Aerial seeding of cover crops into standing soybeans has begun in Iowa. And farmers are asking questions about seeding, in particular regarding the timing that works best.
"For seeding a cover crop into soybeans, aerial seeding should be accomplished before the R6.5 stage of soybean growth," says Brian Lang, Iowa State University Extension field agronomist at Decorah in northeast Iowa. "At the R6.5 stage, soybeans start showing leaf yellowing in the lower half of the canopy, soon to be followed by some leaf drop. We want the seed on the ground before more than 10% of the leaf drop occurs. There are usually nine to 10 days from beginning R6 stage to R6.5 stage."
When is best time to aerial seed cover crops in corn?
For corn, it's not as clear cut as to when it is best to do what with which cover crops, says Lang. The general idea is to wait until the corn canopy would be a week or two away from starting to "open up" and let sunlight in. "This suggests anywhere from just past ½ milk line to initial black layer," he says. "The overriding factor is soil moisture, and not so much whether its ½ milk line or black layer. After initial black layer has formed in the corn kernel, the crop canopy will start opening up allowing sunlight to penetrate to the germinating/emerging cover crop."
As far as what cover crops and seeding rates, the possible combinations are boundless, notes Lang. If this management is new to you, start simple. The NRCS has a basic publication on cover crops with suggested seeding rates and seeding windows. Go to nrcs.usda.gov/Internet/FSE_DOCUMENTS/nrcs142p2_005818.pdf For those looking for more comprehensive information on cover crops, the "catch-all" website is the Midwest Cover Crops Council homepage mccc.msu.edu/.
Corn growing degree days, growth and development
Another question Lang is getting from farmers has to do with growing degree days and measuring corn and soybean maturity. Here are some guidelines he offers:
Northeast Iowa was anywhere from 2,000 to 2,150 GDD from May 1 to Aug. 27 depending on location. On average the crop is about four days behind the long-term normal. Average GDD accumulation per day in late August is approximately 17, early September is approximately 15, mid-September is approximately 12.
Here is a similar guideline that attempts to include both calendar days and average GDD days for a full season corn hybrid in northeast Iowa. Reproductive development by GDD for a 2,650 GDD corn hybrid (~1,250 GDD from R1 to R6):
* R4: Dough stage 1,925 GDD, about 25 days after R1. You can now estimate yield via kernel counts using this tool agry.purdue.edu/ext/corn/news/timeless/YldEstMethod.html .
* R5: Kernels denting 2,075 GDD, about 32 days after R1.
* R5.25 (full dent): ¼ milk line, 2,150 GDD, about 38 days after R1 (start corn silage harvest for bunkers and bags).
* R5.5: ½ milk line, 2,270 GDD, about 48 days after R1 (corn silage harvest for upright silos).
* R5.75: ¾ milk line, 2,445 GDD, about 55 days after R1.
* R6: Physiological maturity (black layer, safe from frost) 2,650 GDD, about 65 days after R1.
"Aerial seeding of cover crops is generally timed for the beginning of black layer stage," notes Lang.
Soybeans; how do you measure growth and development
Most soybean fields are in early R6 stage with some at R6.5 stage (halfway through the stage). Soybean stages are illustrated and defined at: extension.agron.iastate.edu/soybean/production_growthstages.html Below is a similar guideline for a full season variety in northeast Iowa.
Soybeans: Reproductive development for a full season soybean (~70 days from R1 to R7--frost safe):
* R6: Full seed (pods contain green seeds that fill the pod to capacity at one of the four uppermost nodes on the main stem), 20 days to R7.
* R6.5: At R6.5 stage (about halfway through R6 stage) mid-canopy leaves begin to yellow and drop. "Aerial seeding of cover crops should be planned just before or at the beginning of leaf drop," says Lang.
* R7: Beginning maturity (one pod on the main stem has reached its mature tan or brown color), 10 days to R8 (safe from a significant yield reduction if a killing frost occurs at this point).
* R8: Full maturity (95% of the pods have reached their mature color).