Management Strategies to Maximize Skeletal Growth Rate in Dairy Heifers

The objectives of this study were to determine the effects of recombinant bovine somatotropin (bST) and added dietary rumen undegradable protein (RUP) on organ and tissue weights and body composition in growing dairy heifers. A total of 32 Holstein heifers, 3 months of age at the beginning of the study were used in the experiment. Eight heifers were slaughtered at 3 mo of age to determine pre- treatment body composition. The remaining heifers were randomly assigned to treatments (n=6) consisting of 0.1 mg/kg body weight per day of bST and 2% added dietary RUP (dry matter basis) applied in a 2X2 factorial design. A total of six heifers per treatment group (3 each at 5 and 10 mo of age), were slaughtered to determine body composition an organ masses. There was a trend for increased live and empty body weights (EB:W), carcass and non-carcass components for heifers treated with bST or fed RUP. Added RUP increased rumen and reticulum weights whereas administration of bST tended to increase the weights of small and large intestine at 10 months of age by 22 % and 26%, respectively. Spleen, heart, and kidney weights at 10 months of age were increased 36%, 28% and 23% for bST treatments respectively, compared with controls. Rates of ash and protein deposition between 3 and 10 months of age were increased by bST by 7.2 g/d and 28.9 g/d, respectively, while no treatment differences were observed for rates of fat and energy deposition. Bovine somatotropin significantly altered the metabolism of growing heifers in a manner that led to increased protein and ash deposition, and tended to reduce fat percentage, and there was a similar tendency observed with added RUP. This suggests that nutritional and endocrine manipulations could increase growth rates of skeletal and lean tissues without increasing fat deposition in prepubertal dairy heifers.