Pregnancy outcome after insemination of frozen-thawed bovine semen packaged in two straw sizes: A meta-analysis1

The 0.25-mL French straw has been thoroughly studied and compared with the 0.5-mL French straw for about 40 yr. The 0.25-mL straw has some distinct advantages over the 0.5-mL straw in terms of storage efficiency and extender usage. The 0.25-mL straw is more sensitive to temperature change, which may be an advantage in some freezing systems with a slow freezing rate, but provides no advantage when the freezing rate is accelerated to optimize freezing in 0.5-mL straws. Disadvantages of the 0.25-mL straw include increased sensitivity to post-thaw temperature change, slightly more difficult handling, and inferior readability. Fertility of cattle inseminated with extended semen packaged in 0.25-mL straws assessed in 13 studies (>770,000 inseminations) had a weighted advantage of 0.9% (0.7% unweighted advantage) compared with semen packaged in 0.5-mL straws. In 2 studies in which palpated conception rates were obtained, the weighted advantage of the 0.25-mL versus the 0.5-mL straw was 0.2% (0.4% unweighted advantage). Paired t-tests did not detect a significant difference in pregnancy outcome between straw sizes. Logistic regression of all 15 studies (>780,000 inseminations) detected large variation among studies and tended to detect a small advantage for the 0.25-mL straw. Meta-analyses applied to fixed- or random-effect models of all 15 studies indicated the average odds of having a greater pregnancy outcome with the 0.25-mL straw were either 3 or 4% greater. Based on these odds ratios, the expected proportion of difference in pregnancy outcome translated into a difference of 0.74%. These small differences in pregnancy outcomes do not provide compelling evidence that a transition from 0.50- to 0.25-mL straws in the United States is warranted, especially given the added negative aspects of semen handling and the greater potential for technician × straw type interactions among herdsman inseminators.