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 an d 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 (an d 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 pe riod from weeks 3-12, mean concentrations of plasma prolactin, thyroxi ne and insulin in the 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 d uring early pregnancy.