FERTILITY OF STALLION SEMEN PROCESSED, FROZEN AND THAWED BY A NEW PROCEDURE

The fertility of frozen-thawed and fresh semen from 3 stallions was compared in a trial using a randomized block design and 90 mares for 108 cycles. Semen was collected every 3rd day, diluted to 50 .times. 106 sperm/ml with a citrate-based centrifugation medium and centrifuged. The cells were resuspended at 700 .times. 106 progressively motile sperm/ml of added lactose-EDTA-egg yolk extender containing 4% glycerol, packaged by placing 0.55 ml into polypropylene straws and frozen. Semen was thawed by immersion in 75.degree. C water for 10 s. All of the 43 ejaculates collected were frozen, but 21 were discarded because progressive sperm motility was < 35% immediately after thawing or < 40% after 30 min of incubation at 37.degree. C. Semen from the same stallions was collected daily for inseminations with fresh semen. Semen containing 200 .times. 106 progressively motile sperm was added to 10 ml of heated skimmilk extender. Mares were inseminated daily starting on the 3rd day of estrus or when a .gtoreq. 4-cm follicle was detected, whichever came later, and continuing through the end of estrus or for 9 days. Based on palpation per rectum on day 50 postovulation, the pregnancy rates from inseminations during 1 estrus were 50, 56 and 61% with frozen semen and 67, 67 and 61% with fresh semen (P > 0.05) from the 3 stallions, respectively. Mean pregnancy rate with frozen semen was 86% of the rate attained with fresh semen.