Efficacy of conjugated linoleic acid for improving reproduction: a multi-study analysis in early-lactation dairy cows.

The feeding of conjugated linoleic acid (CLA) supplements to early-lactation dairy cows has been shown to decrease milk fat synthesis and possibly improve reproductive performance. However, previously reported studies used too few animals to clearly establish the effect of CLA on reproduction. Our objective was to combine data from these studies to evaluate the association of CLA with time to first ovulation and time to conception using methods of survival analysis and overall success of pregnancy by logistic regression. A database was compiled of individual animal data (n = 212) from 5 controlled studies in which CLA had been supplemented to early-lactation dairy cows. Survival analysis incorporated both semi-parametric models (Cox proportional hazards) and parametric models (log-normal). The probability of cows becoming pregnant increased in a nonlinear manner as trans-10, cis-12 CLA dose increased, with the optimal dose predicted to be 10.1 g/d. At the optimal dose, the probability of pregnancy was increased by 26% compared with those animals receiving no CLA (probability = 91% and 72%, respectively). Similarly, the log-normal model predicted that time to conception was decreased in a nonlinear manner with increasing trans-10, cis-12 CLA dose. The predicted optimal dose was 10.5 g of trans-10, cis-12 CLA/d and at this dose the median time to conception was decreased by 34 d when compared with those cows not receiving CLA (117 vs. 151 d in milk, respectively). The log-normal model was also the best-fit model for time to first ovulation. Overall, this multi-study analysis demonstrated a strong concordance between the nature of the dose response and the predicted optimal dose of trans-10, cis-12 CLA across the 3 reproductive variables evaluated. These results indicate that reproductive performance of dairy cows may be improved by feeding of CLA supplements during early lactation.