1. Residual herbicides and rainfall. Residual herbicides do vary in the relative amounts of rain needed for “activation”, or adequate movement into the soil to reach germinating seeds. Most growers are applying mixtures or premixes of several products, so we’re not sure these differences are as important as the overriding principle here. Residual herbicide treatments need to receive a half to one inch of rain within a week or so after tillage or an effective burndown treatment, to control weeds that can will start to emerge at that time. This varies with timing of application and weather. Summer annual weeds are the target here, and their emergence ramps up in early May, although cold weather can slow this down. So residual herbicides applied in mid-April, prior to most of the summer annual weed emergence, may not need rain as soon after application, compared with herbicides applied in May. Aside from this, residual herbicide activity is not really dependent upon soil reaching a certain temperature. Under more marginal rainfall conditions, it’s possible that herbicides may control the small-seeded weeds that emerge at or just below the soil surface, but be less effective on larger-seeded weeds that can emerge from deeper. In a tilled situation, a timely rotary hoe can be used to remove some of the weeds that are about to emerge (the “white stage”) and buy some time for rain. The good news here is that we have effective POST herbicides to remedy many situations where the residual herbicides are not completely effective.
2. Residual herbicides and crop injury. The concerns here seem to be more about soybean herbicides, which may partly reflect the overall greater safety of residual corn herbicides. Several residual soybean herbicides can cause injury, depending upon when they are applied relative to planting, rainfall, soil type, seeding depth, etc. These include products that contain metribuzin, sulfentrazone, flumioxazin, and chlorimuron. One of the things that has reduced our risk of injury from all of these herbicides is that in no-till soybeans they have usually been applied a week or more prior to planting to accommodate restrictions on 2,4-D ester and dicamba. Application at or after planting increases the risk of injury, as does use in tilled situations. We have increased metribuzin use substantially over the past decade, but injury has been extremely rare due to application prior to planting and use of relatively low rates in combination with other products. We hear more about injury or suspected injury from flumioxazin and sulfentrazone when wet weather delays planting and forces application of residual herbicides after planting. It’s worth noting here also that the Xtend and Enlist soybean systems do away with the wait to plant soybeans for dicamba and 2,4-D, respectively, and more growers may be waiting until after planting to apply burndown/residual herbicides.
In brief, symptoms of these are as follows: chlorimuron – slowed development, stunting, yellowing; flumioxazin and sulfentrazone – necrosis on young leaves and stem, stunting; metribuzin – usually delayed until first trifoliate, yellowing and possibly necrosis on margins of older leaves. Cloransulam, imazethapyr, and imazaquin are generally safer on soybeans than chlorimuron, in situations where injury is a concern. Activity of metribuzin varies considerably with soil texture and organic matter content, so using the labeled rate for soil type is important. Injury from any of these may be more likely when herbicide application is delayed for several days after planting, followed by substantial rain as the soybeans are about to emerge. Labels for products containing flumioxazin state that soybeans should be planted 1 ½ inches deep and herbicide should be applied no later than three days after planting, in an attempt to avoid this situation (does not always work). The good news here is that early injury to soybeans usually does not reduce stand, but may slow early growth and rate of crop canopy development and leave soybeans open to the effect of other stresses. In some of these situations, it can be difficult to sort out how much of the damage is due to herbicide and how much is due to other factors. Yield loss is probably infrequent based on the soybean plant’s ability to compensate for these types of factors.
3. Cold weather and burndown herbicides. We had a fairly warm winter and early spring, followed by the recent month of colder than normal weather. The net result of this is large winter annual weeds, and weather that is currently not terribly conducive for burndown activity. There is not much specific guidance on herbicide labels about cold weather, just general statements about how effectiveness can be reduced under adverse conditions that include cold weather. We expect many experienced applicators may have their own set of rough guidelines on this, or at least gut feelings. Under cold conditions, the rate of herbicide activity declines and also the overall effectiveness. It’s more difficult to define the weather conditions when herbicide should not be applied. These would certainly include periods when frost or freeze is occurring overnight and daytime weather is cool and cloudy (less than about 50). One night of frost followed by a warm sunny day may still allow for decent herbicide activity, if weeds appear sufficiently recovered from the frost. Aside from this we could make a general recommendation to keep applying as long as night and day temperatures are at least 40 and 60 to 70, respectively, although this is still not ideal compared with day temperatures higher than 70 with sun. One way of dealing with this problem is to just wait for a return to warm, sunny weather before applying burndown herbicides. Another is to increase herbicide rates and use a more comprehensive herbicide mixture. For example, adding Sharpen to a mixture of glyphosate plus 2,4-D or dicamba. As with the less than effective residual herbicides under dry weather, burndown herbicide problems can sometimes be resolved with an effective POST treatment of glyphosate, 2,4-D, or dicamba, depending upon the trait system.
4. Reminder about the value of fall herbicides. Fall herbicides are an essential tool for marestail management, but given our current situation of dense, big weeds in no-till fields and potential problems with burndown herbicide effectiveness, it’s worth reminding all of us why fall herbicides started being used in the first place. In the late 1990’s, growers were experiencing problems with dense stands of winter annual weeds such as chickweed that interfered with tillage and planting. One contributor to this was the occasional reduced activity of spring-applied burndown herbicides in cool weather, which resulted in too slow death and dry down of weeds to prevent the problems the weeds caused. Fall-applied herbicides became a solution to this, since they result in almost weedfree spring seedbeds up until the point when giant ragweed and other summer annuals emerge (early May for most of these). As anyone knows who has used fall herbicides, their effectiveness reduces the overall importance of the spring-applied burndown, since it does not have to control a mess of large, overwintered weeds. It’s all just way easier. And issues with cold weather and spring-applied burndown herbicides are therefore less important. For as little as $6 worth of fall-applied herbicide. Something to think about moving forward.