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.