Managers on Kentucky horse farms prefer pastures used for grazing pregnant mares to be composed of Kentucky bluegrass and orchardgrass and little, if any, tall fescue. Most of the tall fescue in Kentucky pastures is "KY 31," and essentially all of that species contains the natural race of the endophytic fungus Neotyphodium coenophialum. This fungus is implicated in late-term pregnancy problems, such as an increase in gestation, dystocia (difficult birth), and agalactia (failure to produce milk). Horse farm managers prefer to remove tall fescue and other problematic and unsightly weeds from the pasture.

Research objectives Determine the control of tall fescue and troublesome weeds from Kentucky bluegrass/orchard grass pastures by 1) selectively killing tall fescue with imazapic herbicide and not harming the desired grasses; or 2) killing all vegetation with glyphosate herbicide and seeding desirable grasses.

Methods and materials A horse pasture composed of about 50% Kentucky bluegrass and 50% tall fescue was selected in 2005 to compare different methods to reduce tall fescue population. Treatments of imazapic and glyphosate, used alone or in combination, were applied at different times in 2005 and 2006. Imazapic treatments were applied with methylated seed oil at 1 qt/A (quart per acre) as per instructions on the label. The selective removal treatments contained only imazapic. Total renovation treatments included glyphosate.

All treatments were applied at a volume of 15 gallons of water per acre. Individual plots were 40 feet wide and 80 feet in length, and each treatment was replicated four times. The entire experimental site was over-seeded with a mixture composed of 60% Kentucky bluegrass and 40% orchard grass on Sept. 7, 2005. Dates of individual treatments are listed in Table 1 (see below).

The experimental site was managed along with the rest of the horse farm and included mowing every seven to 10 days to a height of about 6 inches and fertilization as per soil test recommendation in September. Relative percentage of tall fescue, Kentucky bluegrass, and orchard grass was determined by the point intercept method. Imazapic is sold in Kentucky under the trade names of Panoramic and Plateau. Glyphosate is sold under numerous trade names. The product used in these experiments was Roundup WeatherMax.


Table 1
Treatment Amount per Acre Date Applied % Kentucky Bluegrass % Orchardgrass % Tall Fescue % Nimblewill
Glyphosate 3 qt 7/21/2005 15 b 85 a 0 c 0.4 c
Glyphosate + Glyphosate 2 qt + 7/21/2005 19 b 81 a 0 c 0 c
2 qt 8 25-05
Glyphosate + 2 qt 7/21/2005 13 b 86 a 0 c 0 c
Imazapic 10 oz 6/19/2006
Imazapic 12 oz 5/18/2005 28 ab 40 a 27 b 10.7 b
Imazapic + 10 oz + 5/18/2005 16 b 76 a 3 c 22.5 a
Imazapic 10 oz 6/21/2005
Untreated     36 a 7 c 57 a 2.9 c
All treatments decreased the percentage of tall fescue compared to the untreated areas, which contained 57% tall fescue (Table 1). All combinations of glyphosate and two imazapic treatments in one year reduced tall fescue composition to less than 3%. While the selective removal treatments (imazapic only) and the total renovation treatments (those with glyphosate) greatly reduced tall fescue, both methods have drawbacks. The selective removal technique removes the tall fescue, but other weeds occupy the sites where tall fescue plants died. This can result in weedy pastures in those pastures that contain 50% or more of tall fescue at the time of treatment. The total renovation technique results in an excellent pasture, but it cannot be grazed for several months during the renovation process.

An important factor to consider is what weeds will appear when tall fescue is removed. An example was found in this study. Nimblewill (Muhlenbergia schreberi) is a native, warm-season grass that reproduces by seeds and numerous fine stolons and is not impacted by the weekly mowing regimes. The problem with nimblewill is aesthetic as well as it not being palatable to horses. In heavily grazed pastures, the nimblewill has a competitive advantage over Kentucky bluegrass and orchard grass. Nimblewill rarely grows with tall fescue, but when tall fescue was removed with imazapic, nimblewill emerged and became a serious weed in the pasture and occupied 10-25% of the pastures treated with imazapic.--William W. Witt, MS, PhD

Listing of products implies no endorsement by the University of Kentucky or its representatives. Criticism of products not listed is neither implied nor intended.

Acknowledgement: The author appreciates the cooperation of WinStar Farm, LLC, providing the pastures for this research.

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