The effect of insemination time and sperm dose on pregnancy rate using sex-sorted ram sperm

The objective was to determine the optimum timing of insemination and minimum effective dose rate of sex-sorted ram sperm. Semen from three Merino rams was sorted into high purity X- and Y-chromosome bearing sperm populations. Ovulation was controlled in 732 Merino ewes using PMSG at progestagen pessary removal and GnRH 36 h later. Sorted (S) and non-sorted (NS) doses of 1 or 15 × 106 motile, frozen-thawed sperm were inseminated laparoscopically at 50, 54, 58, 62, and 66 h after progestagen withdrawal. An additional treatment dose of 0.5 × 106 S or NS sperm was inseminated at the 58 h time point (n = 60). Pregnancy was diagnosed by ultrasound at 60-62 d gestation. Both 1 × 106 and 15 × 106 sperm achieved similar pregnancy rates, regardless of sperm type, at 58 h (S1: 46 ± 9.4%; S15: 43 ± 9.3%; NS1: 41 ± 9.2%; NS15: 49 ± 9.4%). However, pregnancy rates were lower (P < 0.05) for doses of 1 than 15 × 106 sperm inseminated at 50 (15 ± 6.3% vs. 36 ± 9.1%), 54 (14 ± 4.4% vs. 55 ± 7.3%), 62 (33 ± 6.9% vs. 54 ± 7.3%), and 66 h (29 ± 8.6% vs. 56 ± 9.5%). There was no difference between S and NS sperm for inseminations with 0.5 × 106 motile sperm at 58 h after PR (15 ± 3.6% vs. 14 ± 3.3%), nor with 15 × 106 motile sperm at all insemination times (49 ± 6.3% vs. 49 ± 6.3%). However, fertility was higher for S than NS sperm at the 1 × 106 dose level (37 ± 6.1% and 16 ± 4.0%). More than 90% of lambs born were of the predicted sex. We hypothesise that the sorting process selects a homogeneous, fertile sub-population of sperm, removing those that are dead, damaged and morphologically abnormal.