Cover crops provide multiple benefits with regards to protecting soil from erosion, improving soil health, and as a component of a nutrient management plan. For those cover crops that over winter and resume growth in the spring, for example, cereal rye and annual ryegrass, an important question is when to terminate that cover crop. That decision should consider the next crop, planting date of that next crop, the spring weather pattern and purpose of the cover crop. For cover crops that have not been planted with the intention of providing a forage harvest, and that are on acres intended for corn grain production, this may be a year to consider early termination of that cover crop.
A driving factor for early termination of cover crops this year is the potential for a drier than average spring and summer. On a recent OSU Extension Ag Crops team conference call, Jim Noel from the National Weather Service talked about weather patterns following an El Nino year. Often the pattern is for the spring and summer months to be drier and warmer than average. At this point, warmer than average weather and plant growth points to an earlier spring. I have talked with several farmers who tell me that our soil moisture is drier than average. If this pattern holds, the risk is a cover crop can take up moisture that should be saved for the cash crop. At the recent conservation tillage conference in Ada I saw data that showed lower corn yields following cover crops in dry years when those cover crops were not terminated early enough. Those cover crops robbed soil moisture leading to delayed germination and slower development that was not made up compared to a corn crop planted with no cover crops or planted into a winter killed cover crop.
Given the risk of or the potential for a drier than average spring and summer, cash grain corn producers should consider terminating cereal grain and annual ryegrass cover crops in the late March to early April time frame. Ideally we would like to see less than 8 inches of growth for either of those crops. I have read several sources that recommend annual ryegrass be terminated at 6 inches or less of growth. The recommended method for early termination is the use of herbicides.
Glyphosate should be effective, especially if day time temperatures are above 50 F, and is probably one of the most economical options. A Purdue Extension publication entitled “Successful Annual Ryegrass Termination with Herbicides” says that producers need to use at least 1.25 lbs. of acid equivalent /acre of glyphosate and possibly up to 2.5 lbs. of acid equivalent /acre of glyphosate under less than ideal conditions for herbicide translocation. Purdue research also shows that mixing 1 oz. of Sharpen with 1.25 lbs. of acid equivalent rate of glyphosate provides the most consistent results in terminating the cover crop. Mark Loux, OSU Extension Weed Specialist says that the Sharpen option only needs to be used where there is a desire for burndown help with marestail or other weeds. The advantage of using Sharpen for this purpose in the tank mix is that it won't reduce the glyphosate activity which can happen with atrazine or 2,4-D in the mix. Dr. Loux’s recommendation is to use a minimum of 1.5 lbs. of acid equivalent/acre of glyphosate for cover crop termination and only include Sharpen if marestail control is needed. For those who might be interested, see the April 14th, 2015 (CORN 2015-08) issue of the CORN newsletter by Mark Loux about cover crop burndown.
For more information about cover crops and termination options talk to a member of the OSU Extension Agronomy Team.