EFFECT OF FEED RESTRICTION ON SERUM SOMATOTROPIN, INSUUN-UKE GROWTH FACTOR-I-(lGF-I) AND IGF BINDING PROTEINS IN CYCUC HEIFERS ACTIVELY IMMUNIZED AGAINST GROWTH HORMONE RELEASING

Feed restriction often increases serum somatotropin (ST) and decreases insulin-like growth fac­ tor-I (IGF-I) in ruminants; however, the mechanisms responsible for this change in ST and IGF-I are not well defined. We investigated the effects of feed restriction on serum ST, IGF-I, IGF binding proteins (IGFBP), insulin and nonesterified fatty acids (NEFA) in cyclic Angus and Charolais heifers (n=15) previously immunized against growth hormone releasing factor (GRFi) or human serum al­ bumin (HSAi). Cows were fed a concentrate diet ad libitum (AL) or were restricted to 2 kg cotton seed hulls (R) for 4 d. Each heifer received each dietary treatment in a single reversal design. As anticipated, GRFi decreased ST, IGF-I and insulin (P<.05). In addition, GRFi decreased serum IGFBP-3 (P<.Ol), but increased IGFBP-2 (P<.Ol). Feed restriction resulted in an increase in serum ST in HSAi, but not in GRFi heifers. Regardless of immunization treatment, feed restriction de­ creased serum IGF-I and insulin, and increased NEFA (P<.Ol). In conclusion, the increase in serum ST levels observed during feed restriction was blocked by active immunization against GRF. How­ ever, feed restriction resulted in decreased serum IGF-I in GRFi heifers in spite of initial low lev­ els of IGF-I (due to GRFi). Although GRFi decreased levels of IGFBP-3 and increased levels of IGFBP-2, feed restriction for 4 d did not alter serum IGFBP.