Meta-analysis of the effects of dietary interventions to alleviate heat stress in lactating dairy cows.

This meta-analysis evaluated the efficacy of various dietary interventions in alleviating the adverse effects of heat stress in lactating dairy cows. A total of 53 studies were included, representing a wide range of geographical locations, lactation stages, and treatment periods. Various feed additives and supplements have been considered, including live yeast (mainly Saccharomyces cerevisiae) and their yeast-derived products, phytochemicals, vitamins, minerals, betaine, AA, and lipids. The data were analyzed according to random-effects models using the metafor package in R software by employing both univariate and multivariate meta-analysis approaches. Analysis of subgroups and covariables (parity, breed, DIM or lactation phases, duration of the interventions, and climatic regions) were performed on the main response variables such as milk yield, DMI, ECM, and milk yield/DMI. The overall estimate showed that dietary interventions improved milk yield (+3.41%), DMI (+2.51%), FCM (+4.75%), and ECM (+5.19%) of lactating dairy cows exposed to heat stress, but feed efficiency metrics (milk yield/DMI and ECM/DMI) were unaffected. All models exhibited high heterogeneity (I2 >90%; Qp-value <0.001), indicating substantial variations from the studies. Notably, the types of feed additives and supplements (treatment group) were significant moderators (P < 0.001) for milk yield, DMI, FCM, and ECM, whereas other factors, such as region and trial length, did not affect the performance metrics. Yeast and derived products and phytochemical products were the most tested products, each exhibited higher lactation milk yield (+3.61% [n = 24] and +3.75% [n = 20], respectively) and DMI (+4.25% [n = 21] and +2.0% [n = 14], respectively) than untreated cows. Further subgroup analysis revealed divergence effects of yeast products in which live yeast (Saccharomyces cerevisiae) and postbiotic improved milk yield by +5.70% [n = 15] and +3.42% [n = 7], respectively but Aspergillus oryzae did not show an effect. Overall, yeast did not affect milk composition but increased CP digestibility (+3.70%) and total antioxidant capacity (+25.1%) but decreased MUN (-27.3%), plasma haptoglobin (-7.85%), and respiration rate (-2.85%). Treatment with minerals + vitamins supplementation improved milk yield (+4.32%; n = 8), DMI (2.52%; n = 8), and ECM (+8.39%; n = 12) but did not affect feed efficiency. Across all dietary interventions and covariates, high heterogeneity (I2 >90%) in most response variables warrants a larger dataset and further investigations to improve the precision and consistency of the results. Nevertheless, the consistent direction of effects across performance and metabolic indicators suggests that dietary strategies, especially with yeast, phytochemicals, and mineral-vitamin supplements, can be used as effective tools to mitigate heat stress in dairy cows, although the effect was not directly on lowering body temperature in most cases. Overall, despite promising results for certain feed additives or supplements, limited studies on some treatment categories require further investigation or product improvement to uncover their potential benefits.