09 Jan Herbicide Rotation
Determine the herbicide classes you’ve been using in your normal rotation, then strive to use different herbicide classes when you rotate to alfalfa.
Herbicide rotation has become a key part of most farmers’ battle against resistant weeds. The most notorious and perhaps most wide-spread weed resistance occurs with glyphosate resistant weeds of so many destructive weed species. At last count there are now at least 56 weed species with documented glyphosate resistance. You might be struggling with resistant broadleaf species of ragweeds, pigweeds, palmer amaranth, kochia, waterhemp or horseweed. Or in some states you can add resistant grassy weeds to the list, including Johnsongrass and some ryegrass variants. The initial rise of resistant variants can go un-noticed within a field, giving the appearance of good herbicidal control. But even a small percentage of resistant plants growing un-checked, can lead to a predominance of weed seed production from a resistant variant. This can make for a drastic increase in resistant weed growth in the following season unless herbicide rotation is done effectively. This table provides a summary of resistant weed species by herbicide classification group. Herbicide Resistant Weeds by Herbicide Site of Action Summary Table (weedscience.org)
Too much reliance on any one herbicide class can hasten the development of weed resistance in your fields. Herbicidal control strategies that utilize two or more herbicides within a given crop, as well as application of different herbicide classes across crops in rotation, are generally recommended. These practices can maintain the efficacy of weed control, and stave off the development of resistant weed species variants in your fields.
As a perennial crop, alfalfa offers an opportunity to rotate into some different herbicide classes not commonly used in typical corn/soybean or sorghum crop rotations. It’s a good idea to determine the herbicide classes you’ve been using in your normal rotation, then strive to use different herbicide classes when you rotate to alfalfa. Double TeamTM Sorghum products also offer a good opportunity for herbicide rotation away from typical herbicide classes used in corn and soybeans. This chart provides a handy reference for looking up most herbicides to determine their classification grouping.
A 2021 joint study between University of Nebraksa and University of Idaho weed scientists was undertaken “to evaluate the efficacy of pre-plant incorporated (PPI), post-emergence, and residual herbicides on annual weeds in newly established” alfalfa. Among fifteen herbicide treatments, the best weed control efficacy was obtained with combinations of application timing. Here is the summary of results from that study…. PowerPoint Presentation (naaic.org)
When you rotate to alfalfa, strive to use different herbicide classes versus your other crops in rotation. The table below can serve as a reference for herbicides labled for alfalfa use, their application timing, herbicide classification group (shown in the table as Site Of Action or SOA), and a few key considerations or restrictions. As always, read and follow herbicide labels for herbicide use requirements. Some herbicides have supplemental labels for individual crop practices or additional state requirements.