Unlike most weed management situations, suppression of the target plant is the desired result, not elimination. This offers several challenges as most management recommendations using herbicides target removal, and little research has looked at the concept of suppression. While there are a range of grasses that will likely be targeted for suppression in Wisconsin, the most common in CRP fields is switchgrass (a warm season perennial grass). I only found one experiment that measured the effectiveness of herbicides at suppressing switchgrass. Unfortunately the experiment was conducted in filter strips between field crops in Mississippi (Rankin et al. 2005), so the results may differ under Wisconsin’s environmental conditions. Data from this project with regards herbicides registered for use in CRP are summarized in Table 1. They found that systemic herbicides like glyphosate, Poast, and Fusilade provided the most visual injury, and only high rates of contact herbicides like Gramoxone provided significant suppression two months after treatment. Ideally these treatments would provide suppression into the following years, but this experiment did not evaluate differences past four months.

 

Table 1.Effectiveness and estimated cost of CRP registered herbicides in suppressing switchgrass in Mississippi (adapted from Rankin et al. 2005).

 

		Cost	Application Timing		% Visual Injury
Herbicide	Rate	$/A	Season	switchgrass height	2 MAT	4 MAT
Glyphosate	0.25 lbs ai/A	$ 2-3	Spring	2 ft	22	14
Glyphosate	0.75 lbs ai/A	$ 8-12	Spring	2 ft	66	40
Gramoxone	10 fl oz/A	$ 2.5	Spring	2 ft	0	0
Gramoxone	2.5 pt/A	$ 10	Spring	2 ft	26	21
Poast	5.3 fl oz/A	$ 2.5	Summer	4 ft	32	-
Poast	21.2 fl oz/A	$ 10	Summer	4 ft	66	-
Fusilade	9 fl oz/A	$ 12	Summer	4 ft	47	-
Fusilade	36 fl oz/A	$ 48	Summer	4 ft	71	-
Glyphosate	0.25 lbs ai/A	$ 2-3	Summer	4 ft	22	-
Glyphosate	0.75 lbs ai/A	$ 8-12	Summer	4 ft	46	-

Based on these results, it appears that several herbicides are available that, UNDER THE CORRECT CONDITIONS should provide effective suppression. However, care must be taken to apply these herbicides correctly as either reduced or enhanced suppression is undesirable and landowners are responsible for assuring that any herbicide application will not be adversely affected. To further complicate matters, CRP management restrictions are imposed that prevent any management activities outside the nesting season at different timings for warm (April 15-May 14 OR August 2-August 15) and cool season (April 1-May 14 OR September 1-October 1) grasses. 

 

The Mississippi study provides a foundation for future work to determine the best herbicide, timing of application, and rate. Results will likely be different in Wisconsin as switchgrass will not be two feet tall between April 1 and May 14^th, therefore I would suspect reduced control at this timing in Wisconsin compared to the Mississippi study. I would, however expect results to be similar with the summer applications of glyphosate, Poast, and Fusilade. Research will evaluate these treatments this summer, but switchgrass height and developmental stage should be similar (close to four feet tall), and these parameters have been shown to be the most important with management of other perennial grasses like quackgrass. I will be evaluating several of these herbicides this spring and fall in addition to other NRCS recommendations, so stay tuned for further results.

 

When selecting an herbicide for this mid-contract management, it is important to consider several factors. If desirable broadleaf plants are present I would recommend the grass specific herbicides as they would provide the least injury to these desirable plants. Contact herbicides like Gramoxone are an option for suppression, and while the Mississippi study showed significant reductions in populations up to 4 months after treatment, it is unlikely that these treatments will cause long-term reductions in populations as they don’t damage belowground rhizomes. Due to the cost of various treatments and fact that this disturbance is supposed to suppress the target species populations enough that this won’t need to be repeated for 5-10 years I would consider systemic herbicides as they will likely be more effective at reducing belowground biomass and competitiveness of these grasses for more than one year. Also realize that the effectiveness of treatments can vary dramatically based on the age of the population to be suppressed. Older stands tend to have more litter present and more stored energy in perennial organs. These two factors can make suppression more difficult and may require higher rates for suppression, or utilization of other management methods than herbicides. Finally one should also consider what other species are present at the site or historically have been present as cover of these species may dramatically increase in cover as a result of management. Whatever herbicide you select, make sure to follow the label, as specific restrictions may exist with some products that don’t exist with others.