Control of cover crops is a key element if used in a no-till farming operation. Without proper management, cover crops, including cereal rye and annual ryegrass, can compete with crops and potentially reduce yields.
“Some weed scientists say that it’s better to stay away from cover crops rather than risk poor control,” says Dan Towery of Ag Conservation Solutions. “In addition, they say some cover crops have the potential of developing herbicide resistance.
“That opinion, however, overlooks what’s actually happened with the increased use of cover crops in the Midwest and elsewhere. And it also overlooks the huge impact of cover crops on improved soil quality, improved nutrient cycling, improved weed control, and higher yields.”
While annual ryegrass can become a problem if it goes to seed, and is not controlled in the following years, the reports of such issues in the Midwest have been scarce, Towery says. Even where mismanagement occurred, an additional application of herbicides usually took care of the problem.
In the vast majority of cases, Towery says being attentive to management is sufficient to get consistent burndown results. He adds that there are a number of important factors in controlling some cover crops:
Weather and the timing of application
Mixing and application of herbicides
Water hardness and pH
Key Control Factor #1. To maximize the uptake of herbicide — most often glyphosate, with additives — the temperature should be warm, preferably above 50 F on days of application and not below 40 F at night. At lower temperatures, Towery says you need to reduce the amount of water carrier to 7 to 10 gallons per acre, essentially bumping the concentration of glyphosate.
“Application is best when applied on a sunny day,” Towery says. “Application should cease in mid afternoon to provide enough to daylight for translocation to occur.
Key Control Factor #2. Use the full rate of glyphosate (at 5.5 pounds per gallon of active ingredient) suggested on the label — 32 ounces per acre — but be sure to check the concentration on the label. Towery recommends adding 8 ounces of 2,4-D to that mixture to help control any winter annuals and to use flat fan nozzles.
“Don’t add atrazine or any herbicide that will cause antagonism, thus reducing chemical uptake,” he says. “For some residual control, one may add 0.33 ounce per acre of Basis.
“After one application, scout your fields in 2 to 3 weeks and spray again if the cover crop is not fully controlled. Occasionally, the field may be entirely brown, but the annual ryegrass starts greening back up. It should be warmer then and control is relatively easy.
Key Control Factor #3. Hard water can lower the effectiveness of glyphosate and some other herbicides. Test your spray water to determine both the hardness and pH.
According to Towery, Monsanto recommends that 8.5 to 17 pounds of ammonium sulphate (AMS) per 100 gallons of water be added to the herbicide mix to “soften” the water.
“After testing your water, you may find the lower amount of AMS is sufficient to achieve the right balance,” he says. “AMS should be added to the tank solution prior to the herbicide and always consult the label for mixing instructions. There may be limitations on the use of fertilizer-based carriers.”
Towery adds that an elevated water pH may also reduce glyphosate effectiveness in cold weather. The AMS will lower the pH by a factor of 1. If the resulting pH is still above 6.0, some growers are adding citric acid or vinegar to lower the pH and improve performance.