Exploratory Meta-Analysis of the Effect of Malic Acid or Malate Addition on Ruminal Parameters, Nutrient Digestibility, and Blood Characteristics of Cattle

The aim of this study was to determine, through meta-analysis, the effects of malic acid/malate addition on ruminal and blood parameters and diet digestibility in cattle. The literature search was conducted in Web of Science, Science Direct, and Google Scholar using the terms "organic acids", "malic acid", "malate", and "bovine". The database was composed of papers published between 1980 and 2023. The average effect of malate/malic acid inclusion was calculated using the "DerSimonian and Laird" random effects model. Meta-regression and subgroup analyses were conducted to explore sources of heterogeneity. Overall, malic acid (MAC) addition did not significantly affect rumen pH (ES = 0.310, <i>p</i> = 0.17), but subgroup analysis showed that malate increased pH (ES = 1.420, <i>p</i> < 0.01). MAC increased rumen propionate (ES = 0.560, <i>p</i> < 0.01) and total volatile fatty acids (VFAs; ES = 0.508, <i>p</i> = 0.03), while reducing the acetate-to-propionate ratio (<i>p</i> < 0.01). Starch and NDF intake were significant covariates affecting pH and VFA-related variables. MAC improved total-tract digestibility of dry matter (DM; ES = 0.547, <i>p</i> ≤ 0.05), crude protein (CP; ES = 0.422, <i>p</i> ≤ 0.05), and acid detergent fiber (ADF; ES = 0.635, <i>p</i> ≤ 0.05). It increased glucose levels (Overall ES = 0.170, <i>p</i> = 0.05) and reduced NEFA (Overall ES = -0.404, <i>p</i> = 0.03). In conclusion, the effectiveness of MAC depends on its chemical form. Improvements in rumen pH, fiber degradation, and blood parameters suggest more efficient energy use and potential metabolic benefits. The influence of diet-related covariates suggests that the response to MAC may vary depending on the nutritional composition of the diet.