Possible roles for prolactin, thyroxine and insulin in the growth of heifers before and during early pregnancy.

Two groups of British Friesian heifers with cyclic ovarian function and the potential for further growth were individually fed a restricted diet at two levels over a 12-week period. Eight heifers receiving the higher level of feed (SP group) and five receiving the lower level (LP group) became pregnant by artificial insemination at week 6. Mean (and SEM) body weight changes in the SP and LP groups were respectively 0.24 (+/- 0.024) kg day-1 and -0.19 (+/- 0.039) kg day-1. Over the period from weeks 3-12, mean concentrations of plasma prolactin, thyroxine and insulin in the five LP heifers were 39%, 67% and 74%, respectively, of those in the eight SP heifers. The results suggest that prolactin, thyroxine and insulin have roles in the growth of heifers before and during early pregnancy.