Effects of grazing system, season, and forage carbohydrates on glucose and insulin dynamics of the grazing horse.

Rotational grazing is a recommended practice for grazing livestock, but little is known about its benefits with respect to grazing horses. The objective of this study was to investigate the effects of continuous (CON) and rotational (ROT) grazing on forage nutrient composition and whether those concentrations influenced circulating glucose and insulin concentrations in the grazing horse. Twelve mature Standardbred mares were paired by age and weight and randomly assigned to 1 replicate of either a 1.5 ha cool-season grass CON or ROT system for a total of 3 mares in each system. Mares on CON were allowed to graze the entire system at all times, whereas mares on ROT were given access to a 0.4 ha pasture section and stress lot where they were confined during inclement weather and slow forage growth. Blood and feces from horses and forage from each system were sampled over one 24-h period in June, August, and October. Blood was assessed for plasma glucose (GLU) and serum insulin (INS), feces for pH, and forage for nutritional composition. Data were analyzed by ANOVA with repeated measures with significance set at P < 0.05. There were no treatment differences for water and ethanol soluble carbohydrates (WSC and ESC, respectively), starch, ADF, and NDF, but CP was lower in ROT compared to CON (P = 0.04). With respect to month, WSC were highest in June compared to August and October, whereas ESC were highest in June compared to only August. Starch was lower in October than in June and August. Concentrations of ADF and NDF were lowest in October compared to June and August. Crude protein was higher in October than June and August. Plasma GLU and serum INS were affected by season and time of day but not grazing system. For all horses, GLU was highest in August (105.6 ± 1.3 mg/dL), whereas INS was highest in October (0.21 ± 0.02 μg/L; P < 0.0001). Fecal pH only varied by season and was highest in August (7.06; P < 0.0001). Few consistent correlations between grazing systems were found with the exception of INS with ESC (R = 0.32 to 0.39; P < 0.04) and INS and GLU with ADF and NDF in August and October (R = -0.31 to -0.48; P < 0.04). In conclusion, grazing system did not affect the forage carbohydrate concentrations or GLU or INS in horses; however, season did have an effect on both forage nutrient content and glucose metabolism in horses.