Serum Prolactin and Growth Hormone in Holstein Bulls

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Abstract
Repeatabilities among three estimates of serum prolactin and growth hormone collected in 71 Holstein bulls within 10 days were .13 and .30. However, befit- abilities of the serum hormone concen- trations were not different from 0. In another experiment of 86 Holstein bulls, serum collected in July contained more than twice as much prolactin as serum collected in January whereas growth hor- mone was unaffected by months. Prolac- tin was unchanged whereas growth hor- mone decreased with age. However, logarithm base 10 of serum concentra- tion of prolactin increased as age ad- vanced. Neither hormone was significant- ly related to predicted difference. We conclude that simply measuring basal serum prolactin and growth hormone has little usefulness for estimating breed- ing values of cattle.