Effects of dietary fat on fertility of dairy cattle: A meta-analysis and meta-regression.

Evidence is increasing of positive effects of feeding fats during transition on fertility and the adaptation to lactation. This study used meta-analytic methods to explore the effects of including fats in the transition diet on the risk of pregnancy to service (proportion pregnant) and calving to pregnancy interval. Meta-analysis was used to integrate smaller studies and increase the statistical power over that of any single study and explore new hypotheses. We explored the effect of fats and diet composition on fertility using meta-regression methods. Relatively few highly controlled studies are available providing detailed descriptions of the diets used that examined interactions between fat nutrition and reproductive outcomes. Only 17 studies containing 26 comparisons were suitable for inclusion in statistical evaluations. Reproductive variables evaluated were risk of pregnancy (proportion pregnant), primarily to first service, and calving to pregnancy interval. Production variables examined were milk yield, milk composition, and body weight. The sources of heterogeneity in these studies were also explored. A 27% overall increase in pregnancy to service was observed (relative risk=1.27; 95% confidence interval Knapp Hartung 1.09 to 1.45), and results were relatively consistent (I(2)=19.9%). A strong indication of a reduction in calving to pregnancy interval was also identified, which was consistent across studies (I(2)=0.0%), supporting a conclusion that, overall, the inclusion of fats does improve fertility. Further exploration of the factors contributing to proportion pregnant using bivariate meta-regression identified variables that reflected changes in diet composition or animal response resulting from inclusion of the fat interventions in the experimental diets fed. Increased fermentable neutral detergent fiber and soluble fiber intakes increased the proportion pregnant, whereas increased milk yield of the treatment group decreased this measure. Unexpectedly, the estimated energy costs of urea production also had a positive association with proportion pregnant. The limited number of suitable studies for the analysis highlights the need for more work to improve understanding of the critical nutritional factors affecting fertility. These factors include specific fatty acids in dietary interventions that contribute to increasing fertility of cows in dairy production systems.