With some cropping regions experiencing dry spring weather, some farmers and applicators are considering spraying burndown applications earlier than normal. But Penn State University weed specialist Bill Curran reminds no-tillers that the activity of all foliar applied herbicides is dependent on air temperature and other variables.

"If you are trying to kill a cover crop or even winter annual weeds, susceptibility of the target species and environmental conditions like temperature, moisture and sunlight play a role in the effectiveness of the treatment," Curran says.

Curran cites a University of Illinois field study that compared glyphosate, paraquat (Gramoxone), and paraquat plus metribuzin (Sencor) for their performance under challenging spring conditions. The herbicides were applied at six different timings in the spring based on daytime high air temperatures ranging from 47 F to 87 F.

Curran says common chickweed and henbit were present in the study.

Overall, temperature had no effect on chickweed control with glyphosate or with paraquat plus metribuzin and both provided 90% control or better regardless of air temperature. However, control increased from about 70% at 47 F to about 90% at 75 F with the paraquat alone treatment.

In contrast, Curran says henbit control was less than 80% with all herbicides until applications were made when daytime high air temperatures were above 75 F. In fact, control with glyphosate and paraquat was less than 50% at temperatures up to 75 F. The paraquat plus metribuzin treatment was somewhat better providing close to 80% control at 75 F.

"This study is a nice example of how not only temperature plays a role in foliar herbicide activity, but also target species and how a soil residual herbicide like metribuzin or atrazine can increase effectiveness, especially with a product like paraquat," Curran says. "Temperature had little influence on common chickweed control with glyphosate; however, application temperature significantly affected glyphosate activity on henbit."

Curran says no-tillers should also consider herbicide rates; overcast conditions at application; adding AMS and other appropriate adjuvants to the spray tank; tip selection; and gallonage for better spray coverage when making spring applications under less-than-ideal conditions.