**1. Introduction**

Information about wheat production in the United States has been recorded since 1867 with respect to hectares planted (black dots on **Figure 1A**) and yield (bars on **Figure 1B**). While production in hectares first increased until 1950, it then decreased as yield in kg per hectare increased.

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**Figure 1.** Hectares of wheat planted (A) and grain yield (B) in the United States (National Agricultural Statistics Service, USDA. Hectare and kg/ha data available at https://www.nass.usda.gov).

Improved genetics, fertility, disease, insect, and weed control options contributed to increased yield. As **Figure 1A** indicates, hectares planted decreased from 1950 to 1955, then stayed relatively constant until 1980, and then began to decline in 2000 and have continued this downward trend into the twenty-first century. However, wheat yield doubled from approximately 1000 to 2000 kg/ha from 1950 to 1980 (**Figure 1B**). Then, from 1980 to 2010, yield increased from 2000 to 3000 kg/ha as the overall number of hectares planted again declined to the 1950s level. The mid-1950s increase is significant as it occurred with the introduction of herbicides. Multiple weed species have become an issue in wheat all across this production region. However, Italian ryegrass [*Lolium perenne* L. ssp. *multiflorum* (Lam.) Husnot] is one of the most common and troublesome weeds in wheat. As a winter forage, Italian ryegrass is planted and then becomes a problematic weed in small grains due to escapes [1–3]. While herbicides can be used to control this weed, there are also herbicide-resistant issues. There are multiple reports of Italian ryegrass herbicide resistance to acetyl CoA carboxylase (ACCase), acetolactate synthase (ALS), and glyphosate herbicides [1, 4]. A commonality for Italian ryegrass resistance is that most of these herbicides mechanisms of action that have resistance issues are generally all postemergence (POST) applied to the weed. In order to have profitable wheat production, applications of preemergence (PRE) herbicides with residual control of Italian ryegrass and other winter weed species would benefit growers. Currently, there are limited herbicides that can be applied at the time of wheat planting.

### **2. Importance**

**Figure 1.** Hectares of wheat planted (A) and grain yield (B) in the United States (National Agricultural Statistics Service,

USDA. Hectare and kg/ha data available at https://www.nass.usda.gov).

192 Wheat Improvement, Management and Utilization

Soft red winter wheat is an autumn-seeded crop in the Mid-South and Southeastern United States where it is double-cropped with cotton (*Gossypium hirsutum* L.), peanut (*Arachis hypogaea* L.), or soybean (*Glycine max* (L.) Merr.). Italian ryegrass is a vigorous erect winter annual native to temperate Europe where it was grown as a forage with reports of its presence in France, Switzerland, and England from 1818 to 1831 [3]. This use led to its migration to the Western Hemisphere with reports by Henderson on its quality [5]. Because of easy establishment, it was adapted for forage production. However, volunteer Italian ryegrass seed can become a weedy plant in small grains such as wheat [2, 3]. It can grow to over 1 m in height protruding above the wheat canopy, producing multiple tillers and seed heads from a single plant (**Figure 2**). It has long, clasping auricles, and awned seeds (**Figure 2**). Over time, it has become a major weed species for this region due to its aggressive growth and seed production. It has consistently ranked as being one of the most common and troublesome weeds in small grains and wheat for over 20 years [6–10]. Stone et al. [11] reported that Italian ryegrass interference with wheat was the result of its greater root density relative to the crop, which creates excess competition for moisture and nutrients. With respect to aboveground development, Ball [12] determined that leaf production rate was greater for Italian ryegrass as compared to wheat. Liebl and Worsham [13] noted that wheat grain yields were reduced by 4% for every ten Italian ryegrass plants per m2 and that declining yields could be primarily attributed to reductions in crop tillering. According to Appleby et al. [14], Italian ryegrass infestations of 29–118 plants per m2 reduced wheat yields between 7 and 50%. Italian ryegrass has similar growth stages to soft red winter wheat (**Figure 3**) and thus competes for resources in terms of space, sunlight, nutrients, and moisture.

**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).

**Figure 3.** Italian ryegrass [*Lolium multiflorum* L. ssp. multiflorum (Lam.) Husnot] in seedling, tillering, and reproductive phases, respectively [photos by Timothy Grey (center) and Sidney Cromer (left and right)].

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