Ovulation rate and survival of fertilized ova in Merino ewes selected for and against multiple births

Abstract

Peppin Merino ewes selected for (T group) and against (0 group) multiple births over a number of generations were used in an experiment designed to study the effect of selection on ovulation rate, response to exogenous gonadotrophin (1300 i.u. pregnant mare serum gonadotrophin—PMSG), and their ability to support one or more embryos of their own or the other group. Fertilized eggs, collected following treatment of donors with PMSG, were transferred within and between groups at a rate of one and three per recipient. Recipients were not treated with PMSG.

The mean ovulation rates of T and 0 recipients were 2.03 and 1.13, and the mean ovulatory response to PMSG of T ewes was almost 50 % greater than that of O ewes (4.76 v. 3.29 corpora lutea).

The survival to birth of transferred eggs was affected by both the selection group of the egg and the selection group of the recipient, and by the number of eggs transferred. More O than T recipients lambed (77 % v. 45 %); and within O recipients, intergroup transfers were more successful than intragroup transfers (94 % v. 54% ewes lambed). A similar but not significant difference was observed in T group recipients (54% v. 38% lambed).

Where three eggs were transferred, neither the selection group of the recipient nor that of the egg had any effect upon the incidence of multiple births. It is concluded that selection has operated primarily on ovulation rate, and there was some evidence of heterosis in intergroup transfers.