Post-thaw viability of bull AI-doses with low-sperm numbers

Use of AI-doses containing low-sperm numbers are increasingly been used to optimise use of elite bulls as well as to accommodate an eventual wider application of sex-sorted semen. Since spermatozoa might, however, suffer from high extension rates, thus compromising fertility, this study evaluated the post-thaw sperm quality of semen from commercial progeny-tested, high-ranked AI-sires whose semen was within acceptable limits of normality, frozen in a split-design to 15 (control, 15M) or 2 x 10(6) total spermatozoa (treatment, 2M) per straw. Assessment post-thaw included computer-evaluated sperm motility (CASA), membrane integrity (SYBR-14/PI), membrane stability (Annexin-V/PI), acrosome integrity (Carboxy-SNARF-1/PI/FITC-PSA), and chromatin integrity (AO of in situ acid-induced DNA denaturation). High extension did not affect the proportions of linearly motile spermatozoa, of membrane integrity or stability nor chromatin integrity, immediately post-thaw. However, high extension clearly affected linear sperm motility following incubation at 38 degrees C for 30 min, sperm viability when assessed by SNARF and, particularly, acrosome integrity of the otherwise viable spermatozoa. Individual sire variation was evident. Fertility was preliminarily evaluated for one of the less affected bulls in a blind field trial. A total of 109 dairy cows were randomly inseminated with 15M or 2M-straws without differences in pregnancy rate between them (47% versus 43%). This similarity in fertility rates, confirmed the in vitro methods used were appropriate for identifying cryosurvival and further suggested the site of sperm deposition was not crucial for the fertility of low-sperm AI-numbers for this particular sire. However, the inter-bull variation seen calls for caution when cryopreserving low concentrations of bull spermatozoa with conventional freezing protocols.

9 DOES STRAW CONCENTRATION AFFECT POST-THAW SPERM QUALITY IN AI BULL SIRES?

There is increasing interest in decreasing the number of spermatozoa per AI dose, owing to the discrimination of threshold concentration and fertility, economical revenues and the use of sexed semen. This study evaluated the quality (as sperm viability, acrosome integrity, membrane stability and chromatin stability) post-thaw of semen collected from four elite AI sires and frozen at 15 × 106 (control) or 2 × 106 spermatozoa (spz) per dose to disclose eventual deleterious extension effects. Semen collected via a.v. from 4 élite AI sires was split-processed and frozen in 0.25 mL plastic straws under commercial conditions, at either 2 × 106 or 15 × 106 spz/straw (the latter as control). The semen parameters were within acceptable limits of normality. The post-thaw samples showed acceptable motility (above 50%). Sperm viability and stability were assessed post-thaw using flow cytometry of cells loaded with SYBR-14 and propidium iodide (PI) for sperm viability (membrane integrity), and with carboxy-SNARF-1 (SNARF; Invitrogen AB, Frolunda, Sweden), PI, and FITC-Pisum sativum agglutinin (PSA, triple stain) for acrosome status. Membrane stability status was measured with Annexin-V/PI, while sperm chromatin condensation and stability were assessed following in situ acid-induced DNA denaturation and staining with acridine orange. No significant differences were seen between the two concentrations regarding sperm motility, plasma membrane integrity (SYBR/PI, SNARF), or chromatin stability. The highly extended semen (2 × 106 spz/straw), however, showed a higher (P < 0.05) frequency of spermatozoa with translocated phosphatydil serine (as detected with Annexin-V), indicating their plasma membranes had become unstable. Also, there were more spermatozoa showing acrosomal damage (PSA). In conclusion, low concentrations of spermatozoa do not properly sustain conventional cryopreservation, although damages appear restricted to the sperm membrane. These changes in the stability of the plasma membrane apparently did not affect the viability of the spermatozoa, although they may negatively affect their lifespan. These findings may have an impact on the care that must be taken when cryopreserving low concentrations of spermatozoa with conventional freezing protocols. This work was supported by the Swedish Farmers’ Foundation for Research in Agriculture (SLF) and FORMAS, Stockholm.