Herbicide resistance screening in Italian ryegrass (Lolium perenne L. ssp. multiflorum) and sustainable weed management strategy for peanut (Arachis hypogaea) in Alabama

Abstract

Alabama has a diverse agriculture with corn, cotton, soybeans, peanuts, and wheat being the major row crops. Weed management is a very important aspect in the successful production of all crops and given the rise in cases of herbicide resistance it is becoming more challenging. Italian ryegrass [Lolium perenne L. ssp. multiflorum (Lam.) Husnot] is a significant weed in winter wheat, corn, soybean, and peanut crops in Alabama. In response to reports of herbicide failure, field surveys were conducted in these cropping systems across Alabama in 2023. The objectives were to document the distribution of herbicide resistance, assess seed morpho-physiological diversity—such as seed length, awn length, seedling length, 100-seed weight, and seed dormancy—and explore the relationship between herbicide resistance status and seed morpho-physiological traits in the collected populations. Populations were evaluated in a greenhouse for sensitivity to herbicides representing three modes of action: an acetolactate synthase (ALS) inhibitor (pyroxsulam), two acetyl-coenzyme-A carboxylase (ACCase) inhibitors (fluazifop and clethodim), and a 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS) inhibitor (glyphosate). Herbicide screenings were followed by dose-response assays of the most resistant ryegrass population for each herbicide at eight rates (0.5, 1, 2, 4, 8, 16, 32, and 64x) compared to a susceptible population at six rates (0.0625, 0.125, 0.25, 0.5, 1, and 2x). Out of 44 populations evaluated, 21%, 11%, 25%, and 2% were found resistant to glyphosate, fluazifop, pyroxsulam, and clethodim, respectively. Resistance levels were confirmed to be 192-, 14-, 90-, and 738-fold for glyphosate, fluazifop, pyroxsulam, and clethodim, respectively. Mutation detection studies revealed specific mutations—D2078G in the ACCase gene, P106S in the EPSPS gene, and R421T in the ALS gene. A significant positive association was observed between seed dormancy and the number of survivors following glyphosate (r = 0.17, P < 0.01) and clethodim (r = 0.16, P < 0.01) treatments. Herbicide-resistant Italian ryegrass poses a serious threat to sustainable crop production in Alabama, necessitating immediate changes and diversification in conventional management practices. The observed diversity in morpho-physiological traits enhances the adaptability of Italian ryegrass, and its correlation with herbicide resistance offers valuable insights for developing effective management strategies. Additionally, Alabama was the third-largest producer of peanuts in the U.S. in 2023. Historically herbicides have been the primary source of weed control in peanuts however increasing cases of herbicide-resistant weeds and lack of commercially available herbicide-tolerant trait technology in peanut cultivars have limited the choices in terms of chemical weed control for growers and therefore two-year peanut study laid out in split-split plot design focused on investigating integrative and individual effect of row spacing, mulch and row orientation on weed control and yield has been conducted at Wiregrass research and extension center, Alabama. Crop rows, oriented north-south in the northern hemisphere, in the summer season up to 55° latitude may receive higher light interception and shade weeds growing in interrow spaces. In this study, crop rows-oriented northeast-southwest yielded 34% greater compared with rows-oriented east-west also early season weed density was greater in rows-oriented north-south and was lowest in east-west orientation which was closely followed by northeast-southwest orientation. Spacing does not have any significant effect on yield or weed density, however, it does improve canopy closure revealed by LAI and NDVI. Plots with mulch do have lower weed density up to 4 weeks after planting (WAP) in 2023 and 2WAP in 2024 compared with plots without mulch. Based on the current study northeast-southwest orientation along with mulch or cover crops may provide yield advantage and better early season weed management in peanuts in Alabama.