ARS | Publication request: Composition of horse diets on cool-season grass pastures using microhistological analysis

Submitted to: Forage and Grazinglands
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: December 9, 2008
Publication Date: March 25, 2009
Citation: Morrison, J.I., Smith, S.R., Aiken, G.E., Lawrence, L.M. 2009. Composition of horse diets on cool-season grass pastures using microhistological analysis. Forage and Grazinglands. doi:10.1094/FG-2009-0325-01-RS.

Interpretive Summary: Tall fescue toxicity in late-term broodmares can be detrimental to both the mare and foal, and is a major concern for the horse industry as a whole. In the United States, an estimated 700,000 horses are managed on pastures containing varying amounts of tall fescue. Since tall fescue is endemic and thrives in the transition zone climate, it is difficult and costly to completely eliminate from existing_pastures(16). Techniques that help horse farm owners and managers know what their animals are consuming when grazing mixed-species pastures could be useful for making management decisions for pastures containing tall fescue. Several techniques are available to evaluate diet composition of grazing animals, but the standard method is to analyze ingested plant material from esophageal fistulas. This invasive technique is not acceptable for equine research, but microhistological analysis of plant fecal fragments provides a useful non-invasive technique. An experiment was conducted to validate the microhistological analysis of plant fecal fragments in horse feces as a non-invasive technique to estimate botanical composition of horse diets on mixed cool-season pastures. Microhistological analysis of feces can serve as a useful research and diagnostic tool in quantifying the proportion of tall fescue, and other species, in the diets of horses.

Technical Abstract: Grazing patterns and diet composition can be difficult to determine with horses, but are important when pastures contain species that have the potential to cause animal toxicity. The objective of this study was to determine the composition of domesticated horse diets when grazing mixed cool-season pastures'using microhistology of fecal samples. Samples of tall fescue, Kentucky bluegrass, and orchardgrass were evaluated for microscopically unique characteristics from plant fragments in fecal material. Grazing studies were conducted in cool-season grass pastures in November 2006 and April 2007 in Lexington, KY. Eight mature Thoroughbred mares were placed in individual paddocks of varying botanical compositions and grazed for six days. Fecal samples from manure piles were taken from each paddock to determine diet composition. There was a high correlation between tall fescue and orchardgrass in the pasture and in the diet (0.90 and 0.80, respectively), but no correlation with bluegrass. For each percent increase of tall fescue or orchardgrass in the pasture, there was a corresponding increase of 0.44% and 0.42%, respectively, in the diet. In conclusion, microhistological analysis of plant fecal fragments showed that horses consume tall fescue and orchardgrass in similar proportions to those found in their pastures.