Understanding maternal contributions to fertility in recipient cattle: development of herds with contrasting pregnancy rates.

Abstract Causes of variation amongst recipients within a herd in their ability to initiate and maintain pregnancy is largely unknown. In order to develop an experimental resource to understand the biology of recipient reproductive performance, each of 155 contemporary yearling heifers received 2 in vitro-produced embryos on 6 separate occasions during a 26-month period. Sixty days after transfer, pregnancy and the number of foetuses were determined ultrasonically and then pregnancies were terminated and the process was repeated. Heifers were ranked on their aggregate pregnancy rate performance, and the highest (High) and lowest (Low) 25 were retained. Mean pregnancy rates of all recipients ranged from 0.20 to 0.67 depending on transfer occasion. The mean±s.e. pregnancy rate of the High and Low sub-herds were 0.76±0.04 vs. 0.11±0.03, respectively ( P