Effect of using liquid semen on fertility in German Holstein Friesian dairy cattle: A randomized controlled clinical trial.

Low fertility rates in lactating dairy cows as well as restricted availability of semen doses of young bulls with high genetic merit are two major problems in the reproduction of dairy cows. By using liquid semen (LS), the number of doses per ejaculate can be increased. One of the challenges of optimizing the reproductive performance of dairy cows is the phenomenon of variable estrus lengths. The objective of this study was to determine whether the use of LS affects pregnancy outcome of dairy cows with delayed ovulation, when compared with frozen semen (FS). A randomized controlled clinical trial was implemented. In a split-sample procedure, 131 ejaculates were processed into LS (Caprogen, LIC, New Zealand) and FS (BioXcell, IMV, France). Both semen types of each ejaculate were inseminated under the same field conditions to cows showing natural or induced heat. Cows and semen type were allocated according to the last digit of the cows identification number (even = frozen semen, odd = liquid semen). Inseminations (n = 667) were conducted after localization of the pre-ovulatory follicle. Determination of ovulation was performed 24 h post AI per transrectal ultrasonographic examination. Ovulations were classified as delayed when the pre-ovulatory follicle was still present at ovulation control. The prevalence of delayed ovulations was 25.2%. Data of 667 inseminations were analyzed with a generalized linear mixed model including semen type (P = 0.016), parity (P = 0.014), backfat thickness (P = 0.006), estrus induction (P = 0.010), ovulation (P = 0.265) and the interaction term 'semen type by ovulation' (P = 0.094). Overall, a higher pregnancy per AI (P/AI) of LS (45.4%) than P/AI of FS (33.7%) was found. In cases of delayed ovulations, use of LS resulted in higher P/AI (46.8%) compared with FS (27.7%; P = 0.017). We concluded that the fertilizing capacity of LS in prolonged intervals from AI to ovulation might be greater when compared with FS and could be an efficient tool to improve fertility of lactating dairy cows with delayed ovulations.