Great article by Rod Swoboda of the Wallaces Farmer about when to cut Alfalfa and when to plant Cover Crops. I'd also add that around eastern Iowa silage cutting should be about done based on moisture in the silage (if the rains allowed cutting) and so you could be planting alfalfa or cover crops now in those fields.
Alfalfa and Cover Crop Notes
Compiled by Rod Swoboda of the Wallaces Farmer
Fall harvest tips for alfalfa—taking the final cutting
Lang says he and his ISU Extension colleagues are also getting questions about alfalfa harvesting management. This time of year, there are always questions regarding when that last cutting of alfalfa hay could be made and still allow enough time to build root food reserves before the first killing frost hits in the fall. The answers to these questions are usually something like… “it’s fine to harvest through the first week of September” and “we recommend harvesting at least six weeks before the killing frost.”
On average, the alfalfa killing frost (25 degrees F or below) in northeast Iowa occurs in the third week of October. So there is minimal risk harvesting alfalfa through about Sept. 10 in northeast Iowa, notes Lang.
Watch number of growing degree days, not the calendar
While those “good old answers” still work, the more correct answer actually deals with
growing degree days (GDD), not the calendar, says Lang. Researchers now define a risk assessment of fall harvest based on alfalfa GDD. The research basically says as long as the plants accumulate at least 500 GDD from harvest to killing frost, the plants will have stored enough root carbohydrate to survive the winter.
A nice summary of this research is available at the following website. The data from Lancaster and Beloit, Wisc. (southern Wisconsin) would apply quite nicely to northeast Iowa, he says. That website is uwex.edu/ces/forage/pubs/Late-Summer-Cutting-Management-of-Alfalfa.pdf.
The Lancaster & Beloit data suggest alfalfa harvested through the first week of September is very low risk of winter injury, having plenty of time to replenish root carbohydrates going into the winter. A September 15 harvest could start providing some risk, and a September 21 harvest even more risk. “These risks do not mean that you will lose the entire stand, but rather would likely lose a percentage of plants and reduction in first crop yield next season because of winter injury and slowed plant recovery in spring,” says Lang.
What if you chose to harvest “after the killing frost”?
Growing Degree Day (GDD) research says as long as the plants do not accumulate more than 200 GDD from after harvest to before the killing frost, the plants should still overwinter just fine. “This means that you do not have to wait for the actual killing frost to occur as long as you are close enough to it when you harvest,” says Lang.
For example, October 15 is a good cut-off date in locations if the killing frost has not occurred yet, it likely will soon, and the weather in late October is usually cold enough that 200 GDD will not accumulate in the time remaining in the fall. A critical issue with harvesting after a killing frost is that little to no regrowth will occur following the harvest, so you want to cut high, leaving a good stubble height (approximately 6 inches) to help trap snow and insulate the plants.
Cover crop time of seeding, does it make a difference?
Lang is also fielding questions from farmers this week regarding cover crops. Aerial seeding of cover crops into standing soybeans usually begins when the mid-canopy leaves start to yellow, he notes. This is around the R6.5 stage of soybean growth.
Some leaf drop begins at this time, and it’s nice to have the cover crop seed under the leaf drop rather than on top of the leaf drop. So, we like to get the seed on the ground before more than 10% of the leaf drop occurs. There are usually ~9 days from beginning R6 stage to R6.5 stage.
For corn, it’s not as clear cut as to when it is best to do what with which cover crops. 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 half-milk line to initial black layer. The overriding factor is soil moisture, and not so much whether its half-milk line or black layer. After initial black layer the 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. If this management is new to you, start simple. 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/.