Effect of energy substrate and days on feed on plasma insulin response in finishing beef heifers

Abstract Text: The objective of this study was to determine the effect of dietary energy source and days on feed (DOF) on plasma insulin concentration and insulin responsiveness when subjected to an arterial glucose challenge. Eight heifers were randomly allocated to 1 of 2 finishing diets consisting of a barley-based control (CON; n = 4; 75.2% barley grain, 6% barley silage, 9.8% canola meal and 9% vitamin and mineral supplement on a DM basis) or a diet where a high-lipid byproduct-pellet replaced 60% of the barley grain and canola meal relative to CON (HLP; n = 4). Diets were formulated to be iso-caloric and iso-nitrogenous, but the CON had greater starch (46.6 vs. 39.5%) and lower ether extract (3.8 vs. 5.7%) than HLP. The 160-d study period was divided into four 40-d periods (P1, P2, P3, and P4). On the final day of each period, 7.57 mmol/kg BW^0.75 of dextrose was infused and the insulin response was analyzed in plasma collected at 0, 5, 10, 15, 20, 25, 30, 45, 60, 90 and 120 min post-infusion. Data were analyzed using a mixed model (fixed effects of treatment, period, and the treatment × period interaction).  Period was included as a repeated measure. The 12-h fasting plasma insulin concentration did not differ (P = 0.40) between the treatments averaging 1.51 µg/L. However insulin concentration increased from P1 (1.17 µg/L; P ˂ 0.01) to P3 (1.81 µg/L) and P4 (1.60 µg/L) with the latter not differing. Area under the curve for insulin following the glucose challenge tended (P = 0.08) to increase with DOF, but did not differ by diet. The peak insulin concentration following the glucose challenge increased from 8.42 during P1 to 11.3 µg/L during P3 (P = 0.048) and the time to attain the peak tended to increase (P = 0.09) with DOF, but was not affected by treatment (P > 0.1). A tendency (P = 0.07) for a treatment × period interaction was observed for peak insulin concentration with HLP attaining a greater peak than CON in all periods except P2. The results of this study indicate that insulin concentration and the insulin insensitivity to a glucose challenge in growing beef heifers increase with advancing DOF and this increase is independent of energy substrate fed. Increasing insulin resistance may be one factor leading to reduced energetic efficiency associated with advancing DOF in finishing cattle.

Keywords: beef, insulin, finishing