The success of your burndown program for glyphosate-resistant horseweed, wild radish, or other problematic weeds is largely dependent on the timeless of your application. Early to mid March is generally the optimal time for burndown applications for multiple reasons. First and foremost, applications need to be made when weeds are relatively small. Targeting your burndown applications to horseweed that is still in the rosette stage (see Figure 1 below) will likely result in the best control possible.

Figure 1. Horseweed at the rosette stage.
rosette horseweed

Additionally, early to mid March burndown applications allow enough time to elapse between burndown and cotton planting to suffice the labeled waiting intervals for many of these herbicides. Applications made during April and/or targeted at larger weeds (see Figure 2 below) may result in suboptimal control and may not allow sufficient time to meet the waiting intervals between burndown and cotton planting for most burndown herbicides.

Figure 2. Larger horseweed.
larger horseweed

The following are recommended burndown tankmixes for control of glyphosate-resistant horseweed (marestail) and continued residual control. Again, it is important to target these applications to small weeds in early to mid March.

Glyphosate* (32 oz/A) + Clarity** (8 oz/A) + Valor (2 oz/A)
*Recommended rate of glyphosate is for products containing 4.5 lbs/gal of glyphosate acid. Adjust rates for acid equivalency if using other glyphosate products with different concentrations of glyphosate acid. See Table 10-8 in 2015 Cotton Information for adjusting rates among various glyphosate products.

**The waiting interval for Clarity is the following: accumulate at least 1 inch of rainfall, then wait 21 days before planting cotton

Glyphosate* (32 oz/A) + 2,4-D** (32 oz/A) + Valor (2 oz/A)
*Recommended rate of glyphosate is for products containing 4.5 lbs/gal of glyphosate acid. Adjust rates for acid equivalency if using other glyphosate products with different concentrations of glyphosate acid. See Table 10-8 in 2015 Cotton Information for adjusting rates among various glyphosate products.

**The waiting interval for 2,4-D is 30 days between preplant burndown application and cotton planting

Glyphosate* (32 oz/A) + Sharpen** (1 oz/A)
*Recommended rate of glyphosate is for products containing 4.5 lbs/gal of glyphosate acid. Adjust rates for acid equivalency if using other glyphosate products with different concentrations of glyphosate acid. See Table 10-8 in 2015 Cotton Information for adjusting rates among various glyphosate products.

**The waiting interval for Sharpen is 42 days and accumulation of at least 1 inch of rainfall between preplant burndown application and cotton planting

See Table 10-3 in 2015 Cotton Information for assistance in selecting burndown herbicides for other common weeds. http://content.ces.ncsu.edu/weed-management-in-cotton.pdf