Meta-analysis of the effects of dietary sources of selenium on lactational performance and oxidative status of dairy cows

Selenium is an essential trace element to maintain normal physiological and immunological systems. The role of various types and forms of Se on the productive performance and antioxidant status of dairy cows is overwhelming in the existing empirical evidence and, therefore, needs to be quantitatively summarized. This meta-analysis evaluated the effects of different types and forms of Se intake on the productive and oxidative status of various physiological stages of dairy cows. A database was built from 29 peer-reviewed publications that reported lactation performance, Se concentrations in the blood or milk, or antioxidant parameters of dairy cows. A total of 139 observations were included in the analysis according to mixed model univariate and multivariate regression analyses by evaluating the Se levels (%), intake (mg/d), Se forms (organic vs inorganic), Se types (Se-selenite, Se-yeast, Se-methionine) and other categorical predictors in the models. Then, the models were evaluated and cross-validated to select the best-fitted models on some important parameters. Se intake, in any form and type, had no relationship with milk yield but was positively associated with fat-corrected milk (FCM) yield due to a trend of linear increase in milk fat yield (g/d). A significant interaction of Se intake × lactation phase was observed. Daily Se intake positively affected the dry matter intake (DMI) of periparturient cows but did not affect the DMI of lactation cows. Increasing Se intake resulted in a linear increase of DMI and moderately increased glutathione peroxidase (GSH-Px) during the periparturient period but had no significant effect during early and mid-lactation of dairy cows. The interaction model of Se intake × Se forms was the best predictor to evaluate Se concentration in blood and milk. The organic Se had a higher transfer rate from diets to blood without a statistical difference between Se-yeast and Se-methionine. In milk, intake of Se-methionine demonstrated a strong linear increase in Se concentration and had a 47% higher slope of transfer rate than Se-yeast (2.47 µg/kg and R2 = 0.91 vs 1.68 µg/kg and R2 = 0.48, respectively). The amount of daily Se intake also curvilinearly affected superoxide dismutase activity and malondialdehyde concentration. Altogether, the predicted models and results of the present meta-analysis provide empirical evidence that organic Se, especially Se-methionine, had a higher transfer rate from diets into the milk than organic Se-yeast. Organic Se might also improve milk protein and fat-corrected milk in lactating cows and antioxidant status in the periparturient period of dairy cows.