Effects of 2.1 and 3.5x10(6) sex-sorted sperm dosages on conception rates of Holstein cows and heifers.

The objective was to compare conceptions rates of Holstein cows and heifers after artificial insemination (AI) with 2.1 or 3.5x10(6) sex-sorted sperm or 15x10(6) conventional sperm. Ejaculates collected from 7 Holstein sires were cryopreserved conventionally at 15x10(6) sperm per dose or sorted to 90% purity for X-chromosome-bearing spermatozoa using flow cytometry and cryopreserved at either 2.1 or 3.5x10(6) sperm per dose. All treatments were processed in an egg-yolk (20%), Tris, glycerol (7%) extender and packaged in color-coded 0.25-mL French straws. Straws (n=700 straws/dosage per sire) were packaged and distributed in aliquots of 12 (4 straws/sperm dosage) to 69 Holstein herds with an across-herd goal of achieving approximately 50% use in heifers and cows. Straw color was recorded in the on-farm recordkeeping system at the time of AI and retrieved by electronic download. Data for cows and heifers were analyzed separately. Among heifers, 6,268 services were retrieved from 45 herds (298+/-4.2 services/sperm dose per sire; range: 244 to 344). Conception rate of heifers was influenced by the sire by treatment interaction. Conception rate of the 2.1 and 3.5x10(6) sex-sorted sperm dosages were comparable in 6 of 7 sires. Conception rate of both sex-sorted dosages were less than those of conventional semen for 6 of 7 sires. Across sires, heifer conception rates for 2.1 and 3.5x10(6) sex-sorted sperm dosages and 15x10(6) conventional dosages were 44, 46, and 61%, respectively. Among cows, 5,466 services were retrieved from 52 herds (260+/-3.3 services/sperm dose per sire; range: 236 to 289). Conception rates of cows were influenced by herd, sire, and sperm dosage. Conception rates of the 2.1 and 3.5x10(6) sex-sorted sperm dosage were comparable for all 7 sires. Conception rates of 2.1x10(6) sex-sorted sperm dosage were less than those of conventional semen for 4 of 7 sires and conception rates of the 3.5x10(6) sex-sorted sperm dosage were less than those of conventional semen for 2 of 7 sires. Across sires, conception rates for 2.1 and 3.5x10(6) sex-sorted sperm dosages and 15x10(6) conventional dosages in cows were 23, 25, and 32%, respectively. In conclusion, these data could not confirm that a meaningful improvement in conception rates should be expected in cows or heifers from increasing sex-sorted sperm dosage from 2.1 to 3.5x10(6) sperm per dose.