Cover crops can provide Mid-South producers with weed suppression, improvements in soil health and erosion control. But University of Arkansas weed scientist Jason Norsworthy urges producers to do their homework before planting one. It’s critical to match your cover crop to region, soil type and crop mix among other factors.
Norsworthy has been researching cover crops since the early 2000s, and more recently has focused on his efforts on the effects of a cover crop on suppression of glyphosate-resistant Palmer amaranth.
His studies have shown that choice of a cover crop is the first and most important decision a farmer makes.
For example, legume cover crops can provide a nitrogen benefit, but can deteriorate quickly, allowing sunlight to reach soil, and actually triggering emergence and growth of weeds. “Where I had hairy vetch and Austrian winter peas as cover crops in research plots, I had more pigweed in than I did where I didn’t have a cover crop.”
Early studies on wild radish, a weed, indicated that it is extremely allelophathic to cotton, and could cause significant stand reductions. “Yet corn was pretty tolerant of it,” Norsworthy said. “Planting into a killed stand of wild radish, we saw substantial weed suppression. The point is that it’s not that hard to find cover crops that have good strong allelophathic properties. But obviously you have to match it with a crop that’s tolerant.”
Two cover crop choices, wheat and cereal rye, have done good jobs of suppressing weeds in both cotton and soybean, noted Norsworthy. Both can be used to salvage a field where Palmer amaranth is out of control, a situation not uncommon in the Mid-South over the past few seasons.
Norsworthy’s research showed that in heavily-infested fields, the one-time use of a moldboard plow, followed by the planting of cereal rye, reduced Palmer amaranth populations by around 75 percent in cotton. “The reason we couldn’t reduce them further was we had to bed the soil for cotton. When we did that, we pulled pigweed seed back up that had been buried previously with tillage.”
Norsworthy added that rebedding was not necessary the second year following deep tillage when a cover crop protected the beds from deterioration.
On soybean, which did not require bedding, Palmer amaranth emergence was reduced 96 percent over two years of research where cereal rye was planted as a cover crop and where wheat was planted, harvested, then planted to soybeans.
“If you go with a cover crop before you have a major pigweed problem, the cover crop is going to help prevent that pigweed problem from blowing out,” Norsworthy said. “If you have a pigweed problem that’s blown out, you can use the moldboard plow one time, then from that point on you can reduce tillage and keep pigweeds in a manageable situation.”
Norsworthy said cover crops can also “take a lot of selection pressure off our preemergence herbicides as well as our postemergence herbicides. If the cover crop itself is basically suppressing that weed, then we won’t have to solely rely on a herbicide to kill pigweed.”
On the other hand, a cover crop does not eliminate the need for any herbicide applications, according to Norsworthy’s studies, and there are extra costs associated with a cover crop. “But from a dollar standpoint, you could pay for the cost of the cover crop from the reduced amount of tillage that you had in the system.”
Norsworthy adds that there are other benefits to a cover crop. “It can I help you reduce soil erosion where you have trouble from wind blasting.”
Norsworthy observed that after killing a cover crop in cotton, “it seemed like the soil held moisture a little longer. We did not have any trouble at all seeding into the stubble with a conventional planter with double disk openers. Obviously our ground speed was a little slower than what would be without a cover crop. But we had a tremendous amount of biomass and we had a lot of success with that.”
Norsworthy believes more testing of cover crops in the Mid-South is necessary. “Some of these cover crops grow better on some soil types than they do on others. I think that’s one issue that we run into.”
Norsworthy noted that wild radish, seven-top turnip and tillage radish, members of the Brassicaceae family, produce glucosinolates which can break down into allelopathic substances which can have varying effects on both crops and weeds. “The amount of stress these cover crops experience has a direct impact on the amount of allelochemicals they produce.”
This may be triggered more readily in sandier soils, according to Norsworthy. “I don’t think we see nearly as much allelochemical production on soil types that are more prone to hold moisture. So again, you have to do a good job of matching your cover crop with your soil type.”
On some soil types there may be a need to fertilize a cover crop to produce enough biomass for weed suppression, Norsworthy added.
Norsworthy is participating in a United Soybean Board project studying the effects of a cover crop on Palmer amaranth suppression in nine states, including Mississippi, Tennessee, Arkansas and Missouri, and several Midwest states. “We’re looking at some of the same cover crops across regions and hopefully will gather information on soil types as well.”
Norsworthy will discuss his cover crop studies in depth at the upcoming Arkansas Cover Crops Conference, July 24-25, at the Hilton Garden Inn, Jonesboro, Ark.
Other speakers will address irrigation in cover crops, pest management in cover crops, crop rotations, economics, presentations from producers using cover crops, choosing a cover crop and management considerations. For more information, visit www.aracd.org.