In a recent issue of Ag Web, sponsored by Farm Journal magazine, an article written by Darrell Smith covered some ideas and advice given by the magazine’s resident agronomist, Ken Ferrie. The following paragraphs caught my eye:

Cover crops can reduce corn yield by acting as weeds in the row and by tying up soil nutrients when they decompose. “If cover crop plants are allowed to grow in the corn row, the corn plants see them as weeds, and it creates stress,” Ferrie says. “Stress lowers yield potential. The longer weeds and corn plants grow together in the row, the greater the reduction in ear size. Even if you take out the weeds, or the cover crop, a few weeks later, the damage has been done. Yield potential has been lost, and you will never get it back.” 

He goes on to say, “Another source of stress on young corn plants is the carbon penalty. When cover crops are killed, the influx of carbon in the residue leads to a higher population of soil microorganisms. They temporarily tie up soil nitrogen and other nutrients, leaving corn plants to go hungry in the critical early weeks. If a cover crop has a high carbon/nitrogen ratio, the longer it’s allowed to grow in the spring, the more residue and the higher the carbon penalty.”

This seems to make sense until you look at a couple of basics:  In most cases, cover crops are planted in the fall,  just after harvest or, increasingly, when the corn is still standing but already matured. (An exception is the relatively experimental “interseeding” of cover crops in the spring, after the corn is about knee high).Thus, the planting of a cover crop in August or September or October would have no bearing whatsoever on yield.

It appears that he may have planted another cover crop in the spring, because the fall planting had winter killed. Then,because of the bad spring weather (2014), he didn’t plant the corn until the end of May, six weeks after normal. He planted into a relatively new cover crop which, of course, would compete for available nitrogen. Then, as it turns out, he didn’t put any ‘starter’ nitrogen on the corn when he planted, but instead waited until weeks later when he sprayed glyphosate to kill the cover crop.

When he concludes that the reason for poor yield was because of “carbon penalty” (residue from dead cover crops creating more microorganisms and thus tying up nitrogen) it may in fact be more due to the anomalies in his experiment that year.

In any case, it’s important to remember to give corn plants a boost of nitrogen when planting – somewhere between 30 and 40 units. And if you’re planting into an existing cover crop, make sure you add the N then, not waiting until you burn down the cover crop, perhaps a month later.