Manage pigweeds with cover crops

Aug 21, 2025

Pigweeds are among the most difficult weeds to manage in row crop systems due to their rapid growth, high seed production and widespread herbicide resistance. During the past two decades, reliance on herbicides for weed management has led to the evolution of resistance in Palmer amaranth populations to several herbicides, particularly ALS -- acetolactate synthase -- glyphosate and PPO-inhibitors. That reality has intensified the need for integrated weed management strategies, including non-chemical practices such as cover crops.

Recent research from the University of Nebraska–Lincoln sheds light on how cover crop management decisions such as planting time, species selection and termination timing influence pigweed -- Palmer amaranth, waterhemp, redroot pigweed and smooth pigweed -- suppression. In a comprehensive meta-analysis of 41 field studies conducted across the United States and Canada from 1994 to 2024, researchers evaluated how various cover crop strategies affected the density and biomass of pigweed species, with an emphasis on Palmer amaranth, waterhemp, redroot pigweed and smooth pigweed.

Cover crops work, decisions matter

The meta-analysis, which included more than 100 site-years of data from across the United States and Canada, found that cover crops significantly reduce pigweed emergence and growth. However, their effectiveness depends on how and when they are managed.

Key findings were taken from the research.

Cover crops reduced the pigweed density by 58 percent in the early season -- zero to four weeks after crop planting, by 48 percent in the midseason -- five to eight weeks after planting, and by 44 percent in the late season -- more than eight weeks after planting.
Cover crops reduced pigweed biomass by 59 percent in the early, 55 percent in the midseason and 37 percent in the late season.
Among cover crop types, grasses and mixtures reduced pigweed density by 60 percent and 77 percent in early season, 53 percent and 59 percent in midseason, and 44 percent and 47 percent in late season. Legume cover crops were effective only during the early season with a 47 percent reduction, while brassicas did not affect pigweed density.
Cover crop residues remaining on the soil surface were more effective for pigweed suppression than incorporation.
Cover crop biomass was a critical factor. Treatments producing greater biomass provided greater pigweed suppression.

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Video: Western Canadian Agriculture: Hard Times Made the World's Best Farmers

Western Canadian agriculture produced the most advanced farmers in the world not through abundance but through adversity. The crow rate fell. The wheat price went nowhere. The brown envelopes stopped.

When the subsidies disappeared, the bad farmers left and the good ones stayed. And the ones who stayed could not just grow wheat anymore. They started growing lentils and canola and peas and flax and faba beans. They built crop rotation. They got serious about agronomy because there was no government backstop. That process produced the Western Canadian agriculture Dennis Bulani describes in this clip: the most advanced, most educated farming culture in the world.

His contrast with Iowa corn and soybean farmers is sharp. At a DeKalb farmer meeting in Okoboji, Iowa, he asked what crop rotation they ran. Beans on corn stubble, corn on bean stubble. How do you fertilize? The co-op agronomist handles it. Have you considered other crops? No need. We make so much money on corn and soybeans. Western Canadian agriculture was never allowed that comfort. And now those Iowa farmers are watching soybean markets lock up with China and corn prices slide, and they do not have the agronomy knowledge or the research base to pivot. Western Canadian farmers adapted on a dime because they had done it before.

Dennis also makes the case that Western Canadian agriculture keeps adapting in real time. Low commodity prices over the past year have pushed growers to look seriously at precision spot-spray technology. He knows a neighbor who bought a sprayer with the seeing-eye system and sprayed only 80 out of 320 acres. As a chemical retailer Dennis acknowledges that will affect his sales. He supports it anyway, because if it advances Canadian agriculture and makes farmers money, that is a good outcome.

The lesson Dennis draws from the tale of two farms: continuous improvement is the only durable strategy. When canola was $22 a bushel some growers went to Arizona instead of the Crop Production Show. When the price came down those same growers came back to the research and the discipline. Products do not go on Rack Petroleum's shelves unless they pass a replicated trial first. That is what Western Canadian agriculture built through hard times: farmers who do the work whether the times demand it or not.

Dennis Bulani is CEO of Rack Petroleum and Ultimate Yield in Biggar, Saskatchewan. Dan Aberhart hosts GTF Productions, Western Canadian Agriculture's foremost live briefing platform and its foremost AI training platform for ag operators