A systematic review and meta-analysis of the effects of inclusion of microalgae in dairy cows' diets on nutrient digestibility, fermentation parameters, blood metabolites, milk production, and fatty acid profiles

Recently, microalgae have been used as protein supplements to improve the productivity of dairy cows. However, the results are inconsistent among different studies. Thus, the aim of this study was to assess the effects of dietary microalgae incorporation on animal performance. The effect of microalgae was assessed by examining the raw mean differences (RMDs) between the treatment (with microalgae) and control (without microalgae) diets using a random-effect model. Heterogeneity was evaluated through meta-regression and subgroup analyses using microalgae species, inclusion level, days in milk, experimental duration, and cow breed as covariates. Microalgae supplementation decreased the intake of dry matter (DM), organic matter, and neutral detergent fiber (NDF). NDF digestibility improved, whereas the acetate:propionate ratio decreased. Milk and lactose yields remained unchanged. Despite a decrease in milk fat, the fatty acid (FA) profile improved, especially considering the increase in conjugated linoleic acid (CLA) C18:2 c9t11, docosahexaenoic acid (DHA) C22:6 n-3, and mono- and polyunsaturated FA (MUFA and PUFA) and the decrease in the n-6:n-3 ratio. The main sources of variation in the responses to microalgal inclusion in cow milk production and quality were the animal breed, microalgae species, and their level of incorporation. In general, the incorporation of 61–100 g kg DM −1 of microalgae improved milk beneficial FA, including eicosapentaenoic acid (EPA) C20:5 n-3 and DHA, and Schizochytrium sp. increased DHA levels. The Holstein and Friesian breeds were characterized by a significant decrease in saturated FA (SFA). As a result, microalgae supplementation could be a sustainable agricultural practice for improving dairy cow milk quality.