Effect of glucose infusion on glucose and insulin metabolism in early- and mid-lactation ewes and goats fed diets differing in starch and highly digestible fiber concentration

This study aimed to test possible metabolic differences between ewes and goats in response to an intravenous glucose infusion. Thirty-six animals, 18 mature Sarda dairy ewes and 18 mature Saanen goats (from 15 to 150 ± 11 d in milk [DIM], mean ± SD; body weight: 49.8 ± 6.8 kg for ewes, 60.6 ± 7.3 kg for goats) were compared simultaneously. In early lactation, both species received the same high-starch diet (HS: 20.4% starch, 35.4% neutral detergent fiber [NDF], on dry matter [DM] basis), whereas from 92 ± 11 DIM both species were randomly allocated to 2 dietary treatments: HS (20.0% starch, 36.7% NDF, on DM basis) and low-starch (LS: 7.8% starch, 48.8% NDF, on DM basis) diets. At 50 and 150 ± 11 DIM, ewes and goats were challenged with an intravenous glucose tolerance test and peripheral concentrations of glucose and insulin were determined 15 min before and 5, 10, 15, 30, 45, 90, and 180 min after glucose infusion. In early lactation, baseline plasma glucose and insulin concentrations tended to be higher in ewes than in goats (glucose: 55.8 vs. 42.9 ± 7.3 mg/dL; insulin: 0.13 vs. 0.05 ± 0.04 µg/L). After glucose infusion, glucose and insulin concentrations were higher in ewes than in goats (278.6 vs. 247.6 ± 13.1 mg/dL; 0.82 vs. 0.46 ± 0.12 µg/L). In mid-lactation, the dietary treatment (HS vs. LS) did not affect glucose and insulin metabolism. Baseline plasma glucose was numerically highest in ewes, while baseline insulin was higher in ewes than in goats (0.39 vs. 0.12 ± 0.099 µg/L). After glucose infusion, glucose concentration did not differ between ewes and goats, while insulin concentration was highest in ewes. Compared with goats, ewes showed in both periods a higher peak insulin, insulin increment, linear insulin area under the curve, insulin resistance index, and lower insulin sensitivity indices. In conclusion, despite the limitations associated with the use of intravenous glucose tolerance test to assess glucose regulation mechanisms, this study indicated large species differences in both early and mid-lactation and a more evident anabolic status in the ewes compared with the goats.