Enriching egg yolk fractions-tris-based extenders with cow skimmed milk improved the cryopreservation of bull sperm.

The effects of substituting whole egg yolk (WEY) in a tris-citrate-based extender with two derived fractions, including a buffer-soluble fraction (BSF) and an ammonium sulfate insoluble yolk fraction, on the freezability of bull sperm were investigated. The BSF and ammonium sulfate insoluble yolk fraction levels of egg yolk in the respective diluents were consistent with their extracted values from an equivalent volume (20.00%) of egg yolk in the control diluent. The extenders were then enriched with 0.00, 5.00, and 10.00% (v/v) of pasteurized skim milk. Semen samples were collected weekly over five consecutive weeks from six adult bulls, and the standard ejaculates were pooled. The pooled semen was subsequently divided into seven experimental extenders and frozen in 0.50 mL French straws. Various sperm quality parameters, including kinematics, acrosome integrity, capacitation, and DNA fragmentation, were evaluated post-thawing. Results indicated that sperm kinematics and the percentage of acrosome-reacted sperm in experimental extenders were not significantly different from those in the control group. The milk-free ammonium sulfate insoluble yolk fraction extender exhibited higher percentages of capacitated sperms, but lower percentages of spermatozoa with intact DNA compared to the WEY extender sperm. The addition of 10.00% milk into the BSF diluent resulted in a significant increase in the proportion of sperm with intact DNA and a notable decrease in the percentage of sperm with partially fragmented DNA compared to the control. In conclusion, a BSF extender enriched with 10.00% cow's skimmed milk is recommended as a substitute for WEY in the cryopreservation of bull semen.