Growth rate and changes of the somatotropic axis in beef cattle administered exogenous bovine somatotropin beginning at 200, 250, and 300 days of age

¹ Department of Animal Science, University of Connecticut, Storrs, CT 06269

^* To whom correspondence should be addressed. E-mail: steven.zinn{at}uconn.edu.

	Abstract

To determine the effects of bovine (b) ST treatment beginning at 3 different ages on the growth rate and components of the somatotropic axis, 40 beef cattle (200 ± 21 d of age) were randomly assigned to 1 of 4 treatments (10 animals/treatment). Three of the treatment groups received bST (33 µg/kg BW) daily beginning at 200, 250, or 300 d of age until all animals reached 400 d of age; the fourth group served as controls (0 bST). Animals were housed in pens (5 animals per pen; 2 pens per treatment) and fed a diet formulated for an ADG of 1.2 kg/d. Feed intake (per pen) was measured daily and BW was determined weekly. Blood samples (10 mL) and ultrasound measurements were collected at 200, 250, 300, 350, and 400 d of age. Serum concentrations of ST and IGF-I were determined by RIA and IGFBP-2 and -3 by ligand blot procedures. Overall, cattle gained 284.0 ± 14.7 kg BW with a treatment by week interaction (P < 0.01), such that during the treatment period, ADG was 11.6, 8.7, and 15.8 % greater (P < 0.05) in cattle treated with bST beginning at 200, 250, and 300 d, respectively, relative to controls during the same time frame. Average DMI was 13.6 % less (P < 0.05) in bST-treated cattle than in controls. Increases in ADG coupled with a reduction in DMI resulted in 11.7, 14.0, and 26.4 % increases (P < 0.01) in feed efficiency (G:F) in bST-treated cattle beginning at 200, 250, and 300 d of age respectively, compared with contemporary controls. Backfat thickness increased (P < 0.05) over time, but the magnitude of increase was less in the bST-treated cattle (treatment by week interaction; P < 0.05). Area of LM increased (P < 0.05) over time, but was similar across treatment groups. Serum concentrations of ST, IGF-I, and IGFBP-3 increased (P < 0.05), whereas IGFBP-2 decreased (P < 0.05), over time. The changes in the components of somatotropic axis were more pronounced in bST-treated cattle compared with controls, with the greatest magnitude of response in animals that began bST treatment at 300 d of age. In conclusion, exogenous bST-induced growth response was greater in animals that began to receive bST administration at 300 d of age in a shorter period (100 d) compared with animals that received bST beginning at 200 or 250 d of age.

Key Words: Age, Beef cattle, Growth rate, Insulin-like growth factor, Insulin-like growth factor binding proteins, Somatotropin