Corn Following a Legume Cover Crop
You finally did it. You took the leap of faith and grew small grains last year. Everything went great – you got your weeds back under control and grew an amazing clover cover crop. Next year you’re going back to corn – but, uh oh, hold on, it’s not business as usual. Now that you’ve grown biological nitrogen with your cover crop you’ll need to adjust your nitrogen plan. And what about terminating that clover before corn? You’ve heard that can be a real chore. PFI members Randy and Willie Hughes, who operate a 5,500 acre split conventional and organic farm in southern WI, joined us for our December small grains shared learning call to address these questions.
Adjusting Your Fertilizer Plan
The first step in deciding how much nitrogen you’ll have to purchase this year for your corn is figuring out how much you already have in the plant matter and the soil from your nitrogen-fixing, legume cover crop. As with different fertilizer products, no two cover crops are created equal in terms of the nitrogen they provide. The amount of nitrogen fixation depends on the biomass produced by the plant and how long it’s been in the field. Luckily, the Hughes provided some rules of thumb that can help put a number to this N source:
|Cover Crop||Biomass||Amount of N|
|1 Year Alfalfa||Over ankle high||100 lbs/acre|
|2 Year Alfalfa||Over ankle high||200 lbs/acre|
|Clover||Way above ankle but below knee||80 lbs/acre|
It’s important to note that all of the legumes listed in the table above are planted in July or August in the year preceding the corn, after wheat is harvested in the Hughes’s operation. “You won’t get nitrogen out of it if it’s only got a couple months of growth,” Randy says, “so it’s got to go in after a small grain.”
But, N in the cover crop is not necessarily correlated directly to available N in the soil that corn can use. So the Hughes designed a study to see which fertilization strategy created the most available N. They compared two different legume cover crop treatments and two manure treatments that varied the time of cover crop planting and manure application. The cover crop treatments were alfalfa planted after oat harvest (summer) or winter wheat with alfalfa drilled into it in the spring. The manure treatments applied 6,000 gallons of hog manure into a non-legume cover crop after wheat (summer) or applied in the spring before soybean planting.
They found, as they expected, that the legume cover crop with more growing time produced more available N, but both cover crop treatments actually had higher N concentrations than either of the manure treatments. The frost seeded alfalfa into wheat resulting in 26 ppm of available N in the soils, alfalfa planted after oat harvest rang in at 20 ppm, hog manure applied after wheat was 14 ppm and spring applied N was only 10 ppm. Through this project the Hughes learned that their green manure strategies were highly effective at providing available N to the subsequent crop.
Terminating the Cover Crop
While you want to give the cover crop as much time to grow as possible to maximize the available N, we also know that killing it before corn planting to avoid yield drag can be tricky. As Willie says, “You’ve got to get it dead or it’ll be a weed for you too.” Their preferred implement is the disc, a fifty foot sunflower 1550, though they say plowing or chiseling could work. They perform 1-2 passes with the disc when the soil temperature is at 45-50 degrees and then plant corn when soil temperature reaches 60 degrees.
One benefit of the nitrogen provided by plowing in the cover crop, often referred to as a green manure, is that the nitrogen is not as susceptible to leaching. “If you get four inches of rain in the conventional world you lose four inches of nitrogen, but with the legume breaking down it doesn’t leach away because it’s not soluble yet.”
When it comes the balance sheet, the Hughes see the benefits. “Small grains have made or saved more money organically than they have conventionally,” Randy says. “You can buy nitrogen conventionally about as cheap as you can grow it, but in organic you can’t.”
Every month we host a shared learning call featuring on growing or marketing small grains. If you’d like to join our next shared learning call, email Alisha@practicalfarmers.org or call 515-232-5661. Learn more about our small grains cost share or other programming at practicalfarmers.org/small-grains-cornbelt .