Over the last two weeks, we have heard of at least four new reports of either Palmer amaranth or its close relative waterhemp. Most of these reports are coming out of the southeastern region of the state in situations where glyphosate did not control targeted pigweed species and now some of these plants are more than 3 feet tall and beginning to flower. Most of these are failures in soybean, but one incident late last week was in established alfalfa.

In this particular situation the farmer noticed the problem earlier in the summer, applied Gramoxone after second cutting which appeared to do a good job controlling emerged plants. Low and behold, at third cutting, the field was loaded again with emerged plants. This is very concerning as we would assume that established alfalfa that is cut every 30 plus days might be a good management strategy for pigweed control. The farmer is going to treat again with Gramoxone after third cutting, but will add the residual herbicide Prowl to the mix.

A paper by Putmam et al. from Kansas State University at the 2013 Weed Science Society Annual Meeting in Baltimore compared a number of dormant and in-between cutting herbicides for control of Palmer amaranth in alfalfa. In this research, Palmer amaranth began emerging on May 1 with 20% cumulative emergence by June 3, 80% by July 8, and 100% emergence by Aug. 5.

This demonstrates how late into the summer this weed can emerge and the need for residual control. In this study, the best late-season Palmer amaranth control was achieved with sequential treatments that included flumioxaxin or diuron. Flumioxazin for alfalfa is sold under the trade name Chateau and must be applied either during the dormant period or as soon as possible after cutting and removing alfalfa to minimize potential injury. Diuron is sold under the tradename Karmex and is not commonly used in alfalfa in our region. 

Several other treatments provided good early-season Palmer amaranth control, but control diminished as the season progressed. This reminds us that Palmer amaranth emerges throughout the growing season and therefore, sequential herbicide treatments with good residual activity may be necessary for season-long control.