5 Seasonal effects on pasture and a group of mares with metabolic syndrome

Metabolic horses are at an increased risk for insulin resistance and laminitis. Previous studies showed a greater risk for horses to develop insulin resistance and laminitis during the spring and summer months. The aim of this study was to determine the effect of season on pasture and insulin sensitivity (SI) in a group of 10 non-pregnant metabolic mares. These mares were previously diagnosed to have metabolic syndrome through various parameters such as body weight, regional adiposity, body condition score (BCS), and glucose and insulin response tests. These mares remained on pasture for the duration of the study but no intake measurements were done. Frequent sampling intra venous glucose tolerance tests (FSIGT) were performed once in April, May, and June for a total of 3 times. Blood was analyzed for glucose, insulin, cholesterol, nonesterified fatty acids (NEFA), triglycerides (TG), tumor necrosis factor α (TNFa), and interleukin 1 β (IL-1β). Pasture samples were taken biweekly, in dates between FSGITS, and analyzed for nutritional content. Data were analyzed by ANOVA for time effect and all significant results had a P-value of < 0.05. Body weight, BCS, and TNFa increased progressively from April to June. These results corroborate previous studies which have shown that an increase in inflammatory factors is associated with obesity. Insulin sensitivity (SI) and β pancreatic response measured as acute insulin response to glucose (AIRg) increased from April to May despite weight increase and returned to April levels in June, perhaps due to increased activity of horses. Basal glucose decreased progressively from April to June. Basal insulin and IL-1β did not vary with time. Cholesterol did not change from April to May but decreased from May to June. NEFA decreased from April to June and TG decreased in May and returned to previous levels in June. Pasture water-soluble (WSC) and ethanol-soluble carbohydrates (ESC) increased from April to May and did not significantly change from May to June. Increased ESC and WSC in pasture may have caused NEFA to decrease, indicating less fat mobilization and higher carbohydrate metabolism. Beta pancreatic response increased to high levels (>338 mU/L per minute) in June and also could have been an effect of the increased amount of ESC and WSC in the pasture which induced a higher pancreatic response to maintain euglycemia. These results confirm a relationship between season and pasture content, and current results could serve to manage horses with metabolic syndrome. Owners should be proactive changing management before spring weight gain to avoid problems associated with insulin resistance.