Herbicides are an important tool for removing noxious or invasive weeds from plant communities, allowing desirable vegetation to respond. However, severely degraded sites where desirable plants are either not present, or remnant populations of desirable grasses are insufficient to recover after invasive plant control may require re-vegetation.
It is important for land managers to have information on the length of time required between applying herbicide treatments and seeding desirable grasses to reduce the potential for seedling injury. The current label for aminopyralid-containing products (e.g., Milestone®) allows for use on established desirable grasses, or the herbicide can be applied in spring followed by fall grass seeding. The potential to seed grasses in late autumn as a dormant fall planting following an autumn herbicide application would allow more flexibility for land managers tasked with re-vegetating degraded sites.
Materials and Methods
Field research trials were established in 2009 to determine if warm and cool season grasses could be planted either in late autumn as a dormant fall planting or in the spring after a September application of herbicide. With a dormant seeding, grasses are planted in the fall just before the soil freezes. The seed remains dormant in the soil until the following spring when they will germinate and grow as soon as conditions are favorable.
Three sites were selected for the study at university research farms with Dr. Roger Becker, University of Minnesota, Dr. Rodney Lym, North Dakota State University, and Dr. Mike Moechnig, South Dakota State University. The study sites were tilled in August to prepare a seed bed and subsequently treated with glyphosate at 32 fluid ounces per acre (fl oz/A) to keep the sites weed free prior to seeding. Herbicide treatments included Milestone at 3, 7, and 14 fluid ounces per acre (fl oz/A), Transline® at 1.5 pints per acre (pt/A), and Tordon® 22K at 1 quart per acre (qt/A). Herbicides were applied in September (Table 1) with a CO2 backpack or bicycle sprayer at 13 to 20 gallons per acre prior to seeding. Cool and warm season grasses commonly planted in restoration projects were selected for seeding. Grasses were seeded in late autumn or spring (Table 1) with a six-row cone seeder with 7 to 12 inch row spacing. Cool season species included intermediate wheatgrass (Agropyron intermedium), Canada wildrye (Elymus canadensis), and green needlegrass (Stipa viridula). Warm season species included big bluestem (Andropogon gerardii), little bluestem (Schizachyrium scoparium), sideoats grama (Bouteloua curtipendula), switchgrass (Panicum virgatum), and indiangrass (Sorghastrum nutans) (Table 2). Experiments were designed as randomized complete blocks with four replications per treatment combination.
Non-treated control plots were hand weeded for the early part of the growing season in 2010 to help ensure seedling success. Visual plant injury, plant count (number of plants per 0.5 meter row), and percent frequency of occurrence were measured in July 2010 at all sites.
Results showed grass establishment was greatest with the spring grass planting date compared to dormant fall planting when averaged across treatments (including the non-treated control) and sites (Figure 1). There was no significant difference in grass establishment in herbicide treated plots compared to the non-treated control when seeded at the dormant fall planting date (Figure 2). The low success in seedling establishment in the dormant fall planting may have been due to unfavorable conditions during winter and early spring.
Results of the spring planting date showed there was no significant difference in either cool or warm season grass establishment (except for big bluestem) in herbicide-treated plots compared to non-treated controls. There was a trend for greater seedling establishment in plots treated with Milestone at 3 and 7 fl oz/A compared to non-treated controls (Figure 3). The number of cool season grasses established in spring plantings ranged from 7.2 to 7.6 plants per 0.5 meter row (plants/0.5m row) in Milestone-treated plots, 6.6 plants/0.5m row in Transline-treated plots, and 5.2 or 5.4 plants/0.5m row for Tordon 22K-treated or non-treated controls, respectively. There was also a trend for the number of established warm season grass seedlings to be less in plots treated with Milestone at 14 fl oz/A, Transline, or Tordon 22K compared to Milestone at 3 or 7 fl oz/A; however, this difference was not significant. Milestone at 14 fl oz/A is twice the maximum broadcast rate of 7 fl oz/A. The percent frequency of warm season grass was greater in plots treated with Milestone at 3 or 7 fl oz/A or Transline at 1.5 pts/A compared to non-treated controls (Figure 4).
Big bluestem was the only grass that had a significant response to herbicide treatment (Figure 5). Seedling establishment was greatest in plots treated with Milestone at 3 and 7 fl oz/A compared to the non-treated control (Figure 5). Big bluestem seedling establishment was significantly greater with Milestone at 3 fl oz/A compared to Milestone at 14 fl ozA, Transline at 1.5 pts/A, or the non-treated control when averaged across locations.
Based on these results a land manager could apply Milestone at 7 fl oz/A or less in early fall followed by a dormant fall planting or early spring planting of these grass species and have successful seedling establishment if environmental conditions are favorable. These data are corroborated by a similar study in Colorado (http://techlinenews.com/articles/2012/12/30/rangeland-grass-species-tolerance-to-pre-plant-applications-of-aminopyralid-and-aminocyclopyrachlor) in addition to other field experiments conducted in the western United States. Results confirm the utility of Milestone herbicide in rangeland grass reseeding programs following invasive broadleaf weed control.
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Milestone is not registered for sale or use in all states. Contact your state pesticide regulatory agency to determine if a product is registered for sale or use in your state.
Tordon 22K is a federally Restricted Use Pesticide.
State restrictions on the sale and use of Transline apply. Consult the label before purchase or use for full details.
Always read and follow label directions.