By Justin McMechan, Extension Crop Protection and Cropping Systems Specialist
With cold spring conditions this year, cover crop plant growth has been delayed and questions have come up as to whether that should affect timing of cover crop termination. In turn, this brings up other questions. Growers walk a fine line between growing cover crops long enough to get the biomass they want without reducing yield in the following grain crop.
There are two primary considerations: the goal for the cover crop and objectives related to a grower’s management perspective. First, are cover crops being grown to achieve soil health objectives (interest in root growth, erosion control), which might argue for an early or timely termination or are they being grown for forage production (top growth), which might call for a later termination date.
Other goals might include for N contribution, which would argue for a later termination date as well. Second, producers will need to consider the effect of cover crop termination date on delayed planting and the potential for grain yield loss.
It is better to stay with a timely termination as temperatures warm up. (See factors affecting cover crop production in the 2016 CropWatch article https://cropwatch.unl.edu/what-can-we-expect-forage-cover-crops in the first section on variable production).
Cover crops need to be planted early enough in the fall (and have the needed light, water, and nutrients) to grow well to survive the winter. Heavy precipitation near harvest last fall delayed planting of cover crops, with many not planted until the third week of October. With the onset of cold, many cover crops, especially small grains, did not grow enough in the fall to survive the winter well.
While we aren’t seeing enough warm-up this spring for sustained growth, we are seeing the lengthening of the photoperiod and much development is progressing. Cereal rye, barley, triticale, and wheat planted in late August and early September have survived, are tillering, and were just starting to elongate, but the production was not there.
Generally, our studies have indicated that cover crops planted prior to October 1, 2017 achieved sufficient growth in the fall to survive the winter well with substantial spring growth. Cover crops planted after October 1 are not faring as well. We also had some cover crops planted this spring on March 23 (more for forage). The seed had germinated by April 9, but they still have not emerged.
NRCS Termination Guidelines
Growth of cover crops in the spring can be fundamental for growers enrolled in the NRCS Environmental Quality Incentives Program (EQIP). This program provides economic incentives to qualifying growers who plant a cover crop as long as they meet NRCS requirements. According to the 2014 NRCS Conservation Practice Standards Code 340, winter annual cover crops should be planted within two to four weeks of harvest to allow for adequate spring growth.
In late October of last year we planted wheat, rye and triticale cover crops in eastern and south-central Nebraska as part of a study on termination timing. On April 10, a portion of these cover crops were terminated with little growth (1-2 tillers per plant) and extended leaf heights of 3 inches. To qualify for a payment, growers enrolled in EQIP would have to allow these cover crops to reach a minimum height of 6-8 inches as indicated in the Nebraska NRCS Cover Crop Code 340.
Growers should also consider the cover crop termination relative to planting a cash crop. In Nebraska, NRCS Termination Guidelines (2014, Version 3) indicate that cover crops in the western and central part of the state (Zone 2) should be terminated at least 15 days prior to planting a subsequent cash crop. In contrast, cover crops in the eastern third of the state (Zone 3) can be terminated at or before planting the subsequent crop. Such guidelines only pertain to cover crops planted into non-irrigated fields to promote best management practices that minimize risk of yield loses due to a lack of soil water.
Insects and Cover Crops
Questions still remain about the role of insects in cover crops systems. Previous research has shown the potential for both pest and beneficial insects. Last spring, some growers in south central and eastern Nebraska experiences problems with wheat stem maggot in wheat and rye cover crops transitioning to corn. In most cases, the cover crop was still alive at the time of corn planting. Growers and consultants are encouraged to scout fields for insects prior to planting a cash crop. For more information see Insects in Cover Crop Systems (April 6, 2018) in CropWatch.
Consideration should be taken when transitioning from rye or wheat to corn as insects can span the host range of these crops. Careful management is necessary as beneficial insects have shown significant potential to reduce primary insect pests in subsequent cash crops. In addition, growers enrolled in EQIP programs should prioritize early planting of cover crops to increase the likelihood of achieving minimum growth requirements prior to spring termination.
Burndown Weed Control Considerations
Glyphosate is the most common herbicide for terminating cover crops, primarily for grass cover crop species such as cereal rye, the most common cover crop species in Nebraska.
Additionally, glyphosate and other burndown herbicides such as 2,4-D, Sharpen, and dicamba are applied for control of winter annual weeds. The most common winter annual weeds in Nebraska are marestail, henbit, field pennycress, prickly lettuce, shepherd’s-purse, downy brome, tansy mustard, and dandelion.
Efficacy of glyphosate can be affected by temperature. When the temperature is below 40°F for an extended time after burndown herbicide application, weed control will most likely be reduced, specifically for a systemic burndown herbicide such as glyphosate or 2,4-D. Additionally, weed control may be reduced under cloudy conditions following an initial temperature drop below 40°F.
The likelihood of reduced weed control due to cool temperatures will vary depending on the target cover crop/weed species, type of herbicide, and rate of application. The ideal temperature for applying most post-emergence herbicides is between 65°F and 85°F; however, that window is not always practical with other spring practices.
Burndown herbicides can be applied at temperatures of 40°F to 60°F, but weeds may be killed slowly. When the temperature is below 60°F, absorption of herbicides such as glyphosate and translocation of herbicides such as 2,4-D are lower compared with applications at higher temperature; therefore, they act slowly.
Be sure to add labeled adjuvants to improve burndown herbicide efficacy. For example, if you are planning to apply 2,4-D, add crop oil concentrate at 1% v/v (1 gallon per 100-gallon spray solution) or non-ionic surfactant at 0.25% v/v (1 quart per 100-gallon spray solution). Glyphosate should be added with ammonium sulfate and crop oil concentrate or non-ionic surfactant. Spray volume should be 15 gallons per acre or more for better coverage when a dense weed population is present.
Burndown herbicides should not be applied unless there are one to two days with temperatures over 50°F to assure good results.
Growers should also consider effects of late cover crops termination on efficacy of pre-emergence herbicides in corn or soybean. Applying pre-emergence herbicide on cover crop biomass, particularly on dryland, may affect herbicide efficacy and length of residual activity. To achieve good weed control throughout the season, we recommend a pre-emergence herbicide, particularly in soybean as we do not have very effective post-emergence herbicide options.
There is always concern about the amount of glyphosate being used to kill cover crops. A 2017 survey in Nebraska showed more than 95% of cover crop growers relied on glyphosate for termination. While it is mostly being applied to grasses, there are six species (mostly summer annual broadleaves) that are glyphosate-resistant.
The concern is that the increased and repeated use of glyphosate could increase the selection pressure and the evolution of winter annual weeds resistant to glyphosate. Therefore alternate herbicides or tank-mixing glyphosate with other burndown products such as 2,4-D, dicamba, saflufenacil (Sharpen), paraquat (Gramoxone), glufosinate (Liberty) could also get the job done while reducing the selection pressure; however, this increases herbicide input cost, so growers would have to factor that into their decision-making process.