Timing Critical for Postemergence Herbicide Applications

Every crop possesses a critical period during which weeds should be controlled to prevent yield loss.  This period varies among crops and regions and can be influenced by weed species, environment, or crop factors such as variety and row spacing.  For example, seedling corn grows faster than seedling cotton, so the critical period of weed control for cotton is longer than for corn.

Because so many factors can influence this critical period of weed control, a good rule of thumb is that the crop should be weed-free for the first four to six weeks following emergence to avoid yield loss.  Starting with a clean field by using a good burndown program, tillage, or a preemergence herbicide is the best way to allow the crop to get ahead of the weeds.

A common practice that evolved with herbicide-resistant crops was to delay the first postemergence herbicide application to allow more weeds to emerge so they can all be killed at once.  Even before widespread problems with herbicide resistance in weeds, this type of logic was misguided.  Yield loss due to weed competition is nearly inevitable when the first postemergence herbicide application is delayed.  Weed control will likely be incomplete because weed density or size was too great at the time of application making the weeds more difficult to control.  Lastly, multiple herbicide applications may be required for complete control.  Realistically, all fields cannot be treated at the ideal herbicide timing due to environmental conditions, logistics, etc., but they should be treated as close to the appropriate times as possible.

Poor control of large Palmer amaranth with Liberty 280 SL

These concepts translate to all crop-weed situations, but the importance of early herbicide application is even more critical in areas with glyphosate-resistant (GR) Palmer amaranth because the early application timing may provide the only opportunity for postemergence control.  The value of residual herbicides for managing GR Palmer amaranth cannot be understated, and in irrigated crops, residual herbicides offer the best opportunity for controlling GR Palmer amaranth.  However, when GR Palmer amaranth is targeted with a postemergence herbicide, you cannot control it too early, particularly in cotton and soybean where the postemergence herbicide options for GR Palmer amaranth are limited.  A PPOase-inhibiting herbicide (Flexstar, Flexstar GT, Prefix) in soybean or Liberty 280 SL in a LibertyLink system will not control GR Palmer amaranth greater than about four inches in height.  The Flexstar label suggests a rate range for controlling Palmer amaranth with four to six leaves, while the Liberty 280 SL label recommends treating four-inch Palmer amaranth.  In either case, Palmer amaranth should be treated when it is very small.  In fields with GR Palmer amaranth, if the majority of plants are four inches in height, then it is a safe bet that a large number of plants will be five to six inches tall and will likely not be controlled with the postemergence herbicide.

In modern crop production, a large percentage of costs are associated with the bag of seed that is planted.  Also, significant yield losses due to weather late in the season have been common over the last five years.  These factors are out of our control for the most part, so we cannot afford to lose yield to early-season weed competition.  Starting with a clean crop and timing postemergence herbicide applications appropriately are things we can control to avoid yield loss and improve profit margins.