Fertility results after different thawing procedures for ram semen frozen in minitubes and mini straws

The effect of different thawing procedures for ram semen frozen in minitubes and mini straws on the fertility of sheep was tested in a field trial in which 727 Norwegian crossbred ewes, aged between six months and five-and-a-half years from nine farms, were inseminated with frozen-thawed semen in natural estrous. Minitubes were thawed at 70 degrees C for 8 s (T70) and mini straws either at 70 degrees C for 5 s (S70), 50 degrees C for 9 s (S50), or 35 degrees C for 12 s (S35). Cervical insemination with 200 x 10(6) spermatozoa resulted in 25-day non-return rates of 78.7, 69.0, 73.6, and 72.9% (overall 73.6%), respectively, and lambing rates of 77.6, 66.1, 71.4, and 68.9% (overall 71.0%), respectively. There was a significantly higher lambing rate for T70 compared to S35 (P=0.03) and S70 (P=0.02), respectively, but not compared to S50 (P=0.29). Age of the ewes (P=0.02), farmers (P=0.02) and the interaction between farmer x straw type/thawing temperature (P=0.01) had a significant effect on the lambing rate. In conclusion, the superior fertility results achieved for minitubes compared to mini straws have to be carefully evaluated in relation to the possible application of a more rational semen production and simplified semen handling at AI, when using mini straws thawed at 35 degrees C.