If you're still in the learning phase on cover crops, you may wonder if you can cut nitrogen rates if you planted a cover crop. Experts say that all depends on the mix of cover crops you planted, the stand you obtained and when you killed the cover corp.
Some cover crops don't produce nitrogen, but may have captured N last fall after the unusual 2012 season. These would include wheat and rye. Many people planted them on fields where corn yields were extremely low to tie up the N so it wouldn't be lost into the air or through tile lines. The N in this case was applied last year, often by sidedress, but never used by the crop.
Dramatic photos taken last fall indicate that the cover crops did their job in tying up N that wasn't used. In some fields that were seeded early after the poor corn crop was removed, the growth was noticeably taller and more flush over the bands that represented where the N was applied.
Other cover crops, including such things as crimson clover and hairy vetch, can actually produce nitrogen if given enough time to grow in the spring before they are killed with a burndown herbicide application.
Larry Huffmeyer, Napoleon, a Syngenta rep and a farmer, planted into some crimson clover in early May, then killed it. However, he also left a couple of small patches. They were still growing in the third week of May. The clover bloomed. Huffmeyer left the small areas on purpose to see how much N the crop could produce if it was allowed to grow longer. He took samples and sent them to the lab for analysis. Depending on what he finds out, he may adjust nitrogen rates on those areas where corn will be planted.
One thing about cover crops, however, is that the N isn't available right away for the plant. That's why early season N applications are still important. The N will be available during the season as the cover crop decays and releases nutrients.
For more info on cover crops, download our free report: Cover Crops: Best Management Practices.