ALPHA-TOCOPHEROL, SELENIUM AND POLYUNSATURATED FATTY-ACID CONCENTRATIONS IN THE SERUM AND FEED OF SPRING-CALVING DAIRY HEIFERS

The objectives of this study were to provide baseline data for alpha-t ocopherol, selenium and polyunsaturated fatty acid concentrations in t he serum and feed of New Zealand dairy cattle, and to assess the likel ihood that abnormal peroxide metabolism has a role in the impaired lac tational and reproductive performance noted in selenium-deficient catt le. Twenty-four Friesian heifers were randomly allocated one of four w inter diets consisting of hay with or without selenium supplementation , or pasture and silage with or without selenium supplementation. A wi nter diet consisting exclusively of hay (alpha-tocopherol concentratio n 19 mg/kg of dry matter) resulted in a pre-calving serum alpha-tocoph erol concentration of 1.2 mg/l compared to 4.5 mg/l for pastured heife rs (p<0.01). The pre-calving alpha-tocopherol concentration for the he ifers fed hay fell into the range considered deficient (<2.0 mg/l), wh ereas heifers fed pasture and silage remained in the range considered adequate throughout the study period. Serum fatty acid concentration, and the proportion of fatty acids that were polyunsaturated, were lowe st in the hay-fed heifers before calving (1.0 mg/ml, 37.1% respectivel y), and remained unchanged following re-introduction to pasture after calving in late July and August. Serum fatty acid concentration did no t increase following the re-introduction of the heifers to pasture bec ause of the unexpectedly low fatty acid concentration (4.8 g/kg of dry matter) of the mature winter pasture. In October, however, the propor tion of fatty acids in serum that were polyunsaturated increased (50%) , as did serum alpha-tocopherol concentrations (greater than 13 mg/l). Mean serum selenium concentrations in the unsupplemented heifers rang ed from 139 to 204 nmol/l, being lowest in October (p<0.01). Supplemen tation with intraruminal selenium pellets (two pellets delivering abou t 3 mg of selenium/day) increased serum selenium concentration and glu tathione peroxidase activity (p<0.01) whereas the type of winter diet had no effect (p>0.05). These results suggest that dairy cattle winter ed on hay can become Vitamin E-depleted, whereas the feeding of pastur e and silage should provide adequate Vitamin E. The pasture offered fo llowing calving during July and August provided a low dietary polyunsa turated fatty acid challenge, suggesting that abnormal peroxide metabo lism is unlikely to be an important mechanism in the impaired performa nce of selenium-deficient adult cattle which calve at this time of yea r.