COMPARISON OF FERTILITY RATES FOLLOWING INSEMINATION WITH DIFFERENT NUMBERS OF SPERMATOZOA PER INSEMINATION DOSE OF FROZEN BOVINE SEMEN

Semen from 62 bulls from two Al-stations, 276 ejaculates in all, was p repared to give final concentrations of 18, 15 and 12 million total sp ermatozoa per insemination dose. A total of 50878 first inseminations were recorded. The 60-day nonreturn rates (NRR) were 71.6, 70.4 and 71 .0% for 18, 15 and 12 million sperm per dose, respectively. NRR were d ifferent between 18 and 15 million (p < 0.05), but not between 18 and 12 million sperm. The NRR for Al-station 1 and 2 differed significantl y (p < 0.05), mean values being 70.5 and 71.4%, respectively. NRR for station 1 were not influenced by sperm number, while those after Al wi th 18 million sperm for station 2 were significantly higher than with 15 million sperm (p < 0.01). Results from medium-fertility bulls were not influenced by reduction of sperm cells per straw from 18 million t o 15 and 12 million. Low-fertility bulls, however, had significantly l ower NRR after Al with 15 and 12 million than with 18 million sperm (p < 0.05). For high-fertility bulls, the NRR for 12 million sperm per s traw were significantly better than for 15 million (p < 0.01), while t here was no statistical difference between 18 and 12 million sperm.