By William Curran, Extension Weed Scientist

It’s been a challenging spring to control some weeds and even cover crops with the cool weather and frequent rain we have been experiencing the last few weeks. In particular, if you don’t successfully control horseweed/marestail prior to soybean emergence, the POST options are few. 

Performance of burndown herbicides for resistant horseweed/marestail control can vary under the weather conditions we've been experiencing. The herbicide 2,4-D ester is one of the more common tankmixes for control of this weed, but the pint rate (3.8 lb acid/gal or approximately 44%) is not completely effective on larger rosettes or plants that are beginning to bolt.

In a cover crop trial at Rock Springs, we observed some exceptionally large rosettes this spring that appear to have survived last fall’s glyphosate burndown treatment (see accompanying image). Our recent application of a pint of 2,4-D ester plus glyphosate does not appear to have done the job on these big guys. A quart/acre or 1.0 lb acid is better, but it requires up to a 30-day waiting period for soybean planting.

The labels for some 2,4-D products specify only a 15-day waiting period between application and soybean planting for rates up to 1.0 lb/A, so check around. The other burndown herbicide that is frequently used for horseweed/marestail control is Sharpen (also a component of Verdict and Optill).

Sharpen is a contact type or Group 14 herbicide that requires adequate spray coverage for optimum performance. On PA soils, you can only apply 1 oz of Sharpen/acre at soybean planting or 1.5 oz 14 days ahead of planting. You should be applying Sharpen in 15 to 20 gallons of water per acre using flat-fan nozzles. Be sure to include MSO plus AMS or UAN in the mix.

If you did not get adequate control of horseweed/marestail in the burndown and soybeans have emerged, it would be “great” if you planted a Liberty Link variety, as Liberty is the go-to product of choice for this situation. In the likely event you did not plant Liberty Link, then FirstRate or cloransulam is the next product to turn to. FirstRate is an ALS inhibitor or Group 2 herbicide.

This is the same class as many products on the market including Classic, Harmony, etc. Unfortunately, Group 2 herbicide resistance is quite common in horseweed/marestail, so this rescue tactic may fail. In our trial last year at Landisville, a two-pass program that included FirstRate with glyphosate POST provided 84% control horseweed/marestail control.

This horseweed/marestail was not ALS resistant. In the same trial, a two-pass program of glyphosate only provided 67% control. Hopefully, the scenario I am describing does not pertain to you and you are sitting happy knowing your horseweed/marestail is already dead.