By Alisha Bower

When asked what benefits cover crops provide for their farms, Jon Bakehouse of Mills County and Bob Lynch of Humboldt County think first of their soil health benefits. But as the list of benefits grows longer, weed suppression makes an appearance.

Lynch has grown cover crops on his farm since 2012. “We used to have field penny cress, fall pannicum and marestail. These winter annuals would come back up as the weather warmed, so we had to fight all spring to keep them under control until we planted our corn or soybeans. Now, our cover crops cover the ground during this time, suppressing those winter annuals,” he says.

Bakehouse struggles with different weed species in southwest Iowa. While planting a cover on one of his fields, he ran out of seed halfway. This led him to experiment: One part of the field had a stand of rye on it following his corn harvest, while another patch was left bare. “We terminated our rye in April or May. And when we came back a couple weeks later to do another preplant herbicide pass, the waterhemp where the rye had been was only 3 or 4 inches tall, while the waterhemp was at least a foot tall where no rye was planted.”

He says that little “experiment” taught him a lesson: “If nothing is growing in your ground, Mother Nature will grow weeds.”

Another function of some cover crops is allelopathy, or the ability of one plant to secrete chemicals into the soil around it that affect adjacent plants. Cereal rye and other winter cereal grains, sorghums, brassicas (rapeseed, mustards, radishes) and buckwheat all have documented allelopathic effect on weeds. According to Lynch, this is good news for growers: “You don’t have to have a perfect stand of cereal rye. You can have 2- to 4-foot gaps in the stand, and you still won’t get weeds because of rye’s pathological effect.”

Possible cost savings
Relying on cover crops to suppress weeds may have some very real implications for cost savings. Dialing back on herbicide or tillage passes will lower input costs. Lynch, for example, used to apply a herbicide cocktail to his fields before he planted corn or soybeans in order to suppress weeds. “Now, I only use glyphosate to burn down the cover,” he says, “I don’t use 2-4,D or others that I used to have to use, and I’m getting the same level of control. I hope in the future to let the rye grow a little longer and taller, and then after the burndown not have to apply any additional herbicides.”

Bakehouse also notes from his cover crop experience that the efficacy of the herbicide was much better on smaller weeds where the rye had been. He hopes that with a little more experimentation, he too can cut down his herbicide applications significantly — if not entirely — by mechanically terminating his cover crops and then cleaning up remaining weeds only as needed.

Beyond cost savings, finding alternative weed control methods other than herbicides is becoming more of a necessity with every season that passes. As herbicide-resistant weed populations grow stronger and stronger, the forward-looking farmer is exploring alternative methods of weed control. Techniques such as managing cover crops for weed suppression will help slow the development of herbicide resistance and provide options to farmers who have already exhausted their chemical alternatives.

What does the science say?
Because Practical Farmers of Iowa (PFI) has heard many accounts from its members — like Lynch and Bakehouse — about the weed control benefits of cover crops, the group is looking into more of the specifics from research trials throughout the Upper Mississippi basin. This project will pull and combine data from research on corn and soybean systems conducted in Iowa, Illinois, Indiana, Michigan, Minnesota and Wisconsin. Each individual study compared corn-soybean rotations with and without cover crops to explore the percent control that can be achieved through this nonchemical method. PFI staff will pool and analyze all of the data across these studies — a technique called meta-analysis — to see if the results are generalizable across the region as a whole and if other management lessons emerge.

The research aggregated in this meta-analysis looks at many different varieties of both cover crops and weeds — 23 different chronic weeds and 14 species of cover crops in total. Cover crops studied included brassicas such as radish, pennycress and yellow mustard; legumes such as hairy vetch, clovers and medics; and winter cereal grasses such as rye and wheat. The analysis, set to be completed this summer, will explore two main research questions:
• What is the weed suppression potential of cover crops?
• Was there any impact on the corn or soybean yield in those cover cropped systems?

If there is sufficient data, PFI will also investigate if cover crop varieties, termination methods and dates have any impact on weed populations or main crop yields. 

This information can help farmers better manage their cover crops, allowing them to better estimate and apply herbicides at only the needed rate, saving money and combating herbicide resistance. As a result, farmers should be better able to quantify the return on investment from their cover crops. A full research report with the results of the meta-analysis will be published at practicalfarmers.org this summer.

Different termination strategies
Practical Farmers of Iowa member Darwin Pierce performs research trials with the organization on the farms he manages. For the past two years, he conducted an experiment with different cover crop strategies — no cover crop, early termination, late termination and no termination of cereal rye covers — going into soybeans. He collected data on the weed pressure in each of these different management systems and found that the cover cropped plots had significantly less weed pressure both before soybean planting and after. The rye that was not terminated had total weed control, but even the early-terminated rye plots had 40% weed control over the no-cover crop plot.

The increased weed control from cover crops allowed Pierce to skip herbicide passes. “Cover crops cost about $40 per acre,” he says. “I saved two passes of Roundup where we didn’t terminate at $15 to $20 a pass. So we’re at about breakeven,” he says. And he reports that they saw no difference in soybean yields between the different management plots. In fact, in one year they saw a half- to 1-bushel advantage on plots that had been cover-cropped with rye.

Bower is Midwest cover crops associate for Practical Farmers of Iowa in Ames. Contact her at [email protected].