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, p = 0.17), but subgroup analysis showed that malate increased pH (ES = 1.420, p < 0.01). MAC increased rumen propionate (ES = 0.560, p < 0.01) and total volatile fatty acids (VFAs; ES = 0.508, p = 0.03), while reducing the acetate-to-propionate ratio (p < 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, p ≤ 0.05), crude protein (CP; ES = 0.422, p ≤ 0.05), and acid detergent fiber (ADF; ES = 0.635, p ≤ 0.05). It increased glucose levels (Overall ES = 0.170, p = 0.05) and reduced NEFA (Overall ES = −0.404, p = 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.

In this study, we confidently evaluated the effectiveness of using infrared spectroscopy for assessing silage, and observed the results obtained with portable equipment. Several factors alter this ensiled material's composition and quality, and monitoring is necessary to create an accurate diet formulation. Analysis using near-infrared spectroscopy (NIRS) this technique is the wavelength of the near-infrared region and measuring the difference in the amount of energy emitted by the equipment and the energy reflected by the sample to the detector, be measured by different bands create a spectrum of this sample. In the nutrition of ruminants, mainly confined animals, silage is the main source of fiber used in their diet and the NIRS technique has been widely used to evaluate its chemical composition the digestibility, within other parameters. Use with benchtop equipment in the laboratory or with portable equipment offers real-time results on the farm, allows diet variability to be monitored, and assists in decision maki. However, further research into the use of this equipment to assess its accuracy and which type of equipment is best to use is recommended.

The aim of this study was to determine, through meta-analysis, the effects of malic acid/malate supplementation on ruminal and blood parameters and diet digestibility in cattle. The effect of malic acid/malate addition was evaluated using the effect size (ES) method. Two sensitivity analyses were performed: subgroup analysis and meta-regression. The subgroups were defined as follows: “acid,” representing animals supplemented with malic acid, and “salt,” representing animals supplemented with malate. Diet-related covariates were included in the meta-regression analysis. Overall, malic acid (MAC) supplementation did not significantly affect rumen pH (ES = 0.310, P = 0.17), but subgroup analysis showed that malate increased pH (ES = 1.420, P < 0.01). MAC increased rumen propionate (ES = 0.560, P < 0.01) and total volatile fatty acids (VFAs; ES = 0.508, P = 0.03), while reducing the acetate-to-propionate ratio (P < 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, P ≤ 0.05), crude protein (CP; ES = 0.422, P ≤ 0.05), and acid detergent fiber (ADF; ES = 0.635, P ≤ 0.05). It increased glucose levels (ES = 0.170, P = 0.05) and reduced NEFA (ES = -0.404, P = 0.03). In conclusion, MAC effects were influenced by its chemical form, improving rumen pH, CP, NDF, and ADF digestibility. MAC increased VFAs, particularly propionate, lowered the acetate-to-propionate ratio, raised blood glucose, and reduced NEFA. Diet-related covariates were important to explain the between-study heterogeneity.

This meta-analysis and meta-regression evaluated how malic acid supplementation modulates rumen fermentation and its consequences for growth performance, nutrient digestibility, and carcass traits in lambs. Effect sizes (ES) were estimated using a random-effects model. Dietary composition was explored by meta-regression as a key source of heterogeneity, and subgroup analyses were used to compare free malic acid (FMA) and malate. Ruminal pH was not affected by malic acid supplementation. In contrast, total volatile fatty acid concentration increased with malic acid supplementation, particularly in studies using FMA. No effects were detected for propionate concentration, whereas acetate concentration increased (ES = 0.502; p = 0.036). A tendency toward a reduced ruminal acetate proportion was observed (ES = −0.683; p = 0.072). Malic acid supplementation tended (p = 0.057) to increase body weight gain (BWG; ES = 0.325) and final body weight (FBW; ES = 0.234). Malic acid supplementation did not affect carcass traits or overall nutrient digestibility. Meta-regression consistently identified fiber intake-related variables as major moderators of the effects of malic acid. Overall, the effects of malic acid supplementation on lamb performance appear to be primarily driven by its modulation of rumen fermentation and strongly conditioned by dietary context.