Glucose response of horses grazing alfalfa, cool-season perennial grasses and teff across seasons

Elevated nonstructural carbohydrate (NSC) values in pasture forages can cause adverse health effects in horses diagnosed with equine metabolic syndrome, laminitis and obesity. However, many owners still want pasture access for these horses. The objectives of this study were to determine the impact of different forage species on blood glucose concentrations of horses throughout the grazing season. Research was conducted in May (spring), July (summer), September (fall), and October (winter), 2016 in St. Paul, MN. Alfalfa (legume), a mixed orchardgrass and Kentucky bluegrass (cool-season perennial grass mixture), and teff (annual warm-season grass) pastures were grazed by 6 adult horses (BCS ± SD: 5.5 ± 1.0; BW ± SD: 479 ± 49 kg) who were randomly assigned to one of the 3 forage types in a Latin-square design. Teff and cool-season perennial grass pastures were fertilized with 45 kg nitrogen ha-1 before the spring grazing; the alfalfa pasture required no fertilizer. Horses received a 24 h hay washout (mixture of the forage species) followed by a 12 h fast before data collection. Jugular catheters were inserted 1 h before the start of the trial and horses had access to pasture for 8 h each day from 0800 h to 1600 h. Jugular venous blood samples were collected from each horse before being turned out to pasture (0 h) and then at 2, 4, 6, and 8 h following turnout. Plasma samples were collected and analyzed for glucose. Corresponding forage samples were taken at 0, 2, 4, 6, and 8 h by hand harvesting duplicate and representative areas throughout each pasture. Forage samples were dried at 60°C for 24 h, ground, and sent to a commercial laboratory to assess forage nutritive values. Seasons were analyzed separately and data were analyzed using PROC MIXED in SAS with P ≤ 0.05. Diurnal variations were observed in NSC for each species with NSC values increasing throughout the day (P ≤ 0.03). Nonstructural carbohydrate values varied across species in the summer and fall (P ≤ 0.02) with higher NSC values observed in cool-season perennial grasses compared with teff. Plasma glucose concentrations varied among different forage types during spring at 8 h (P = 0.02) and winter at 2 h (P = 0.03) and 4 h (P = 0.01) with consumption of cool-season perennial grasses promoting higher plasma glucose levels compared with alfalfa and teff, respectively. These results suggest minimal changes in plasma glucose when horses grazed different forages species with varying levels of NSC. Future research will evaluate serum insulin levels to further assess the metabolic effects of horses grazing different forage types.