II: Effect of Cooling Rate and Duration of Freezing Point Plateau on Boar Semen Frozen in Mini- and Maxi-Straws and Plastic Bags

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Abstract
The post-thaw motility and the acrosome integrity of semen from 4 boars frozen with a programmable freezing machine, in mini (0.25 ml) and maxi (5 ml) plastic straws and in 10 × 5 cm TeflonR FEP-plastic bags (0.12 mm thick, 5 ml), were compared. The freezing of the semen was monitored by way of thermocouples placed in the straws and the bags. Three freezing programmes were used, namely A: from + 5° C, at a rate of 3° C/min, to −6° C, held for 1 min at –6° C, and followed by a cooling rate of 20° C/min to −100° C; B: a similar curve except that there was no holding time at −6° C and that the cooling rate was 30° C/min, and C: from +5°C to −100° C, with a cooling rate of 35° C/min, followed by storage in liquid N2. Despite the treezing curve assayed, both the mini-straws and the bags depicted much shorter freezing point plateaus as compared to the maxi-straws. Post-thaw sperm motility as well as the amount of normal apical ridges were equally significantly higher when semen was frozen in mini-straws or in bags than in maxi-straws. Significant differences in these post-thawing parameters were obtained between the freezing curves used. The stepwise freezing procedure A appeared as the best alternative for boar semen, considering this in vitro evaluation.