Abstract
One week after weaning, 16 animals were immunized (IMM) against human-globulin (hG-IMM) and 16 against SRIF conjugated to hG (SRIF-IMM) in Freund's complete adjuvant. Subsequent boosters in Freund's incomplete adjuvant were made on days 14, 28, 56 and 73 of the experimental period. On day 79, half of the animals within each immunization treatment were infused continuously with human GRF(1-29)NH2 (3.33 ng min-1 kg-1 BW) and the other half with saline over a 32-day period. GRF increased (P=0.01) body weight gain by 12%, whereas SRIF immunization decreased (P=0.01) weight gain by 7.5% during the infusion period (days 79-111). However, the effect of SRIF immunization on growth rate was not significant (P>0.10) when considered over the entire duration of the experiment (days 0-111). Feed intake, feed efficiency, carcass characteristics, nutrient digestibility, nitrogen retention and metabolizability of energy were not affected (P>0.10) by SRIF immunization or GRF infusion. However, GRF infusion increased pancreatic activity of amylase (P<0.05). In conclusion, GRF, unlike SRIF immunization, could be used to improve growth performance of fast-growing corn-fed bull calves.
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