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Growth, Mammary, Reproductive, and Pituitary Hormone Characteristics of Holstein Heifers Fed Extra Grain and Melengestrol Acetate¹

Department of Dairy Science, Michigan State University, East Lansing 48823

ABSTRACT

Abstarct: Rate of growth was studied in 140 Holstein heifers fed standard amounts (.9 kg/day, control) or extra amounts (4.5 kg/day) of grain with or without .45 mg melengestrol acetate (MGA) from 2.5 months of age to initial breeding size (120 cm withers height). Heifers fed extra grain exhibited first estrus at an earlier age (P<.01) than the controls (7.5±.1 versus 8.7±.2 months), but body weight and withers height at first estrus were unaffected by amount of grain or MGA. Heifers fed extra grain attained 120 cm withers height at 11.4 months of age, whereas controls required 12.5 months (P<.01). Melengestrol acetate did not affect ages when the heifers attained 120 cm withers height. The interval from first estrus to 120 cm withers height (about 3.5 months) was not affected by nutrition or MGA. Although MGA did not affect skeletal growth, it increased body weight about 10% at 120 cm withers height, but only with extra grain (P<.01) and only after about 5.5 months of age.

Neither amount of grain nor MGA affected weights of the mammary glands either at first estrus or at 120 cm withers height. At 120 cm withers height, mammary deoxyribonucleic acid (DNA) and ribonucleic acid (RNA) were elevated over 60% (P<.01) by MGA especially in heifers fed extra grain. Pituitary prolactin was unaffected, but MGA depressed prolactin in blood plasma, especially in heifers fed standard grain. Melengestrol acetate fed before pregnancy did not affect subsequent milk production.

Melengestrol acetate increased the number of large ovarian follicles, reduced uterine DNA, increased uterine RNA, and increased endometrial cell height. Neither pituitary luteinizing hormone nor follicle-stimulating hormone was influenced by MGA or quantity of grain fed. On the average, all heifers calved by 2 years of age, and age at conception, services per conception, weight at calving, withers height at calving, and weight of calf were uninfluenced by MGA or amount of grain. In terms of age at first calving, neither extra grain nor MGA offers appreciable gain relative to standard grain alone.

¹ Published with the approval of the Experiment Station Director as Paper 5284. This research was supported, in part, by a grant-in-aid from The Upjohn Company, Kalamazoo, Michigan.

² Present address: Ohio Agricultural Research and Development Center, Wooster 44691.

³ Present address: M. C. Franklin Laboratory, University of Sydney, Camden, New South Wales, Australia.

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