Effect of storage duration, storage temperature, and diluent on the viability and fertility of fresh ram sperm

Cervical artificial insemination (AI) in sheep with fresh semen yields a much higher pregnancy rate than when frozen-thawed semen is used, and consequently frozen semen is only acceptable for laparoscopic insemination. The short life span of fresh semen is a major constraint on the use of AI in genetic improvement programs for sheep. The main objective of this study was to examine the effects of storage conditions on viability and fertilization ability of fresh ram (Ovis aries) semen up to 72h postcollection. Experiment 1 was designed to evaluate the effect of diluent type (standard skim milk, AndroMed, OviPro, and INRA 96) and storage temperature (5 degrees C and 15 degrees C) on the motility and viability of fresh ram semen. Storage temperature, irrespective of diluent, had a significant effect on both motility and viability. Storage at 5 degrees C maintained acceptable motility and viability up to 72h compared with that of storage at 15 degrees C. In Experiment 2, the penetrating ability of fresh ram semen, diluted in either skim milk, AndroMed, or INRA 96, was assessed using artificial mucus. Flat capillary tubes containing artificial mucus were suspended in 250muL semen at a sperm concentration of 20x10(6)/mL. Semen was stored at 5 degrees C and tested after 6, 24, 48, and 72h. There was a significant diluent by time interaction. In Experiment 3, the fertilizing ability of fresh ram semen stored at 5 degrees C was evaluated in vitro. Fresh semen (diluted in either skim milk, AndroMed, or INRA 96) was added to matured ewe oocytes at 6, 24, or 72h after semen collection. Cleavage rate was recorded at 48h postinsemination, and blastocyst development was recorded on Days 6 to 9. There was a significant treatment effect on cleavage and blastocyst rates; insemination of semen stored for 24h resulted in higher rates than those for storage at 72h. In Experiment 4, the fertilizing ability of fresh ram semen was evaluated in vivo. Semen was diluted in INRA 96, stored at 5 degrees C, and used to inseminate ewes on the day of collection or at 24, 48, and 72h postcollection. Multiparous ewes were cervically inseminated at a synchronized estrus. Fertility rate decreased linearly (P<0.001) up to 72h after semen collection.