Chilled storage of ram semen improves with the addition of egg yolk and glycerol to milk-based extenders

This study aimed at comparing in vitro, ultra‐heat‐treated (UHT) skim milk and INRA‐96^®‐based extenders supplemented or not with 5% egg yolk and/or 2% glycerol on sperm quality parameters along 72 h of preservation at 5°C, using a factorial design. Semen from six healthy mature Merino rams was pooled and extended in each medium using a split sample procedure (six replicates) and chilled. Subjective motility (SM) (%), membrane integrity (MI) (%) and uncapacitated spermatozoa (US) (×10⁶ spermatozoa/AI dose) were used to assess the semen quality at 0, 12, 24, 48 and 72 h of preservation. UHT‐based extenders yielded better (p < 0.05) SM and MI than INRA‐96^®‐based extenders (59.7% vs 57.9%; 60.2% vs 55.8%, respectively) but similar numbers of US (64.2% vs 62.3 × 10⁶ sperm/AI dose, respectively) along the preservation time. Egg yolk–glycerol or just egg yolk as additives improved (p < 0.05) the results compared with the base extenders without additives or just with glycerol. The sperm parameters assessed decline slowly from 0 to 48 h, with a sharp decline (p < 0.05) at 72 h of preservation. In conclusion, UHT and INRA‐96^® were similar as base extenders, and the addition of 5% egg yolk plus 2% glycerol or just 5% egg yolk improved the quality of ram semen preserved at 5°C, at least for 48 h. The combination of egg yolk–glycerol might provide extra protection in case of fluctuation of temperatures below 5°C, commonly seen under field conditions.