North Carolina cotton producers have made great strides with managing glyphosate-resistant pigweed over the last several years. Although it can be expensive and inconvenient, we have the tools we need to do a good job with managing this challenging pest. However, as we have learned, we can get a lot more bang for the buck in our herbicide program by being timely, especially with post-emergence applications. Using the same products in the same aggressive approach, improving timeliness can drastically improve weed control, reduce handweeding expenses, and can prevent resistance to other herbicides. Several factors including weather, equipment limitations, equipment breakdowns, and size/scope of a producer’s operation, can affect how timely we can be, however, striving to improve timeliness within reason will help every producer’s bottom line and sustainability.

For early May planted cotton, pre-emergence residual herbicides are likely beginning to break down. Therefore, timely post-emergence applications will soon be needed to provide 1.) overlapping residuals, 2.) control of SMALL emerged weeds, or 3.) BOTH!! To view a few of the most commonly recommended herbicide programs, please see our earlier post on cotton.ces.ncsu.edu (https://cotton.ces.ncsu.edu/2015/04/weed-management-programs-for-nc-cotton-in-2015-collins-york/) and/or the 2015 NC Cotton Information Book (http://content.ces.ncsu.edu/cotton-information/).

Overlapping residuals is a weed management concept that is proven to be successful. This approach allows for a rotation of modes of action within a season and ensures the presence of continued residual herbicide activity to prevent weed emergence from burn-down until canopy closure. Timely post-emergence applications are critical to achieving this. However, using the same products, delayed post-emergence applications allow for gaps in time for pigweed to emerge. The photo below illustrates how a significant number of pigweed can emerge during the time that pre-emergence herbicides breakdown and the first post-emergence application, if that post-emergence application is delayed by just one week. Spraying bare-ground in a timely manner is a phenomenon that we need to embrace.

The Value of Overlapping Residuals

Overlapping residuals can greatly reduce the number of pigweed that emerge, however we still need to deal with those that do emerge in a timely manner. There are a few herbicide options to deal with emerged pigweed (see previous article mentioned above). One of the most commonly used and effective options is Liberty (if Liberty-tolerant varieties are used). However, this product is only effective when used in a very TIMELY manner. The point we want to make here is that we BADLY need to target SMALL pigweed when using Liberty. SMALL is defined as the largest pigweed in the field is 3 inches tall or less. Spraying 4 inch weeds or taller is not a sustainable practice and can lead to resistance, despite some claims that taller weeds can be managed with Liberty. The timeliness factor is really more important for pigweed versus other weed species, simply because pigweed can grow at a rate of 1 inch or more per day in optimal conditions. Therefore, it does not take much time at all for an emerged pigweed to reach a size that is uncontrollable with Liberty. The photos below illustrate what can happen when targeting larger pigweed with Liberty versus a timely application targeting small weeds. In the first photo, notice that the smaller weeds are killed, however pigweed greater than 3 inches were burned back but not killed. The second photo is a close-up on how larger pigweed can initially be burned back with Liberty, however when these weeds are not completely killed, new vegetative tissue can arise from lower buds and plants will regrow.

The Effect of Liberty on Large vs Small Pigweed

Regrowth of Pigweed when Liberty is Targeted at Larger Weeds

No one likes to handweed, nor the added expense of such. Targeting small pigweed with Liberty using TIMELY applications can drastically reduce the number we have to handweed. Now imagine that pigweed becomes resistant to Liberty in the future, and it is therefore no longer an option for effective control!!!!!!!!!