June 09, 2019

Interpretive Summary: Grazing system, season, and forge carbohydrates on glucose and insulin dynamics of the horse

Interpretive Summary: Effects of grazing system, season, and forge carbohydrates on glucose and insulin dynamics of the grazing horse.

By: Dr. Thomas Powell

Rotational grazing systems are known to be optimal for grazing livestock. Their environmental and animal benefits are well documented. However, in the area of equine nutrition and grazing habits, there is much less known about the usefulness of rotational grazing.

Researchers at Rutgers University and the University of Maryland recently published a study in the Journal of Animal Science examining this issue. They compared the effects of conventional and rotational grazing on forage nutrition composition and subsequently on the circulating glucose and insulin levels of the grazing horse.

A total of twelve Standardbred mares were assigned in groups of three to either a conventional or rotational grazing system treatment. In the conventional treatment, horses were allowed free access to a 1.5 hectare lot. For the rotational system, the same size lot was subdivided into four 0.35 hectare lots and a 0.6 hectare acre stress (dry) lot. The lots were seeded with a cool season blend of tall fescue, Kentucky bluegrass and orchardgrass. Horses were started on the systems in August, 2014. Three seasonal trials were conducted in June, August and October, 2015.

For each trial, samples were taken seven times in a 24-hour period. Forage samples were obtained for nutrient analysis. Blood and feces were collected from each horse to determine plasma glucose and serum insulin and fecal pH.

Aside from a decrease in crude protein for the rotational system grasses, there were no differences in the grass nutrient composition between treatments. Sward height was increased in the rotational treatment. Predictable changes were observed across the nutrient composition reflecting variation in seasonal temperature and precipitation. Likewise, plasma glucose, serum insulin and fecal pH for the horses were not affected by treatment but did fluctuate with the ebb and flow of seasonal changes in forage nutrient composition.

The researchers stated that while the use of rotational grazing did not affect glycemic or insulinemic factors in the horses, the study was limited to 14 months of total grazing time. They postulate that pasture degradation by overgrazing could have a greater impact over time. The seasonal variations in forage nutrients and the subsequent effect on glucose and insulin in the horse reinforce current recommendations that horses with metabolic disorders have grazing minimized or eliminated, regardless of grazing management system.