Feeding a standard commercial concentrate does not impact metabolic responses to an oral sugar test in yearlings

Abstract

Cereal grain-based concentrates in modern equine diets have elevated dietary starch. Meals providing starch >1.1 g/kg BW increase metabolic responses in healthy, adult horses; however, there is limited data in young horses. It was hypothesized that yearlings on a commercial starch concentrate (CS) would exhibit higher responses to an oral sugar test (OST) than those on a low starch/high fiber (HF) concentrate. Quarter Horses (14 ± 1 mo, 363 ± 30 kg) were acclimated to a HF concentrate (9.9 g/kg BW per day, 8.5% starch, 36.6% NDF DM) and coastal Bermudagrass hay (15 g/kg BW per day, 2.4% starch, 64.4% NDF DM) in individual loafing runs for 21 d. Following the 21-d backgrounding, blood samples (D0) were collected and, after stratifying by BW, sex, and age, yearlings were randomly assigned to one of 2 isocaloric diets meeting nutritional requirements: HF (7 fillies, 8 geldings) or CS (28.7% starch, 24.7% NDF DM; 8 fillies, 7 geldings), plus grass hay split into 2 feedings. After 24 d on dietary treatments, an OST (0.15 mL/kg BW) was administered, and insulin and glucose levels were measured at rest (D24) and 60 min post-OST (T60). Insulin and glucose were quantified via RIA and ELISA, respectively, by the Cornell University Animal Health Diagnostic Center laboratory. Data were analyzed on Graph Pad using a Welch ANOVA with Dunnett's post hoc comparisons. Significance was placed at P < 0.05 and data are presented as means ± SD [95% CI]. There were no differences in resting insulin (T0; HF 17.1 ± 4.3 µIU/mL [14.5, 19.7], CS 19.2 ± 4.7 µIU/mL [16.5, 21.9]) or glucose (HF 102.2 ± 10.2 mg/dL, CS 102.5 ± 10.8 mg/dL) between groups after 24 d of feeding (P > 0.2). Glucose concentrations increased following the OST for both HF and CS (P < 0.03), but there was no difference between treatments (P > 0.9). Neither HF (T60 17.9 ± 2.7 µIU/mL, [16.3, 19.5]) nor CS (17.6 ± 3.6 µIU/mL [15.5, 19.7]) insulin concentrations increased post-OST (P > 0.9). Surprisingly, insulin concentrations in HF horses were greater at D24 than D0 (11.2 ± 1.9 µIU/mL [10.1, 12.3], P < 0.01), whereas D24 was not different from D0 in CS horses (15.3 ± 2.1 µIU/mL [14.1, 16.6], P > 0.1). These results were unexpected as the CS (1.4 g/kg BW starch per meal) exceeded the previously established threshold (>1.1 g/kg BW) yet failed to provoke a metabolic response to the OST. The previous threshold was developed from data from mature horses, and young horses (<5 yr old) may have a higher starch tolerance than mature horses. Future research should focus on establishing appropriate starch thresholds for young horses to optimize nutritional management practices and help prevent the development of metabolic disease.