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

Although on-farm limitation of heat stress through environmental modifications, such as using shade, fans, and mechanical cooling systems, are routinely implemented and have helped to reduce heat stress, the ability of dairy cows to dissipate body heat under chronic heat stress is minimal. Therefore, other interventions are critical under such conditions. This includes elevating the energy density of the diet, improving feed efficiency by stimulating rumen fermentation through direct-fed microbials, and improving the oxidative and physiological status of heat-stressed cows by supplementing with feed additives containing antioxidant properties. A number of nutritional interventions using feed additives such as direct-fed microbials, phytochemicals (also known as plant-derived bioactive compounds), minerals, vitamins, probiotics, and ionophores have shown promising benefits. These additives possess antioxidant, antimicrobial, and anti-inflammatory properties, which may help alleviate or prevent the adverse effects of heat stress in dairy cows by reducing oxidative stress, modulating nutrient metabolism, and enhancing immune function. However, results are variable, and therefore to better understand the discrepancies among published studies, the authors cited conducted a meta-analysis aimed to evaluating how and to what extent different feed additives or supplements mitigate 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, amino acids, and lipids. The data were analysed according to random-effects models using the metaphor package in R software by employing both univariate and multivariate meta-analysis approaches. Analysis of subgroups and co-variables (lactation number, breed, days-in-milk [DIM] or lactation phases, duration of the interventions, and climatic regions) were performed on the main response variables such as milk yield, dry matter intake [DMI], fat-corrected milk [FCM], energy-corrected milk [ECM], and milk yield/DMI.

The overall estimate showed that dietary interventions improved milk yield by 3.41%, DMI by 2.51%, FCM by 4.75%, and ECM by 5.19% in lactating dairy cows exposed to heat stress, but feed efficiency metrics (milk yield/DMI and ECM/DMI) were unaffected. All models exhibited high heterogeneity, indicating substantial variations between 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 their derivatives and phytochemical products were the most tested products, each exhibited higher milk yield (+3.61% and +3.75%, respectively) and DMI (+4.25% and +2.0%, respectively) than untreated cows. Further subgroup analysis revealed divergence effects of yeast products in which live yeast (Saccharomyces cerevisiae) improved milk yield by 5.70% [n = 15], but Aspergillus oryzae per se did not show an effect. Overall, yeast did not affect milk composition but increased crude protein digestibility by 3.70% and total antioxidant capacity by 25.1%, and it decreased MUN by 27.3%, plasma haptoglobin by 7.85%, and respiration rate by 2.85%, respectively. Furthermore, minerals + vitamins supplementation improved milk yield by 4.32%, DMI by 2.52%, and ECM by 8.39%, but did not affect feed efficiency.

Conclusions: Across all dietary interventions and covariates, high heterogeneity 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 recommended as effective tools to mitigate heat stress in dairy cows, even though the effect was not directly on lowering body temperature in most cases. Overall though, 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.