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

Abstract

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 36h 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 66h after progestagen withdrawal. An additional treatment dose of 0.5×106 S or NS sperm was inseminated at the 58h 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 58h (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 66h (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 58h 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.