Duration of High-Concentrate Diet Prior to Forage-Finishing II: Effects on Glucose and Insulin Levels Under Challenge, and Gene Expression of Beef Steers.

Twelve Angus-influenced steers (227 ± 10.1 kg) were used to evaluate the effect of duration of exposure to high-concentrate based diet post-weaning prior to forage-finishing on plasma insulin and glucose levels under challenge and lipogenic gene expression. Steers were randomly assigned to one of four feeding treatments: 0 days on high-concentrate (CNT), 40 days on high-concentrate (40D), 80 days on high-concentrate (80D), and 120 days on high-concentrate (120D) followed by finishing to 485 kg on pasture. The high-concentrate diet consisted of cracked corn, corn gluten feed, chopped hay, and mineral supplement; whereas high-quality pasture included winter annuals (ryegrass, oats), alfalfa, and non-toxic fescue. Weight, and blood samples were collected at 21-d intervals. Muscle biopsies taken from the right longissimus dorsi (LM) when steers started the feeding period and at their respective days on treatment. Intravenous glucose tolerance tests (IVGTT) were performed at 40d intervals until 160d. At 24 h postmortem, carcass characteristics and a rib section (11-12 rib) from the left side was collected for proximate analysis. There was an interaction between treatment and time for plasma glucose concentration (P=0.03), whereas there was no interaction and no difference across treatments for plasma insulin concentration (P>0.20). There was no difference across treatments for initial glucose concentration during IVGTT (P=0.86) whereas adjusted area under the curve for glucose concentration linearly decreased as treatment increased (P=0.022); CNT and 120D being significantly different at 120d (P=0.008). There was no difference across treatments for initial insulin concentration during IVGTT (P=0.46), however 120D had a greater initial insulin concentration than CNT at 120d (P=0.024). Adjusted area under the curve for insulin was not different across treatments (P<0.40). Gene expression of acetyl CoA carboxylase, stearoyl CoA desaturase, and glucose transporter 4 were not different (P>0.21) between CNT and 120D at 120d. Fatty acid synthase mRNA expression increased 3.2-fold (P<0.05) for 120D compared to CNT and the ΔCT values for FASN correlated (-0.80; P<0.017) to the quantity of de novo lipogenesis products within the LM at the time of harvest. The consumption of a high-concentrate based diet immediately post-weaning may alter insulin and glucose dynamics, and lipogenic gene expression of cattle finished in a pasture-based system.