Effects of different growing diets on performance, carcass characteristics, insulin sensitivity, and accretion of intramuscular and subcutaneous adipose tissue of feedlot cattle.

Forty-eight individually fed crossbred steers (British and British x Continental; BW=296+/-16.7 kg) were used to evaluate effects of different growing diets on changes in accretion of intramuscular (IMF) and subcutaneous (SCF) adipose tissues, insulin sensitivity, and carcass traits. Dietary treatments were AL-LC (a low-corn diet fed to allow cattle ad libitum access to feed), AL-HC (a high-corn diet fed to allow cattle ad libitum access to feed), LF-HC (a limit fed high-corn diet with the energy intake equal to that provided by AL-LC), and AL-IC (a diet with approximately the midpoint daily energy intake between AL-LC and AL-HC). Steers received treatments until d 56, after which all groups were fed AL-HC until d 140. Real-time ultrasound and BW measurements were taken every 28 d, and 3 glucose tolerance tests (GTT) were conducted on d 0, 28, and 56 of the growing period to assess insulin sensitivity. Based on ultrasound IMF and SCF readings during the growing phase, AL-HC and AL-IC increased accretion of IMF (P=0.01), and AL-LC and LF-HC diets resulted in less accretion of SCF (P<0.01) compared with other treatments. During the finishing period, accretion of IMF (P=0.13) and SCF (P=0.81) did not differ among treatments, which diluted differences in overall (d 0 to 140) accretion of IMF (P=0.28) and SCF (P=0.52), such that final real-time ultrasound measures of IMF and SCF did not differ (P >or= 0.36) among treatments. Actual carcass marbling scores, however, were greater for the AL-HC and AL-IC treatments (P=0.02), and 12th-rib fat thickness tended (P=0.08) to be greater for AL-HC and AL-IC groups. Based on incremental area under the curve and area over the curve as indicators of insulin release and glucose uptake, respectively, no differences (P >or= 0.10) in insulin sensitivity were observed among treatments. Our results suggest that high-corn diets increase growing phase accretion of IMF and SCF; however, these differences were not related to differences in glucose and insulin kinetics.