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Effects of Recombinant DNA-Derived Somatotropin and Dietary Energy Intake on Development of Beef Heifers: I. Growth and Puberty^1,2,

University of Kentucky⁴, Lexington 40546-0215

Abstract

The effects of dietary energy intake and somatotropin (STH) on growth and puberty were studied in 40 Angus heifers. At an average age of 7 mo (208 ± 8 d), heifers were assigned to four treatment groups: 1) vehicle (V) + high energy (HE; 2.68 Mcal ME/kg DM), 2) recombinant DNA-derived STH (20.6 mg/d; s.c.) + HE, 3) V + low energy (LE; 2.22 Mcal ME/kg DM) or 4) STH + LE. Animals remained on treatments until 15.5 mo of age. Body weights (BW), hip heights (HH) and areas of pelvic openings (PA) were measured every 28 d and backfat thicknesses (BF) were measured every 56 d. Plasma progesterone was measured in blood samples taken three times per week beginning at 9 mo of age to determine age at first ovulation. Heifers fed HE were heavier (P < .01), gained faster (P < .01) and had greater BF (P < .01) than those fed LE. Animals treated with STH gained faster (P < .01) and were heavier (P < .05) between 12 and 15 mo of age than V-treated heifers. Heifers treated with STH also had less BF (P < .05) and a tendency for a greater (P = .08) increase in HH than in V-treated heifers. Somatotropin interacted with energy (P < .05) and age (P < .01) to influence PA. Somatotropin increased (P < .01) PA in heifers fed the HE diet but not in those fed the LE diet. The effect of STH on PA became more pronounced as animals became older. Number of heifers reaching puberty by 13 mo of age was greater for animals on HE than for animals on LE (P < .01). Treatment with STH did not influence number of animals attaining puberty.

¹ This paper (No. 88-5-210) is published with the approval of the Director of the Kentucky Agric. Exp. Sta.

² We thank David Rock of American Cyanamid, Princeton, NJ for providing recombinant DNA-derived bovine somatotropin.

³ Reprint requests.

⁴ Dept. of Anim. Sci.

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