EFFECT OF SPERMATOZOAL CONCENTRATION AND NUMBER ON FERTILITY OF FROZEN EQUINE SEMEN

Information on the number of motile spermatozoa needed to maximize pre gnancy rates for frozen-thawed stallion semen is limited. Furthermore, concentration of spermatozoa per 0.5-mL straw has been shown to affec t post-thaw motility (7). The objectives of this study were 1) to comp are the effect of increasing the concentration of spermatozoa in 0.5-m L straws from 400 to 1,600 x 10(6) spermatozoa/mL on pregnancy rate of mares, and 2) to determine whether increasing the insemination dose f rom approximately 320 to 800 million progressively motile spermatozoa after thawing would increase pregnancy rates. Several ejaculates from each of 5 stallions were frozen in a skimmilk-egg yolk based freezing medium at 2 spermatozoal concentrations in 0.56 mt polyvinyl-chloride straws. Half of each ejaculate was frozen at 400 x 10(6) cells/mL and half at 1,600 x 10(6) cells/ml. Insemination doses were based on post- thaw spermatozoal,motility and contained approximately 320 x 10(6) (32 0 to 400) motile spermatozoa or approximately 800 x 10(6) (800 to 900) motile spermatozoa. Sixty-three mares were assigned to 1 of 4 spermat ozoal treatments (1--low spermatozoal number, low concentration; 2--lo w spermatozoal number, high concentration; 3--high spermatozoal number , low concentration; 4--high spermatozoal number, high concentration) and were inseminated daily. Post-thaw spermatozoal motility was simila r for cells frozen at both spermatozoal concentrations (P>0.1). One-cy cle pregnancy rates were 15, 40, 28 and 33%, respectively, for Treatme nts 1, 2, 3 and 4. Packaging spermatozoa at the high concentration ten ded to increase pregnancy rates vs packaging at the low concentration (37 vs 22%; P=0.095). Furthermore, when the lower spermatozoal number was used, there tended (P<0.1) to be a higher pregnancy rate if sperma tozoa were packaged at the higher concentration. There was no increase in pregnancy rates when higher numbers of motile spermatozoa were ins eminated (27 vs 31%; P>0.1). Based on these results, a single 0.5-mL s traw dose containing 800 x 10(6) spermatozoa should be used and each i nsemination dose should contain approximately 320 x 10(6) motile sperm atozoa. Fertility trials utilizing other freezing extenders are necess ary before recommending a single 0.5-mL insemination dose for all free zing extenders. (C) 1998 by Elsevier Science Inc.