants were removed by March [9] .

Growers rely on selective herbicides for downy brome management in wheat. A majority of these herbicides belong to five different ALS-inhibiting herbicide families, including sulfonylureas (SU), imidazolinones (IMI), triazolopyrimidines (TP), pyrimidinylthiobenzoates (PTB), and sulfonylamino-carbonyl-triazolinones (SCT). However, the intensive use of these ALS-inhibiting herbicides has resulted in the evolution of ALS-resistant downy brome biotypes [16] . For instance, a downy brome biotype with cross-resistance to imazamox (IMI), propoxycarbazone-sodium (SCT), and pyroxsulam (TP) has recently been reported from an IMI-tolerant winter wheat field in Montana [17] . Similarly, downy brome biotypes with cross-resistance to primisulfuron, sulfosulfuron, imazamox, and propoxycarbazone have been documented from Kentucky bluegrass fields in Oregon [18] [19] , suggesting the need to revisit effective weed control programs and investigate alternative herbicide chemistries for downy brome management.

Pyroxasulfone is a relatively new soil-applied premergence (PRE) herbicide that acts as a root/shoot growth inhibitor in germinating seedlings of both grassy and broadleaf weeds [20] . It is registered in several crops including wheat, corn (Zea mays L.), soybean [Glycine max (L.) Merr.], sunflower (Helianthus annuus L.), and dry bean [21] [22] [23] . Pyroxasulfone applied PRE in wheat controls grass weeds such as, Italian ryegrass (Lolium perenne ssp. multiflorum) and rigid ryegrass (Loliumrigidum) [21] [24] [25] . However, there is very little information available on the effectiveness of pyroxasulfone for downy brome management in wheat. The objectives of this research were: 1) to evaluate pyroxasulfone applied PRE standalone or followed by (fb) a standard POST herbicide program; 2) compare the effectiveness of pyroxasulfone with other standard preplant (PP) glyphosate-based herbicide programs (standalone) or fb a POST herbicide program; and 3) compare the efficacy of pyroxasulfone formulation, rate and application timing for crop safety and season-long downy brome injury and grain yields in IMI-tolerant (Clearfield®) winter wheat.

Three separate field experiments were conducted at the Montana State University Southern Agricultural Research Center (MSU-SARC) near Huntley, MT, during 2012 through 2016 to achieve the aforementioned objectives (1, 2, and 3). Soil was a Fort Collins clay loam (fine loamy, mixed, superactive, mesic Aridic Haplustalfs) with a pH of 7.8% and 2.1% organic matter. Monthly mean air temperature (˚C) and total precipitation (mm) were recorded at a weather station located approximately 1 km from the test site. Plots were 3 m wide by 9 m long. An IMI-tolerant winter wheat cultivar “Clearstone CL 2” was seeded 2.5 cm deep in 19-cm rows at a rate of 67 kg∙ha⁻¹ using a no-till drill in the first fortnight of October. Plots were fertilized with N-P-K using a gravity-fed broadcast spreader according to the MSU fertilizer guidelines for winter wheat production [26] . The test site for each experiment had a natural uniform infestation of downy brome at densities of 60 to 65 plants∙m⁻². All herbicide treatments were applied with a CO₂-pressurized hand-held sprayer equipped with four flat-fan spray nozzles (TeeJetTP8001, TeeJet Technologies, Wheaton, IL 60189) calibrated to deliver 94 L∙ha⁻¹ at 276 kPa. Herbicides used in the three experiments are presented in Table 1. The herbicide application rates were selected based on the labeled rates in IMI-tolerant winter wheat. In all experiments, a nontreated check was included for comparison. The winter wheat crop was harvested using a small-plot combine (Wintersteiger Ag) and wheat grain samples were cleaned, and yields were adjusted to 13% moisture.

Field experiments were conducted in 2012/2013 and 2013/2014 winter wheat growing seasons, and were set up in a randomized complete block design with four replications. The study included three fall-applied preemergence (PRE) standalone treatments, two spring-applied postemergence (POST) standalone treatments, and two PRE followed by (fb) POST treatments. PRE standalone treatments included pyroxasulfone at either 89 or 178 g∙ai∙ha⁻¹ in comparison to propoxycarbazone at 29 g∙ai∙ha⁻¹ (local standard), and were applied immediately after winter wheat planting on October 12 in 2012 and October 7 in 2013. POST treatments included imazamox at 44 g∙ai∙ha⁻¹ or pyroxsulam at 18 g∙ai∙ha⁻¹, and were applied on May 10 in 2013 and May 14 in 2014 when winter wheat and downy brome had broken their winter dormancy and resumed growth (approximately 4- to 5-tiller stage of winter wheat). Injury data on crop and downy brome were visually assessed on a scale of 0 to 100%, with 0 representing no injury and 100 representing complete plant death at 2, 5, and 8 weeks after the POST (WAPOST) herbicide application.