Abstract

Pastures in the south of Chile have been shown to be selenium deficient, and as selenium is a component of deiodinase type 1, which is necessary for the synthesis of triiodothyronine (T3) at peripheral levels, the effect of selenium deficiency on the blood concentration of theses hormones in cows was studied. Twelve pregnant Friesian cows were randomly allocated into two homogeneous groups of six animals each-selenium supplemented and selenium deficient. The selenium deficient diet consisted of 11.5 kg of hay, 500 g of soya bran, 150 g of mineral mix (without selenium), 500 g of fat for animal feeding, and was given from day 15 of lactation until the end of the study. A commercial concentrate (Cosetan) and urea was also given according to lactation requirements (up to 5 kg and up to 120 g, respectively). The selenium content was 0.05 ppm of dry matter (equivalent to 18% of the daily requirements). The supplemented group was injected subcutaneously with 1 mg Se/kg, using the commercial product Deposel (1 ml/50kg). Blood samples were obtained by coccygeous vein venepuncture before supplementation (prepartum basal values) and thereafter every 15 days. GSH-Px activity in plasma was measured using a HITACHI 4020 spectrophotometer. Serum T3 and thyroxine (T4) concentrations were measured by electrochemiluminiscence. ANOVA, Tuckey test and Student's t-test were used to establish the significance of the intra group differences, with p = < 0.05 being considered significant. There were no significant differences in blood values of T4 in either the supplemented or non-supplemented groups during pregnancy and lactation. However the blood values of T4 decreased significantly (p = < 0.05) during days 30-60 of lactation to values below the reference range. The T3 serum concentrations in the supplemented group of cows were significantly lower (p = < 0.05) at 60, 90 and 150 days of lactation than cows that had been supplemented with selenium.