Duration of High-Concentrate Diet Prior to Forage-Finishing I: Effects on Animal Performance, Glucose/Insulin Levels, Carcass Traits, and Tissue Fatty Acid Composition of Beef Steers.

Thirty-two Angus-influenced steers (255 ± 15.1 kg) were used to evaluate the effect of duration of exposure to high-concentrate based diet post-weaning prior to forage-finishing on performance, plasma insulin and glucose levels, carcass characteristics and tissue fatty acid composition. 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 on pasture to 485 kg. The high-concentrate diet consisted of cracked corn, corn gluten feed, chopped hay, and mineral supplement; high-quality pasture included winter annuals (ryegrass, oats), alfalfa, and non-toxic tall fescue. Weight and blood samples were collected at 21-d intervals. At 24 h postmortem, carcass characteristics and a rib section (11-12 rib) from the left side was collected for proximal analysis, and a section of subcutaneous adipose tissue collected for histology cell sizing. Due to harvesting steers at equal final body weight, there was no difference for total gain or total average daily gain (P>0.19); however, hot carcass weight and dressing percentage increased linearly (P<0.031) with time on concentrate. There was no difference across treatments for ribeye area or yield grade (P>0.11) but 12^th-rib fat thickness and KPH fat quadratically and linearly increased (P<0.015), respectively. Marbling score and total lipid of the longissimus dorsi (LM) linearly increased with time on concentrates (P<0.001). This increase in lipid within the LM resulted in a linear decrease with increased time on concentrate for moisture (P<0.001). Crude protein content of the LM differed quadratically (P=0.001), maximizing in steers from 80D whereas 120D had the least. Within the LM there was no difference across treatments for saturated or monounsaturated fatty acids (P>0.27), whereas n-6 polyunsaturated fatty acids (PUFA) differed quadratically (P=0.002), maximizing at 80D and n-3 PUFA linearly decreased as time on concentrates increased (P<0.001). These changes in n-6 PUFA and n-3 PUFA resulted in a linear increase of n-6:n-3 PUFA ratio as time on concentrate increased (P<0.001). There was no difference across treatments for plasma glucose concentration (P>0.64), whereas there was an interaction between treatment and time for plasma insulin concentration (P=0.012) with a quadratic difference across treatments with the greatest levels occurring in CNT and 120D treatments. The consumption of a high-concentrate based diet immediately post-weaning increases dressing percentage, carcass weight and marbling deposition in cattle finished subsequently on forages.