Cover crops are well known to provide benefits to the soybean crop like reducing nutrient losses and erosion, suppressing weeds and sometimes suppressing diseases. However, many farmers balk at using cover crops ahead of corn due to a perceived risk of potential yield loss.
Developing management practices that lead to a corn yield increase with cover crops drives Scott Nelson’s research with the Iowa Soybean Association (ISA) Research Center for Farming Innovation. “The reason we believe there is potential for corn to yield more following cover crops relates to soil water,” says the ISA Agronomy Network Director. “More soil water in the profile during reproductive development has been documented by several studies for corn following cover crops. The reason is simple: Dead cover crop roots make great channels to absorb summer rains compared to conventionally-tilled fields.”
Why, then, do farmers sometimes experience lower yields in their cover crop production compared to conventional tillage? “We believe one of the reasons is nitrogen availability,” Nelson says. “Cover crops do a very good at sequestering nitrogen during early stages of yield formation. We don’t necessarily need higher rates of nitrogen in corn following cover crops. The nitrogen needs to be managed so there is adequate early-season nitrogen available to the crop during critical early yield formation stages.”
In ISA’s small-plot and on-farm research, funded by the Fluid Fertilizer Institute, scientists have observed significant advantages for corn following cover crops when 30 to 50% of the corn crop's nitrogen need is supplied as nitrate before the V6 growth stage. “In our studies, early-season nitrogen applied with the planter, early sidedress, and post-planting broadcast applications of urea plus AMS provided nearly a 20-bushel advantage compared to the no-cover crops comparison,” Nelson says.
In on-farm comparisons, Nelson and his team observed a 10-bushel advantage for early-season nitrogen in corn following cereal rye compared to no early-season nitrogen applications. This multi-year research is being repeated across several locations.
“For corn following a cover crop, our recommendation for farmers is to plant green when the cereal rye is eight to 12 inches tall,” Nelson says. “Just don't let it get much bigger than that before you terminate it because corn is not very shade tolerant.”
The timing of when to terminate depends on the spring. If weather conditions are good, farmers may terminate before planting. However, Nelson recommends letting the cover crop grow to get the most benefit and not terminating when it’s two inches tall.
Planter setup is also critical to maintain or increase yield. Nelson spends a lot of time with farmers talking about the planting operation when seeding into cover crops. “One of the biggest reasons some farmers incur a yield loss is because the planter setup isn’t optimized to handle cover crop residue, leading to shallow seeding and lost yield.”
One system that offers a viable alternative to planting green is strip-till. Nelson has seen tremendous results with farmers who seed their cover crop before or during a strip-till pass. “This is a viable option even on poorly-drained soils,” he adds.
Along with yield increase benefits from cover crops, six years of side-by-side trials prove better soil health. “Last year, we studied organic matter levels in both the zero to six and six- to 12-inch soil layers, and we saw a higher organic matter increase in the deeper layer, about a 0.3% average increase across all locations. So, we're just beginning to understand subsoil organic matter improvement, which could be quite meaningful to farmers as more organic matter stores more moisture in that six to 12-inch layer,” Nelson adds.
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