Impact of Distinct Freezing Curves on the Quality of Ram Sperm after Thawing

Automated equipment with customized freezing curves can be used to cryopreserve ram sperm, but none is considered a standard.

This study compared the post-thawing quality of ram sperm frozen using a conventional freezing curve and two controlled-rate curves.

Six ejaculates were collected from four rams (n = 24). In the conventional curve (110 min), sperm was cooled at -0.3 to -0.5°C min^-1 until 5°C, stabilized for 60 min and exposed to liquid nitrogen (LN₂) vapor for 10 min (-80°C) before submersion. The slow-customized (SC) curve (126.2 min) used a rate of -0.25°C min^-1 until 5°C, stabilization for 60 min and a rate of -20°C min^-1 until -120°C before immersion in LN₂. Rates for the fast-customized (FC) curve (75 min) were: -0.3°C min^-1 until 5°C; -3°C min^-1 until -10°C; -5°C min^-1 until -35°C; and -4°C min^-1 until immersion in LN₂ (-43°C).

Velocity in a straight line and beat-cross frequency were lower for spermatozoa frozen with the FC than with the conventional curve (P < 0.05). The FC curve resulted in more membrane and acrosome damages than the other curves (P < 0.05). Mitochondrial membrane potential was lower with the SC than with the other curves (P < 0.05).

The conventional curve was more efficient than both tested automated freezing curves. The FC curve may be an alternative to the SC curve due to the shorter processing time.