Samples of blood from ten pairs of split embryo, monozygous twin Holstein heifers were collected twice a week from January 1987 through December of 1989. The members of a given pair were from different surrogate mothers. Half of each sample was stored at -70{\textdegree}C and the other half was stored but subsequently and periodically assayed for the concentration (Ng/ml) of growth hormone, insulin, T-3, T-4, cortisol, progesterone, LH and FSH; this paper is concerned with the first four hormones listed. The differences among pairs were significant at the .0001 level of probability for growth hormone, insulin, T-3 and T-4 respectively. The components of variance associated with differences among pairs were 0.63, 1.59, 173 and 0.29 for growth hormone, insulin, T-3 and T-4 respectively. The corresponding components of variance for differences between animals of the same pair were 0.06, 0.15, 134 and 0.11 respectively. The magnitude of the component of variance among pairs is from 1.2 to 10.5 times that for variation between members of the same pair. The concentration (Ng/ml) of growth hormone decreased essentially linearly from 6.0 {\textpm} 0.25 at 8 months of age to 2.0 {\textpm} 0.25 at 24 months and subsequent ages. The mean for the 2003 samples assayed was 3.11 {\textpm} 0.25. Insulin increased essentially linearly with age from 12.1 to 18.5 (Ng/ml) with a mean of 15.11 + 0.11 for the 1170 samples. T-3 decreased at a decreasing rate from 210 (Ng/ml) at eight months to 170 (Ng/ml) at 24 months; the mean of the 1170 samples assayed was 181 + 0.92. T-4 decreased essentially linearly with age from a mean of 6.7 (Ng/ml) at 8 months to 5.6 (Ng/ml) at 24 months, the mean of the 1030 samples assayed was 6.19 t 0.03 (Ng/ml). Sampling of high and low pairs of dams and daughters in several large commercial herds is in progress to estimate additive genetic variance and genetic covariance with production.

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