Effect of folic acid supplementation on lactation performance of Holstein dairy cows: A meta-analysis

The purpose of this meta-analysis was to evaluate the effect of folic acid (FA) supplement on milk production and composition in Holstein cows, which still remains controversial over published studies. China National Knowledge Infrastructure, PubMed and Science Direct databases were used for searching possible eligible studies. The screening criteria were as follows, participants were Holstein cows, interventions were FA supplement, comparisons contained blank control, outcomes included dry matter intake (DMI), milk yield, milk fat, milk protein and milk lactose contents. Data were analyzed using the Stata 15.1 software, and sensitivity test, meta-regression and subgroup analysis were conducted when heterogeneity existed in outcomes. A total of 14 studies, involving 384 cows, were included in this meta-analysis. The results indicated that, with the supplement of FA, milk yield was distinctly increased by 1.41 kg/d (P = 0.001), milk fat and milk protein were also significantly raised up by 0.73 g/kg (P < 0.001) and 0.84 g/kg (P = 0.004) respectively, whereas DMI and milk lactose had no obvious change (P > 0.05). Heterogeneity was only discovered in milk yield and milk protein, but sensitivity test showed the results were relatively stable. Year, initial cow weight and supplementation method were the three covariates that strongly explained the heterogeneity in milk yield after meta-regression and subgroup analysis, and so as year did in milk protein. In conclusion, FA supplement increased milk yield, milk fat and milk protein contents in Holstein cows, but had no significant effect on DMI or milk lactose content. Besides, we propose a new hypothesis that the effect of FA supplement could be mediated through their effects on ruminal microflora.