Insulin response to feeding forage with varying crude protein and amino acid content in horses at rest and after exercise

This study assessed the insulin response to forage intake with varying crude protein (CP) content in horses at rest and after exercise. Six geldings were fed three grass haylage-only diets for 7 days according to a 3x3 Latin square design. On day 7, blood samples were collected before and for 120 min after feeding 15% of the daily allowance before exercise (feeding A) and after standardised exercise ( feeding B). Feed samples were collected before each feeding. Dry matter (DM) and nutrient content varied (DM: 37-58%, water-soluble carbohydrates minus fructans (WSC-f): 3-12% of DM, CP: 10-15% of DM) which resulted in a variation in nutrient intake within haylage batches. Based on individual CP and WSC-f intakes, intake groups were therefore formed (low and high CP intake; <= and > 180 g CP/100 kg body weight, respectively and low and high WSC-f intake; <= and > 100 g/100 kg body weight, respectively). Amino acids were analysed and intakes were generally higher in the high CP group than in the low CP group. An ANOVA model including horse, CP group and WSC-f intake explained 95% of the variation in plasma insulin response compared to 87% using a model including horse and WSC-f group alone. The plasma insulin area under curve (AUC) following feeding A tended to be higher in the high CP group than in the low CP group (P=0.08), but there was no difference after feeding B. Plasma glucose AUC was not affected by CP group (P>0.05). The study indicates that the post-prandial plasma insulin response in horses fed a forage-only diet is increased by high WSC-f intake but may also be increased by high CP intake, at least at rest. However, due to the low number of observations further studies are needed.