I received one report of a cereal rye control failure this week as temperatures warmed and rye growth resumed. Some follow-up questions and photos showed that concerns regarding burndown applications during cool, early-spring conditions have merit.
A southeast Iowa farmer applied 1.5 lb acid equivalent glyphosate (~43 oz Roundup PowerMax) with a residual herbicide that has contact activity on emerged plants. Spray volume was 15 GPA, using a 50:50 mix of 32% urea-ammonium nitrate (UAN) and water. The farmer sprayed the herbicide on a warm day in early April, but the temperatures turned cold (highs < 57°F and lows < 40°F) for the following 7 days.
While the cool temperatures probably contributed to the control failure, the application methods also likely reduced the activity of glyphosate. UAN has foliar activity on plants, and also can increase the postemergence activity of herbicides. Damage to the foliage by UAN and residual herbicides can reduce absorption and translocation of glyphosate.
The picture above shows severe leaf burning and spots on some leaves from the spray droplets, but the green growth in the center of the plant is new and vigorous. This photo is courtesy of a southeast Iowa farmer.
We know that many farmers include UAN in these mixes and have successful cover crop termination, but this practice can increase the likelihood of termination failure.
The following best management practices will help reduce likelihood of control failures:
- Whenever possible, spray in sunny conditions when daytime temperatures are above 60°F, the cover crop is actively growing, and nighttime temperatures will be above 40°F. Mid-morning and early afternoon applications are better than late-afternoon applications.
- Follow herbicide label instructions for appropriate adjuvants, mixing order, and application instructions including spray volume, nozzle type, and environmental considerations.
- Avoid cutting herbicide and adjuvant rates. We suggest using 1 lb acid equivalent glyphosate for cereal rye.
- Include only what is necessary for the cover crop termination as the addition of off-label adjuvants, other herbicides, or liquid fertilizers may reduce spray efficacy, especially during periods unfavorable for herbicide activity. If any of these are included, glyphosate rate may need to be increased.
- Consult an expert when you’re not sure! Better safe than sorry.