Association between yeast product feeding and milk production of lactating dairy cows: Multilevel meta-analysis and meta-regression

To provide an overview of the association between commercially available yeast product (YP) supplementation and milk yield (MY) in lactating dairy cows, multilevel meta-analyses were performed on 99 trials from 49 peer-reviewed studies. Associations were evaluated using random effects models to examine the standardized mean difference (SMD) between the YP and control treatments. Associations were also quantified using the weighted mean difference (WMD). YP supplementation was associated with increased milk yield (+ 0.69 kg/d), milk fat content (+ 0.06%), milk fat yield (+ 0.04 kg/d) and milk protein yield (+ 0.02 kg/d). A positive association between YP and MY was not observed in primiparous cows during mid- and late lactation or in cows fed a high neutral detergent fiber diet. The improvement of MY in cases of YP supplementation was higher if YP supplementation started before calving (+ 0.79 kg/d) compared to after calving. The multivariate analyses showed that YP supplementation was associated with an increase in MY but not with an increase in dry matter intake (DMI). This provides strong evidence that greater milk production observed in supplemented cows is not a result of increased DMI.