Abstract:
Breed and sire differences in sperm cryosurvival have been noted, with negative implications for sperm
cryobanking and assisted reproduction programs. This study hypothesised that these differences could be
modified by using lower molecular weight cryoprotectants. Therefore, the effect of replacing glycerol
(GLY) with ethylene glycol (EG) on differential cryosurvival of semen from two Sanga cattle breeds
(Mashona vs. Tuli) was determined. Three to five ejaculates were collected from each of ten bulls (3-8
years) by electro-ejaculation, diluted in three Tris-egg yolk extenders (Triladyl®, 7% GLY-based and 7%
EG-based) and evaluated for sperm motility, viability and morphology at three time periods (fresh – 0 h,
pre-freeze – 4 h and post-thaw). Tuli bulls produced larger (11.8±0.31 ml vs. 8.5±0.38 ml) and more
concentrated ejaculates of lower fresh semen quality. Breeds differed across time for motility and
morphology, but not viability. Mashona bull semen had significantly higher motility and normal
morphology values at each sampling time. Bulls classified as poor freezers had lower concentration
(0.70±0.09 × 109
sperm/ml vs. 1.37±0.10 × 109
sperm/ml), sperm motility index (SMI, 35.0±3.4 % vs.
67.8±2.1 %) and viability (69.7±1.1 % vs. 75.7±1.0 %) compared to good freezers. Maintenance of semen
quality by GLY and EG did not differ between breeds, poor and good freezers, or age groups. The
interaction breed by extender across time did not reach statistical significance for all variables. The study
revealed that bull and breed variation in sperm quality and cryosurvival is not modified by replacing GLY
with EG, suggesting that cryostress tolerance of sperm may be under control of mechanisms other than
differential response to GLY cytotoxicity.
