INFLUENCE OF POSTWEANING GROWTH RATE ON PUBERTY AND OVARIAN ACTIVITY IN HEIFERS

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Abstract
Thirty-six Holstein heifers were randomly assigned at the weight of 136 kg to two groups: a fast-growing (FR) and a moderate-growing (MR) feeding regime for an initial phase of 100 days, followed by a final phase ending with ovariectomy after puberty, during which half of each group was subjected to the other feeding regime. The effect at puberty of the FR was a nonsignificant reduction of 16.3 days in age, and a significant increase (P < 0.01) of 26.2 kg in weight when considering the feeding regimes of the initial phase of growth. When the same treatments were imposed during the final phase of growth, heifers on FR were 52.0 days younger at puberty (P < 0.01) and 10.0 kg lighter (P > 0.05) than heifers on MR. Follicular development, in terms of follicular fluid weight, and diameters of the largest and second largest follicles 15 days after estrus, was greater in the ovary bearing the corpus luteum than in the other ovary. The diameter of the second largest follicle was greater in heifers on FR when imposed in the final phase of growth. Injection of Vetrophin at the estrus preceding ovariectomy did not change the ovulation rate, but increased the percentage of small-sized follicles and decreased the percentage of medium-sized follicles. The number of corpora albicantia observed at ovariectomy indicated that 76.5% of the heifers ovulated prior to puberty without exhibiting a standing estrus. Of those exhibiting an estrus at first ovulation, 33.3% had a first estrous cycle of less than 10.0 days in length.

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