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.