Equine chorionic gonadotrophin administration to rams improves their effectiveness to stimulate anoestrous ewes (the "ram effect").

Ewes' response to ram effect is related to the strength of androgen-dependent ram signals. Experiment 1 aimed to determine if the administration of a single dose of 1000IU of eCG to rams three days before joining them with ewes enhance their ability to stimulate females. Based on the results of Experiment 1, in a second experiment rams received two doses seven and three days before their introduction to females. In Experiment 1, rams treated or not with eCG were joined with ewes, and estrous was recorded until Day 5 (Day 0=rams and ewes were joined), and from Day 15 to Day 23. In addition, serum testosterone concentration was measured in all rams in the first recorded period. Testosterone values were greater in eCG-E1 than in Con-E1 rams on Days 0 and 2. The percentage of ewes in estrus was similar in both groups. In Experiment 2, rams were treated with two doses of eCG on Days -7 and -3 or remained as untreated controls. Estrous was recorded until Day 5, and pregnancy rate on Day 46; testosterone was measured in samples collected from all rams. Testosterone concentration was greater in eCG-E2 than Con-E2 rams from Day -5 to Day 1, and tended to do so on Day 2. More eCG-E2 than Con-E2 ewes came into estrus and became pregnant. It was concluded that treatment of rams with two high doses of eCG before joining them with anestrous ewes, enhanced their ability to induce ewes' cyclic activity (the "ram effect").