Selenium supplementation in cattle: Transfer to milk and effect on udder health

Soils in many regions of the world have low selenium (Se) content due to an uneven distribution of Se in the earth's crust. Selenium deficiency has been associated with a wide range of costly disorders in cattle, such as retained placenta, lowered milk production, reduced growth rate, altered immune responses and reduced disease resistance. Mastitis is likely to provide the most convincing evidence for the influence of Se on disease resistance. The aim of this thesis was to evaluate the effect of Se status and Se supplementation on milk Se concentration, and on indicators of udder health. A systematic review and meta-analysis of the effect of oral Se supplementation on milk Se concentration in cattle indicated that milk Se concentration increased, on average, 0.16 μmol/L, with a significant heterogeneity among studies. North American cows supplemented with Se yeast had higher milk Se concentration when measured approximately 75 d after the beginning of supplementation. This information provided the basis for tailoring daily animal requirements in order to enhance Se intake of consumers of dairy products. Mean cow-level milk Se concentration was considered marginal in 14% of dairy cows from Atlantic Canadian dairy herds. The risk of having a marginal Se status was highest when cows were pastured during the grazing season. Selenium status was not associated with the overall odds of new intramammary infections (IMI) in the dry period. However, new IMI caused by Streptococcus spp. and by other Gram-positive pathogens increased with higher milk Se concentration. Bulk tank milk Se from Canadian herds was not associated with milk production parameters or bulk tank somatic cell count (BTSCC). However, higher values of BTSe were associated with lower risk of being a Staph. aureus-positive herd. In pastured herds in Chile, whole blood activity of glutathione peroxidase (GPx) increased after Se supplementation beginning 45 days before calving, no matter what source of Se was used. Supplementation did not affect udder health in the subsequent lactation in multiparous cows, indicating that dietary basal Se intake (approximately 20% of the current recommendation) was adequate for preventing subclinical mastitis in pasture-based cows in Southern Chile. The effect of prepartum supplementation with a single injection of barium selenate or organic Se on the risk of IMI and SCC around calving in pastured first-lactation dairy heifers was studied in Chilean dairy herds. Supplementation was accompanied by a reduction in the prevalence of IMI and SCC at calving, but did not affect the risk of new IMI and SCC in milk during the balance of the first month of lactation. In conclusion, milk Se concentration does not appear to be a principal determinant of udder health in Atlantic Canadian dairy herds, or milk production parameters in Canadian dairy herds. However, higher values of BTSe may be associated with lower risk of being a Staph. aureus-positive herd. In cows raised in pastoral systems in Southern Chile, an intake of approximately 1 mg Se/cow/d was adequate for preventing subclinical mastitis during lactation. However, pre-calving Se supplementation appears beneficial for preventing subclinical mastitis shortly after calving in heifers grazing low Se pastures.