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Influence of Age at First Breeding on Growth, Reproduction, and Production of Well-Fed Holstein Heifers^{1, 2,}

Department of Dairy Science, University of Wisconsin, Madison

ABSTRACT

Thirty-six purebred Holstein heifers were divided into three equal groups (I, II, and III) and bred the first estrus on or after 18, 14, and 10 months of age, respectively. All heifers were uniformly well-fed and managed under loose housing conditions throughout the entire experimental period.

Although there were marked within-group differences in breeding efficiency during the first service period, between-group differences were not statistically significant. Difficulties occurred more frequently at first calving and were significantly more severe in the youngest group of heifers. There were no adverse effects of early breeding on reproductive performance during the second service period.

Growth, as measured by a comparison of body weight gains during specific stages of pregnancy and lactation and average height at withers at 18, 24, and 30 months of age, was not adversely affected by early first breeding and subsequent lactation.

The average 305-day, 4% FCM yields of the heifers which calved at average ages of 27.9, 24.2, and 20.4 months were 11,087, 9,590, and 9,500 lb, respectively. Differences between these first lactation yields were not statistically significant.

¹ Published with the approval of the Director of the Wisconsin Agricultural Experiment Station.

² These data are part of a thesis presented by the senior author in partial fulfillment of the requirements for the Doctor of Philosophy Degree, University of Wisconsin.

³ This investigation was supported in part by a research fellowship from the Wisconsin Alumni Research Foundation, Madison, Wisconsin.

⁴ Present address: Post-doctoral Training Program in Endocrinology, Department of Zoology, University of Wisconsin.