Successfully raising corn after a cover crop requires timely cover crop termination and N fertilization. Commonly, farmers terminate a cover crop 2-3 weeks prior to planting corn but generally do not need to apply any more N than if they did not use a cover crop. Last year, PFI farmer-cooperator Dick Sloan attempted “planting green”: planting his corn into a cereal rye cover crop that was terminated just two days prior. In Sloan’s case, he saw a 5 bu/ac yield reduction compared to where he terminated the cover crop two weeks prior to planting corn yet stands were equal between the two treatments (Cover Crop Termination Date Ahead of Corn). This past growing season, farmer-cooperators Dick Sloan and Tim Sieren compared terminating their cover crops approx. 3 weeks prior to planting corn with terminating their cover crops within 3 days of planting corn. They also investigated N fertilizer timing and rates across the cover crop termination dates.

You can read the full report of this project here: N Fertilizer Strategies for Corn Following Cover Crop.

How was the trial conducted?

This study was conducted by Dick Sloan in two fields near Rowley in Buchanan County and Tim Sieren in one field near Keota in Washington County.

Cover crops used

• Sloan, corn following corn field: winter wheat, oats, winter barley and rapeseed
• Sloan, corn following soybeans field: winter wheat, winter barley and cereal rye
• Sieren: cereal rye

In addition to comparing cover crop termination dates before corn planting, Sloan compared “low” and “high” N rates in both of his fields (Table 1) while Sieren assessed N application timing and form (applying 140 lb N/ac in each of his treatments) (Table 2).

Findings

At either farm, when the cover crops were allowed to grow until the May 5 termination (1-3 days before planting corn on either farm), the cover crops produced over twice as much aboveground biomass as the cover crops terminated on Apr. 17 (Figure 1).

“I was amazed at how well the corn planted into tall, thick covers,” Sloan said.

Across both farms, terminating the cover crop near the time of corn planting (3 or 1 DBP) often resulted in a yield reduction compared to when the cover crop was terminated about 3 weeks prior to corn planting (21 or 19 DBP) (Tables 3, 4 and 5). Higher N rates or varying the N strategy at the farms did not appear to overcome the yield reducing effects of terminating the cover crop near corn planting. The exception came where Sloan followed corn with corn and applied fall hog manure in addition to applying N fertilizer at corn planting and side-dress (Table 3). In this instance, terminating the cover crop three days before planting corn resulted in no yield drag compared to when he terminated the cover crop 19 days before planting corn.

Sieren applied 140 lb N/ac to all treatments but varied when he applied and in what form. Regardless of N strategy, he scored top returns when he terminated the cover crop 19 DBP (Table 5). Sieren saw greatest returns to N fertilizer strategy when the cover crops were terminated 19 DBP and yields were also greatest. Nitrogen costs were greatest and returns on investment were least when the cover crops were terminated 1 DBP.

“I really expected that this would be an example of ‘Don’t do stupid stuff’, which it kind of is!” Sloan said. “Just the physical environment those young plants grew out of, the shading and crowding of all that residue standing there for weeks, it’s not something a farmer would do. But it was a much closer competition than I expected. The corn after corn plot (Table 3) did not have the same level of competition between covers and corn (Figure 1), so it’s not the cover crop termination date, it’s the amount of cover crop biomass you’re planting into.” The cover crop in the corn-following-corn field at Sloan’s was comprised of winter wheat, oats, winter barley and rapeseed. The lack of cereal rye in that mix may have contributed to the less amount of biomass produced than in the corn-following-soybean field where the cover crop was winter wheat, winter barley and cereal rye.

For more details on this trial, read the full report: N Fertilizer Strategies for Corn Following Cover Crop. This project was supported by the Iowa Department of Agriculture and Land Stewardship, Division of Soil Conservation and the Natural Resources Conservation Service of the USDA.

For more information about this study and other studies as part of PFI’s Cooperators’ Program, contact Stefan Gailans at stefan@practicalfarmers.org