Variation in plasma growth hormone during first parity in lactating cows

¹ Danish Institute of Agricultural Sciences, PO-Box 50, DK-8830 Tjele, Denmark
² Departamento de Ciencia Animal, Universidad Politécnica de Valencia, Camino de Vera s/n, Spain

^* To whom correspondence should be addressed. E-mail: peth{at}dca.upv.es.

	Abstract

This study analyzed genetic and phenotypic variation in plasma GH during lactation in first parity dairy cows. The heritability and repeatability were examined using an algorithm for separation of basal and peak concentrations and different power transformations. Blood samples were obtained 17 times during first parity in 85 Holstein, 67 Red Dane, and 62 Jersey cows and assayed for GH. Each breed comprised 2 genetic groups, thus, a total of 6 genetic groups were defined. Across genetic groups, cows were assigned to 1 of 2 TMR rations with either low or normal energy concentration. The separation algorithm identified only 4.0% of the plasma GH concentrations as peaks. After excluding peak concentrations, the repeatability of GH during lactation was improved. A log-transformation was found appropriate for GH. The log-transformed GH concentrations for lactating dairy cows had heritability ranging between 0.14 in early lactation to 0.08 in mid and late lactation. The repeatability was 0.24 in early lactation and increased to between 0.58 and 0.61 in mid and late lactation. We conclude that for GH concentrations in lactating cows which are sampled infrequently the exclusion of peak values to obtain a basal GH concentrations was not effective in clarifying phenotypic or genetic effects.

Key Words: somatotropin, dairy cows, lactation, breeds, heritability

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