Studies on the artificial insemination of Merino sheep. I. The effects of frequency and season of insemination, age of the ewe, rams, and milk diluents on lambing performance

Abstract

(1) Results of large-scale artificial insemination of Merino sheep (2346 ewes) on a private stud are presented, together with data for seven factorial tests designed to determine effects on fertility, following insemination at one or two successive oestrous periods, of season of insemination, milk diluents, age of ewe, individual ram, and frequency of insemination within a single oestrous period. (2) Insemination in the autumn is more effective than in the spring (79.7 v. 54.7 % of inseminated ewes lambed to insemination over two cycles; P < 0.001). (3) Results for maiden ewes in autumn or spring are 15–20% less than those for older ewes (P < 0.05 to < 0.001). (4) Ewes inseminated with semen of rams producing semen of indistinguishable quality, as gauged by standard field methods, may differ in fertility at a single insemination (P <0.01), but this difference disappears if ewes which return are inseminated by semen of the same rams at the next oestrous period. (5) Dilution of semen to 1.4 with heated cow milk – skim or whole – does not depress fertility. (6) Ewe-milk diluent depresses fertility following a single insemination (68.6 v. 47.3%; P< 0.01). (7) Insemination twice in the one oestrous period may increase fertility by 8–9% (70.3 v. 60.9%; P, N.S.). Such an increase is not worth the additional effort. (8) Fertility with diluted semen is comparable with that reported from eastern and central Europe, provided that ewes are inseminated during the autumn.