# A Meta-Analysis of the Association Between Live Yeast Supplementation and Lactation Performance in Dairy Cows Under Heat Stress

## TL;DR

This study finds that adding live yeast to the diets of heat-stressed dairy cows can improve their milk production and feed intake.

## Contribution

The study provides a meta-analysis showing that live yeast supplementation improves lactation performance in heat-stressed dairy cows.

## Key findings

- Live yeast supplementation increased dry matter intake and milk yield in heat-stressed dairy cows.
- Milk fat, protein, and lactose yields increased significantly, though percentages remained largely unchanged.
- The results suggest live yeast is a practical nutritional strategy to mitigate heat stress effects in dairy cows.

## Abstract

Heat stress is a major challenge in dairy farming because it reduces feed intake, lowers milk yield, and can change milk composition, leading to economic losses and lower productivity. A practical nutritional strategy to support dairy cows during heat stress is live-yeast supplementation. The aim of this study was to evaluate whether live-yeast supplementation improves the performance of dairy cows exposed to heat stress. To address this question, we gathered results from previously published studies and analyzed them using a meta-analysis, a method that pools findings from multiple studies to estimate the overall effect. The results showed that supplementing the diet with live yeast in heat-stressed dairy cows increased dry matter intake and increased milk yield. In addition, the total yields of milk fat, protein, and lactose were higher. Overall, these findings suggest that live-yeast supplementation may be a practical nutritional option to help dairy cows maintain milk yield during heat stress.

Heat stress is a major management challenge in dairy cows, leading to lower dry matter intake (DMI), reduced milk yield (MY), and changes in milk composition. A practical nutritional strategy is supplementing diets with live yeast (Saccharomyces cerevisiae) during hot periods. This meta-analysis evaluated the effects of live-yeast supplementation on DMI, MY, and milk components in heat-stressed dairy cows. Bibliographic searches for the period 2000 to 2025 were conducted. Based on the inclusion and exclusion criteria, 14 studies were selected. For each outcome, effect sizes were reported as the standardized mean difference (SMD) and the raw mean difference. Analyses were conducted using a random-effects model, and heterogeneity was assessed using the Q test and the I2 statistic. Based on the SMD, live-yeast supplementation of the diets of heat-stressed dairy cows increased both DMI (p = 0.003) and MY (p < 0.001). Milk fat percentage (MFP; p = 0.104) and milk lactose percentage (MLP; p = 0.530) were not significantly affected, whereas milk protein percentage (MPP) tended to increase (p = 0.078). In contrast, milk fat yield, milk protein yield (MPY), and milk lactose yield (MLY) all increased significantly (p ≤ 0.001). Heterogeneity was significant only for DMI, MPY, and MLY. Overall, based on the available evidence, supplementing heat-stressed dairy cows with live yeast can improve DMI and MY, and increase milk component yields, without materially altering component percentages.