By Amy Sullivan
A new meta-analysis of 41 cover crop research studies examines which cover crop management practices are the most effective at smothering these pigweeds, thanks to a team of researchers at the University of Nebraska-Lincoln, led by Ph.D. student Vipin Kumar.
The studies included 595 observations spanning from 1994 to 2024 throughout the southeastern, northeastern, and midwestern U.S., and Ontario, Canada. Kumar’s analysis revealed that grass cover crops and cover crop mixtures are king at suppressing weeds in the Amaranthus family, even at a lower biomass than the recommended 4,400 pounds per acre.
When compared to no weed control treatment, cover crops on average reduced pigweed weed density and biomass by 59% in early-season (up to four weeks after planting), by 55% in mid-season (five to eight weeks after planting), and by 37% in late-season (more than eight weeks after planting).
The meta-analysis also uncovered these best management practices for maximizing pigweed suppression from cover crops season-long: Use drill-planting methods, delay termination, and leave cover crop residue on the soil.
Cover Crop Planting and Biomass
Drill-planting cover crops on average halved pigweed density across the entire season, while broadcasting did not affect weed density at all. And while previous research has suggested a cover crop biomass of at least 4,400 pounds per acre is needed to suppress weeds, Kumar’s meta-analysis shows that a cover crop biomass of roughly 3,639 pounds per acre can reduce pigweed density by 50%. “We found that we can suppress Amaranthus species weeds with slightly less biomass, one reason being that this weed species has small seeds.” Kumar notes.
To achieve the necessary cover crop biomass in drier, colder regions, Kumar suggests interseeding crops for longer growth, as evidenced by a Nebraska study which found that shifting planting dates from mid-October to mid-September allowed a 316% biomass increase in a rye and oat mixture.
Similarly, terminating cover crops just before, during, or right after planting will allow for more biomass accumulation. But Kumar warns that delaying cover crop termination to after planting could result in reduced cash crop yield and soil moisture availability and complicate nutrient management, depending on the cover crop in use (particularly, grass cover crop species). (See more GROW coverage of upper Midwest research on the timing of cover crop termination and yield impacts here.)
Integrated Weed Management
Kumar found that using chemical, mechanical, or a combination of cover crop termination methods showed similar pigweed control rates, suggesting that farmers can tailor their termination method to what works best for them. However, leaving cover crop residue on top of the soil overall provided the most effective weed control (63% density reduction).
When cover crops were used with preemergence and/or postemergence herbicide treatments, they were not found to reduce pigweed biomass or density. But cover crops still play a vital role when used with herbicide treatments. “Because cover crops reduce the number of weeds being sprayed with herbicides, they can potentially reduce selection pressure for herbicide-resistant Amaranthus weeds,” Kumar explains.
Grass species and cover crop mixes were the best-performing cover crops, providing season-long pigweed control. Legume cover crops only provided early-season weed control, but could be used to maintain soil health. “If your main goal is weed suppression, a grass species is best,” Kumar explains.
Cash Crops
Cover crops accomplished varying levels of pigweed control depending on the cash crop they were used in.
In cotton and soybeans, cover crops on average reduced weed density throughout the season by roughly 57% and 46%, respectively. In vegetable crops like cucumber, tomato, and pumpkin, cover crops decreased pigweed density by 92% in early-season, but had no effect in the mid- and late-season.
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