Abstract

An intensive evaluation of the hormones that are primarily involved in regulating reproductive function in cows treated with progestins was undertaken and comparisons were made to cows with a functional corpus luteum. Cows with a functional corpus luteum were found to have reduced circulating luteinizing hormone (LH) and oestradiol than cows treated with a single progesterone releasing intravaginal device (PRID), norgestomet (progestin used in Syncro-Mate-B synchrony) or melengestrol acetate (progestin used for oestrous synchrony in the USA). If greater doses of progesterone than that released from a single PRID are used, concentrations of LH and oestradiol are like those of control cows in the midliteal phase of their oestrous cycle. In addition, conception rates are significantly improved when greater doses of progesterone are used compared to that of cows in which oestrus is synchronised with a single PRID. We propose that one reason for reduced conception with oestrous synchrony programmes presently in use is the failure of the progestins to effectively mimic the function of the corpus luteum.

JH, Niezen, TS Waghorn, GC Waghorn, and WAG Charleston

Proceedings of the New Zealand Society of Animal Production, Volume 53, , 235-238, 1993