Meaghan Anderson and Bob Hartzler

Meaghan Anderson and Bob Hartzler show a waterhemp plant, which was part of the Iowa State Fair Weed Identification Contest. Anderson suggests farmers take advantage of cover crops to reduce waterhemp populations. Photo by Jean Caspers-Simmet.

While cereal cover crops need to be sprayed at least two weeks ahead of corn planting, that same cover crop termination can wait until planting time for soybeans.

Waiting to spray cover crops in soybeans could provide an extra opportunity to reduce waterhemp and Palmer amaranth populations.

“Once we are going to get the cover crop killed in a timely manner, consider how you might be able to make the cover crop work for you,” said Meaghan Anderson, Iowa State University Extension and Outreach field agronomist in central Iowa.

Anderson spoke during a webinar hosted by University of Minnesota and Iowa State University Extension.

Land Grant University recommendations include terminating a cereal grain cover crop at least 10-14 days prior to planting corn. For soybeans, the cereal grain cover crop can be terminated at soybean planting.

There are a few herbicides that someone could use to kill a cereal grain cover ahead of soybeans, but glyphosate remains the most effective and consistent herbicide choice.

Anderson said it’s important to follow crop insurance cover crop termination guidelines.

“Talk with your crop insurance agent to make sure you are aware of when that cover crop needs to be terminated so it doesn’t interfere with any crop insurance you may have,” she said.

Another reason to correctly time cover crop termination is to minimize any interference with the cash crop. Nutrient tie-up is a concern, and biomass can potentially interfere with planting. In addition, the cover crop may harbor disease pathogens and insects that can jump over from the dying cover crop to the emerging corn plants.

“We would advise that the cover crop be brown and dead before planting corn,” she said.

At seeding time, soybeans seem to tolerate biomass better than corn.

One opportunity that occurs with growthy cover crop ahead of soybeans is weed suppression, she said.

“If we focus on heavy seeding, making sure we have even ground cover in the fall, then when we turn around in the spring we may have enough of that cover crop present that we can actually get some weed suppression with the cover crop in particular for small-seeded weeds, like Palmer amaranth and waterhemp,” Anderson said.

Her research, as well as research by others, has found that about 7,500 pounds per acre of dry cover crop biomass is enough to provide consistent weed suppression. Lower amounts of biomass can also provide at least partial weed suppression.

In the upper Midwest, achieving 7,500 pounds of biomass might be tricky, although 4,000-5,000 pounds may be feasible, she said.

“Seeding rate, seeding date, and termination timing are all important factors that play into how much biomass a cover crop can produce,” she said. An early-planted fall-seeded cereal grain should tiller before going into dormancy in the fall and provide ground cover in the spring. Cereal rye leaves dormancy at 38 degrees.

Anderson looked at several seeding rates used at the same mid-September seeding date in Iowa and found that by mid-May (spring 2014), the biomass across plots was similar due to tillering of plants at lower seeding rates.

“In general, if the standard seeding rate for cereal rye is 0.75-1 bushel per acre, higher seeding rates of 1.5-2 bushels per acre will usually result in a more consistent stand that would be more likely to suppress weeds,” she said.

In central Iowa, some farmers plant soybeans in mid- to late-May and terminate the cover crop a few days before or after soybean planting. If the cover crop burndown tank-mix includes a residual herbicide, then germinating weeds could also be killed for a while.

She warned, though that applying a tank mixture to a thick and green cover crop could intercept the herbicide rather than providing a soil residual. It is also rare; though, that the cover crops are perfectly even across the soil, so the residual herbicide is often necessary to suppress weeds germinating in open areas.

“Farmers are trying to find a way around this by applying the residual earlier, when the cover crop is smaller and more herbicide can reach the soil, or by focusing on growing even stands of cover crops to make sure they can delay the residual herbicide until a post-emergence herbicide application is made – after the cover crop is killed and after the soybeans have emerged,” Anderson said.

Adding cover crops to the seeding schedule opens up a whole stream of opportunities for farmers – from reduced soil erosion, to weed suppression and improved soil tilth and fertility. There are as many ways to correctly use cover crops as there are farmers in the world. Finding solutions that work is part of what makes farming so interesting.