Now is the time to start planting cover crops. Here are some things to consider prior to planting:
- Herbicide carryover from spring and summer applied herbicides in corn, soybean, and small grains may be an issue.
- Feeding or grazing a cover crop mean that it is now a forage crop and herbicide rotational restrictions apply.
- Fall burndown applications prior to cover crop seeding should be carefully evaluated based on cover crop species and weather conditions.
Persistence of herbicides in the soil depends on a variety of factors including rainfall, temperature, and soil properties such as type, pH, and organic matter content. Breakdown of herbicides in the soil typically occurs faster in warm soils receiving abundant rainfall.
There are only a few herbicides that explicitly restrict seeding of cover crops on their labels; they are Harness, Harness Xtra, Hornet, and Python WDG. Rotational restrictions tend to focus more on the safety aspect of using crops to feed or graze livestock and not necessarily on persistence/carryover concerns. Remember that if a farmer chooses to plant a cover crop before the rotational restriction has expired then the farmer assumes the risk for that seeding.
Penn State Extension Weed Scientists, Bill Curran and Dwight Lingenfelter, have an excellent article discussing herbicide persistence. There is also a handy tablethat includes the most common corn and soybean herbicides and their likelihood of carryover injury to grass and broadleaf covers or in Penn State's Agronomy Guide (Table 1.10-6). For more about herbicide carryover research, read the No-Till Farmer's interview with Bill Curran and Kevin Bradley, Extension Weed Scientist at University of Missouri.
When a cover crop is harvested for feed or grazed, then the herbicide rotational restrictions for the preceding cash crop need to followed. Penn State's Agronomy Guide lists rotational restrictions for some legume and small grain crops seeded after corn and soybean herbicides (Table 2.2-17 and 2.4-15). University of Wisconsin Extension has a fact sheet with charts of corn, soybean, and small grain rotational restrictions for 15 different forage crops. There are also fact sheets for winter ryelage after corn silage and spring-seeded forages.
Fall burndown programs in no-till systems may be somewhat limited by the cover crop species. In the Agronomy Guide (Table 2.2-6), efficacy of five fall-applied options are listed:
- Autumn Super +2,4-D- not a good option for cover crops based on the rotational restrictions (e.g. Autumn Super- Barley requires 15 inches of rain and 9 months) Glyphosate - no carryover concerns but only one mode of action, not ideal for herbicide resistance management
- Glyphosate + 2,4-D - okay for grass cover crops, need to wait 30 days before broadleaf cover crops
- Glyphosate+Clarity/Banvel- not recommended at less than 120 days after application
Another option to accommodate cover crops is to focus on a spring burndown that includes an herbicide with residual activity.