Responses of Somatotropin, Insulin, Prolactin, and Thyroxine to Selection for Milk Yield in Holsteins

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Abstract
The objective was to determine the effects of selection for milk yield in Holsteins on blood plasma concentrations of somatotropin, prolactin, insulin, and thyroxine. Two genetic groups, one selected since 1964 for milk yield and one an unselected control, were used for this purpose and averaged 9878 and 7402 kg milk, respectively. All cows initiating normal lactations between August 1984 (29 selection and 23 control) were blood sampled at 42 and 161 d postpartum. On these days, three blood samples were taken at 3.5-h intervals from each cow via jugular venipuncture for radioimmunoassay. Data were analyzed as spit-plots with repeated measures. Whole plot effects included genetic group, season of sampling, and parity. Subplot effects included stage of lactation and sample number within day. Somatotropin concentration increased, insulin concentration decreased, and prolactin and thyroxine concentrations remained unchanged with selection for milk yield. Thyroxine and insulin concentrations tended to decrease with age. Somatotropin concentrations decreased from peak to midlactation, whereas insulin and thyroxine concentrations increased.