There are several challenges to controlling Palmer amaranth in CRP, including:
1) Most infested CRP fields used seed mixes containing native forbs and grasses. Controlling the Palmer while preserving the forb component will be difficult, if not impossible.
2) Group 4 herbicides (growth regulators, synthetic auxins) are the logical choice in CRP since they will preserve the native grasses; however, they are likely to eliminate many of the forbs. These products are not particularly strong on Palmer amaranth and other weedy pigweeds, especially when used on larger plants (>4-6 inches). Many applications likely will be made on large Palmer amaranth because CRP fields will be viewed as lower priority than crop fields, and identifying Palmer amaranth among the established perennials will be difficult while the weeds are small.
Milestone (aminopyralid) is a ‘newer’ Group 4 herbicide from Dow AgroSciences. Milestone has the advantage over other Group 4 products (e.g. 2,4-D and dicamba) in that it is safe on many native forbs. However, there is limited information on its effectiveness on Palmer amaranth or other weedy pigweeds.
I conducted a greenhouse trial comparing Milestone (2.3, 4.7 and 7.0 oz/A) to 1 lb ae 2,4-D LVE/A. Plants ranged from 4 to 10 inches in height at application, and each treatment was applied to five plants using a greenhouse track sprayer.
I normally don’t like using photos to document herbicide activity, but in this case, I think a picture is worth a thousand words. The trial reinforces that neither 2,4-D nor Milestone are especially strong on Palmer amaranth. Two weeks after application, 1 lb 2,4-D had killed 3 out of 5 plants (Fig. 1), whereas with 7 oz Milestone only 2 of 5 plants were dead (Fig. 2). The lower rates of Milestone did not kill any Palmer amaranth. Use rates of Milestone for spiny amaranth and other annual and herbaceous perennials are 4 to 7 oz/A. 2,4-D is labeled at rates of 1-2 lb ae/A in CRP. Rates higher than the 1 lb used in this trial should improve consistency. From this experiment it appears that the increased selectivity provided by Milestone should be weighed against a slight decrease in activity on Palmer amaranth.
Eradicating Palmer amaranth from fields where it was introduced in 2016 should be the goal in this and future years. Due to its tolerance to most herbicides, it is unlikely a single application will provide complete control of Palmer amaranth. An intensive scouting program will be essential to locate Palmer amaranth in fields at susceptible growth stages and to evaluate the effectiveness of all control strategies. Eradication will require several years of 100% control, thus control strategies must incorporate effective herbicides and other management strategies (hand rogueing, mowing), scouting, and, ultimately, prevention of Palmer amaranth seed production.