Fertility of unfrozen and frozen stallion spermatozoa extended in EDTA-lactose-egg yolk and packaged in straws.

The fertility of frozen-thawed semen was compared with that obtained using fresh semen extended in skim milk. Semen for freezing was obtained in June from four stallions of unknown fertility; two ejaculates were collected 1 to 2 h apart every 3 or 4 d. The gel-free fraction of the ejaculate was mixed 1:1 with a glucose-EDTA solution (disodium ethylenediaminetetraacetic acid) and centrifuged at 650 x g for 15 min. The spermatozoa were resuspended in an EDTA-lactose-egg yolk extender containing 5% glycerol. Semen was frozen in .5-ml French straws containing 250 x 10(6) progressively motile spermatozoa before freezing. Only for 31% of the 54 ejaculates frozen was post-thaw spermatozoal motility greater than or equal to 50% of the percentage of progressively motile spermatozoa observed during evaluation of the neat semen. Spermatozoa in second ejaculates apparently were more susceptible to the adverse effects of dilution and centrifugation than spermatozoa in first ejaculates. Only samples containing greater than 30% progressively motile spermatozoa after freezing and thawing (at 38 C) were used for insemination. In June and July, 101 mares were inseminated daily with semen from one of three stallions beginning on d 2 and continuing through the end of estrus for one cycle. Mares were inseminated with semen in one straw or with 250 x 10(6) progressively motile spermatozoa extended in 10 ml of skim milk. Because of the poor survival of spermatozoa that had been frozen and thawed, mares inseminated with frozen-thawed semen received 100 to 130 x 10(6) progressively motile spermatozoa as compared with 250 x 10(6) progressively motile spermatozoa for mares inseminated with fresh semen. One cycle pregnancy rates, based on rectal palpation 50 to 60 d after ovulation, were 29% using frozen-thawed semen and 66% using fresh semen (P less than .05). Values for individual stallions were 19, 24 and 47% with frozen semen and 47, 61 and 67% with fresh semen. Routine use of frozen stallion semen is not recommended at this time.