Merriam-Webster's dictionary defines a weed as a plant that is not valued where it is growing and is usually of vigorous growth — especially one that tends to overgrow or choke out more desirable plants.
Of course, in production agriculture, weeds are highly adapted and compete with cash crops for resources like water and sunlight. However, university research and farmer experience has shown cover crops as a viable tool to outcompete weeds for these resources, especially during the off-season.
"Cropping systems and cover crops certainly offer ops for breaking up the weed cycle and certainly suppressing winter annuals," says Keith Glewen, University of Nebraska Extension educator. "Growers that planted a cover crop, specifically cereal rye in a timely manner last fall, they'll tell you they have a lot fewer issues with marestail and other winter annual weeds."
Covers crowd out weeds
Rodrigo Werle, Extension cropping systems specialist at the University of Nebraska's West Central Research and Extension Center in North Platte, has been researching the use of cover crops as a tool in the greater toolbox of weed suppression, and the costs and benefits of later cover crop termination.
"What we've seen so far is really good suppression of common winter annual weeds — tansy mustard, henbit, pennycress. The biggest one, the most troublesome is marestail," Werle says. "In particular, we've seen good suppression where you have a good stand of cereal rye in a corn-soybean rotation. That's because of its aggressive growth habit. And even if you plant it kind of a late, it's able to catch up in the spring. It doesn't mean you have 100% suppression, but it will even reduce marestail population and plant size compared to where you don't have a cover crop. You reduce marestail population and the plant size."
But cereal rye can also help suppress early-emerging warm-seasons like kochia. If the rye was established early enough and has enough biomass, it can help slow the development of kochia seedlings. With enough biomass, cereal rye can even act as a barrier to help suppress pigweed emergence. Meanwhile, studies at WREC have shown an increase in beneficial insects from cover crops — beneficial in that those insects feed on weed seed, reducing the weed seed bank.
However, Werle adds, it's important to remember that cover crops are not a panacea — just a tool in the grower's weed management toolbox. "A combination of residual herbicide and cover crop biomass can be a very powerful tool for weed management. It reduces emergence but gives a wider window to come back with post program. Weeds that grow back under biomass don't grow as fast," he says. "It's not going to be the miracle we're all hoping for, but it can be an important tool in our toolbox. It's going to have direct impact on winter annuals and an indirect impact on summer annual weeds like pigweeds."
Diversity brings benefits
An extended crop rotation can also help suppress weeds. In western Nebraska, yellow field peas have increased in popularity in the last few years, and researchers like Werle have found that field peas, which are planted in early spring, can also help suppress early-emerging weeds.
"That helps with kochia suppression, and diversification also forces you to completely change your herbicide program," Werle says. "Spraying glyphosate in a corn-wheat-fallow rotation is convenient, but with resistant kochia, that strategy isn't going to work anymore. The producer adds a crop, and it's not just a cover crop, because we're harvesting it for a profit. It's a short-season legume with less water demand, it closes the canopy early, suppresses weeds in spring and summer, and you use a diversified herbicide program that you wouldn't use otherwise."
Although eastern Nebraska is extremely efficient at producing corn and soybeans, there's also potential for diversified cropping systems in this part of the state. At the Eastern Nebraska Research and Extension Center near Mead, Glewen is experimenting with a three-year rotation, following corn with a cereal rye cover crop, followed by soybeans, followed by wheat, followed by a cover crop mix, which could be grazed in late fall or early spring, followed again with corn. This ensures there is always a crop growing and helps suppress weeds.
"There are multiple reasons for looking at a rotation beyond corn and soybeans. In some cases, short term, it's hard to think about any other crop than corn and beans. They're probably the two most available markets for growers," Glewen says. "But there are more benefits to including a small grain in a three-way rotation than the price you get at the market. You're breaking up the weed cycle. You may see an advantage associated with spreading out labor, opportunities with manure applications and nutrient management. And in some places in Nebraska, you might store moisture in the profile for the following crop."