RELATIONSHIP BETWEEN BLOOD-SERUM INSULIN-LIKE-GROWTH-FACTOR-I CONCENTRATION AND POSTWEANING FEED-EFFICIENCY OF CROSSBRED CATTLE AT 3 LEVELSOF DIETARY-INTAKE

Blood serum concentration of IGF-I was analyzed to determine its relat ionship with individual postweaning feed efficiency (gain/feed) of 36 crossbred steer calves fed at three levels of feed intake (n = 12 at e ach level). Diets consisted of a corn silage-based growing diet for 84 d followed by a 91% concentrate finishing diet for 56 d. Dietary inta ke levels were at 80, 90, or 100% of ad libitum. Diets were formulated to ensure equal daily intake of protein, vitamins, and minerals acros s intake treatment levels. Intake was measured daily; ADG, DMI, and fe ed efficiency were calculated at 28-d intervals, through d 140. Indivi dual weights and serum samples were collected at the beginning of the study and at 28-d intervals thereafter. The IGF-I concentrations were determined with a RIA. Data were analyzed as a multivariate split-plot in time. Imposed dietary intake restrictions did not affect serum IGF -I concentration (P =.90) or individual feed efficiency (P =.36), even though the least squares means for IGF-I concentration tended to decr ease and the feed efficiency means tended to increase under the restri cted intake levels. Serum IGF-I concentration, ADG, and feed efficienc y were affected (P <.001) by collection date. Residual correlations be tween IGF-I concentrations at adjacent 28-d sampling times averaged .7 2. Diet intake level x sampling time interactions existed for ADG (P = .02) and feed efficiency (P <.001). Positive residual correlations of .28(P <.001) and .16 (P =.07) existed between IGF-I and ADG and betwee n IGF-I and feed efficiency, respectively. Regression analysis indicat ed that a 1 ng/mL increase in serum IGF-I concentration was associated with a .00135 kg/d increase in ADG (P <.001) and a .0001 kg gain/kg f eed increase in feed efficiency (P =.04). These results support the hy pothesis that serum IGF-I plays a role in growth and in efficiency of feed utilization in beef cattle.