The effectiveness of burndown herbicides can be reduced in cold weather, and we expect to observe sub-optimum performance from herbicides applied late last week or early this week due to the cold conditions. At the same time, soil conditions in many fields are suitable to allow traffic, and rain is forecast for later in the week, so it’s reasonable to expect that herbicides will be applied before the return of warmer weather. We can try to provide some guidance to maximize herbicide activity, but activity can be fairly unpredictable during periods of adverse weather. It is therefore difficult for us to be certain that any of these suggestions will ensure effective control where applicators choose to spray this week.

1. The best strategy is to delay herbicide applications until the return of warmer weather. Even waiting until the end of this week may allow for an improvement in herbicide activity. Our general suggestion is to avoid applying herbicides when nighttime temperatures are below 40 degrees. Allowing for several days of warmer weather prior to application may result in more effective control than applying on the first warm day. By “warm”, we mean daytime temperatures in the high 50s to low 60s, and nighttime temps above 40. Weed scientists from North Dakota (a relatively cold place) suggest that when night temperatures fall below 40 degrees, herbicides should not be applied unless temperatures during the day are above 60.

2. The effect of cold on herbicide activity is likely to vary by weed species and size. We expect that burndown treatments may retain much of their effectiveness on small summer annual weeds that have emerged within the past several weeks. Conversely, activity can be greatly reduced on overwintered winter annuals, biennials, and perennials. For some weeds, such as chickweed and purple deadnettle, burndown treatments may eventually work, but activity may be extremely slow. The problem with slow activity is that there can still be a green mat of weeds at the time of planting, which can interfere with crop establishment and harbor insects. We expect that control of tough no-till weeds such as wild carrot, dandelion, dock, and large marestail will generally decrease. In a study we conducted several years ago at OARDC Northwest Ag Station near Custar, control of dandelion with glyphosate/2,4-D decreased to near zero when herbicides were applied during a period of cold weather at the end of April. For the tougher no-till weeds, failure of an initial herbicide application to control a weed can result in a situation where the weeds become hardened off and even less susceptible to subsequent herbicide applications (i.e. they get “ticked off”).

3. Now for the hard part – specific suggestions for herbicide treatments applied this week. Probably the best advice we can give to maximize activity of burndown treatments is to combine several herbicides that work well together, and increase herbicide rates. Whether following this advice will result in effective control during cold weather is debatable, and waiting for the return of warmer weather is likely to be a more successful strategy. Single ingredient applications, such as glyphosate or 2,4-D ester applied alone, may be affected more by weather compared with application of mixtures of glyphosate and 2,4-D ester. Apply a mimimum of 0.75 lb ae/A of glyphosate and 1.0 lb ai/A of 2,4-D ester where there is adequate time between application and soybean planting, and use a glyphosate rate of 1.1 to 1.5 lb ae/A where the 2,4-D is applied at rates lower than 1.0 lb.

4. Control in soybeans may be improved by applying glyphosate plus 2,4-D ester plus one of the following: Canopy EX, Canopy DF, Synchrony XP, Valor XLT, Sonic, Authority First, or Gangster. While the Valor component of Valor XLT and Gangster may improve the speed of burndown in these types of mixtures, our research shows that Valor can antagonize the activity of glyphosate/2,4-D on dandelion unless chlorimuron or cloransulam is included in the mix. This antagonism may be exacerbated under cold conditions. We would be cautious about full rates of metribuzin (Sencor, etc) with glyphosate/2,4-D this week for the same reason. Conversely, Gramoxone is most effective when applied with 2,4-D ester plus metribuzin or Canopy DF. This might be a week where Gramoxone is a better choice than glyphosate in some fields as long as we get some sun and some daytime temps in the 50’s (the theory here being that because Gramoxone’s activity does not rely on translocation in the plant, cold night temperatures have less effect on it than on glyphosate). We suspect, however, that Gramoxone treatments in soybeans may not be any better than glyphosate/2,4-D treatments on truly tough no-till weeds under cold conditions.

5. Due to the synergism between Gramoxone and atrazine, the combination of these herbicides may be more effective than systemic herbicides in no-till corn under cold conditions. Combinations of atrazine plus Gramoxone (plus 2,4-D for certain weeds) are even more effective than combinations of Gramoxone plus Sencor. A list of the most effective corn burndown programs in OSU research for control of a variety of common broadleaf weeds includes the following: Lexar/Lumax; glyphosate + atrazine; Basis/Resolve + atrazine; dicamba + atrazine; Gramoxone + 2,4-D + atrazine; and Radius/Balance + atrazine + glyphosate. We assume that, under cold conditions, we would be more likely to observe antagonism between glyphosate and atrazine on the more tolerant and larger weeds. Lexar and Lumax have been among the more consistent burndown treatments in OSU research, but we assume their activity could also be reduced under cold conditions. We observed more effective dandelion control with a combination of Lumax plus 2,4-D ester compared with Lumax alone in one study in northwest Ohio, so the addition of 2,4-D ester may help maintain control under cold conditions.

6. Use the appropriate spray adjuvants. We suggest using true ammonium sulfate (AMS) with glyphosate-containing treatments under the current weather conditions, especially where the effectiveness of an AMS substitute product is unknown (see article that follows). While the labels for many “loaded” glyphosate products allow the use of additional nonionic surfactant, we doubt that this will overcome the effect of cold weather of glyphosate activity. However, it can still be a good practice to add nonionic surfactant to certain glyphosate products that contain less surfactant. The addition of crop oil concentrate or 28% to glyphosate-containing mixtures may reduce the effectiveness of glyphosate. Burndown mixtures that do not contain glyphosate should generally be applied with crop oil concentrate, and the addition of some 28% may also improve control. In no-till corn, mixtures of Gramoxone plus atrazine are most effective when the spray solution contains a high concentration of 28%, or 28% is used as the spray carrier.

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