Relationships among Mammary Nucleic Acids, Milk Yield, Serum Prolactin, and Growth Hormone in Heifers from 3 Months of Age to Lactation

Abstract

Nucleic acid content was determined in the left halves of udders (removed surgically) of 28 Holstein heifers at 5 months of age and in contralateral halves of udders after 60 days of lactation. Mammary deoxyribonucleic acid increased 89 times and ribonucleic acid increased 415 times between 5 months of age and 60 days of lactation. Nucleic acid contents of udder-halves at 5 months were positively correlated (r=.05 to .27) with similar mammary tissue measurements after 60 days of lactation and milk production of contralateral udder-halves. Correlations of nucleic acids after 60 days of lactation with milk yield were consistently larger than similar correlations between nucleic acids at 5 months of age and milk yield.Serum concentrations of prolactin and growth hormone were measured at many intervals between 3 months of age and 26 days of lactation. Phencyclidine hydrochloride (an anesthetic) caused release of prolactin. Various estimates of serum growth hormone among animals at different physiological states were positively correlated with each other, but similar correlations involving prolactin were consistently low and erratic. Correlation coefficients between various estimates of serum hormone concentrations and mammary nucleic acids or milk yield were low and often negative. The relatively low magnitude of correlation coefficients precludes use of these measures of mammary development or of serum prolactin or growth hormone in prepubertal heifers as reliable means of predicting future lactational performance.