There is very little documented research on the effect of environmental conditions on specific herbicides. We know that temperature and moisture probably do influence performance of some herbicides, but the research required to make solid statements has not been done, for the most part. That said, you will find a lot of anecdotal information concerning a number of common herbicides. Many extension services have published this information. Here are some pointers that might help get the most out of your herbicide.
MOST HERBICIDES LIKE WARM TEMPERATURES
A North Dakota State University (NDSU) publication says the ideal temperature for most herbicides is 18 to 30C. Temperatures below 18C will slow the speed of weed kill and may increase the risk of crop injury. The NDSU publication says growers should pay particular attention to temperature when using the following herbicides:
ALS grass herbicides tend to work better in warm, dry conditions.
ACCase “Fop” grass herbicides are an exception. They are more effective under cool conditions, whereas other ACCase grass herbicides work better under warm conditions.
Spraying Achieve under cool or cold conditions can result in injury to wheat, especially if there is a significant rainfall shortly after application.
Cold temperatures following an application of the ALS broadleaf herbicides Sencor and bromoxynil can cause crop injury. Delay spraying these products until daytime temperature exceeds 15C.
Cold temperatures can reduce weed control of Basagran, Liberty and paraquat.
Cold temperatures following an application of 2,4-D, MCPA, dicamba, clopyralid, fluroxypyr, and glyphosate will slow weed kill.
LV ester formulations of 2,4-D, MCPA and dicamba can vapourize at temperatures above 21C and vapour drift can result in damage to neighbouring susceptible crops.
A Swedish study found warm weather increased the efficacy of dichlorprop, but decreased the performance of tribenuron-methyl. However, precipitation and warm soils increased the performance of both herbicides.
In the United States, a number of ALS-inhibiting broadleaf herbicides such as Ally carry precautionary statements on the label suggesting the potential for crop injury if applied during prolonged cool weather and wet conditions.
website of four northern U. S. state grain growing associations, lists a number of other herbicides affected by temperature. These include:
Puma is more effective on wild oats in cool weather than when applied in warm/hot conditions.
Wild oat control by Puma is reduced by drought stress.
Achieve and other ALS grass herbicides work better in warm, dry conditions.
Assure II, Poast, and Select control grasses better in warm weather when grasses are actively growing.
Addition of MSO-type adjuvants increase herbicide phytotoxicity in very humid and hot weather.
The Saskatchewan Agriculture publication “Herbicide Considerations for Dry Conditions” states: “Most contact herbicides will become more active as temperature rises.” This includes Group 5, 6, 7, 10, 14 products. However, high temperatures also increase the risk of a herbicide causing crop injury. As a result, the Saskatchewan publication suggests that applying these products in the evening may limit the risk of crop injury, yet still enhance the performance of these contact products.
SUNLIGHT CAN INACTIVATE SOME PRODUCTS
According to NSDU publications:
Sunlight will inactivate Trifluralin and Eptam unless these products are incorporated.
“Dim” grassy herbicides including Achieve, Poast and Select will be degraded by sunlight if left in non metal spray tanks for extended periods of time or if applied at mid day. Use of an oil-based adjuvant increases absorption speed and lessens the risk of breakdown of these products. Another option is to spray these products late in the day when the risk of breakdown of the product by sunlight is reduced.
Bob Hartzler, extension weed specialist at Iowa State University, says intense sunlight can decrease herbicide performance while a long period of low sunlight intensity can increase herbicide performance. “Changes in light intensity can impact leaf cupping and leaf alignment resulting in a decrease in the leaf area exposed to the herbicide application,” he says. “Prolonged periods of low light intensity also results in a reduction in the wax on a leaf surface, so under these conditions not only will herbicide uptake be greater but there can be an increased chance of crop injury under these conditions.”