title: Effect of grazing pasture with a low selenium content on the concentrations of triiodothyronine and thyroxine in serum, and GSH-Px activity in erythrocytes in cows in Chile.
abstract: OBJECTIVE
  To determine the effect of grazing pasture that had a low selenium (Se) concentration on serum concentrations of triiodothyronine (T3) and thyroxine (T4), and erythrocyte glutathione peroxidase (GSH-Px) activity in dairy cows.
  
  
  METHODS
  Forty pregnant Friesian cows were grazed on pasture that contained 0.03-0.04 ppm Se on a dry matter (DM) basis. Two months before parturition, 20 cows were randomly selected and treated with 1 mg Se/kg bodyweight subcutaneously, as barium selenate (Group Se-S). The other group (Se-D) was not supplemented. Blood samples were taken before supplementation (-60 days) and 30, 60, 90, 180 and 270 days after parturition, for determination of concentrations of T3 and T4 in serum, and GSH-Px activity in erythrocytes.
  
  
  RESULTS
  Erythrocyte GSH-Px activity in the Se-D group was <60 U/g haemoglobin (Hb) throughout the experiment. Supplementation increased (p<0.05) activities to >130 U/g Hb throughout lactation. Mean serum concentrations of T4 in Se-D and Se-S cows increased from 23.7 (SEM 0.7) and 23.4 (SEM 0.8) nmol/L, respectively, in the prepartum period to 69.6 (SEM 0.1) and 67.6 (SEM 0.2) nmol/L, respectively, at 180 days of lactation (p<0.01), and no effect of Se supplementation was evident. Serum concentrations of T3 in Se-D cows decreased (p<0.05) from 1.6 (SEM 0.1) nmol/L prepartum to 1.0 (SEM 0.2) nmol/L at the beginning of lactation, and remained lower (p<0.05) than those in the Se-S cows which did not decrease after calving and ranged from 1.9 (SEM 0.1) to 2.4 (SEM 0.2) nmol/L throughout lactation.
  
  
  CONCLUSIONS
  Serum T3 concentrations decreased during early lactation in unsupplemented cows grazing pastures low in Se (0.03-0.04 ppm) and both serum T3 and erythrocyte GSHPx activities were consistently lower throughout lactation compared with Se-supplemented cows. Se supplementation had no effect on serum T4 concentrations.
authors.affiliation: Instituto de Ciencias Clinicas Veterinarias, Universidad Austral de Chile, Casilla 567, Valdivia, Chile. pcontre1@uach.cl
authors.forename: P A
authors.initials: PA
authors.lastname: Contreras
authors.forename: F
authors.initials: F
authors.lastname: Wittwer
authors.forename: R
authors.initials: R
authors.lastname: Matamoros
authors.forename: I M
authors.initials: IM
authors.lastname: Mayorga
authors.forename: G
authors.initials: G
authors.lastname: van Schaik
mesh_terms: D000821:Animal Feed; D000822:Animal Husbandry; D000818:Animals; D002417:Cattle; D002677:Chile; D003612:Dairying; D004912:Erythrocytes / Q000187:drug effects* / Q000201:enzymology; D005260:Female; D005979:Glutathione Peroxidase / Q000187:drug effects* / Q000378:metabolism; D011247:Pregnancy; D012643:Selenium / Q000008:administration & dosage / Q000032:analysis / Q000494:pharmacology*; D013974:Thyroxine / Q000097:blood / Q000187:drug effects*; D016896:Treatment Outcome; D014284:Triiodothyronine / Q000097:blood / Q000187:drug effects*
chemical_list: D014284:Triiodothyronine; D005979:Glutathione Peroxidase; D012643:Selenium; D013974:Thyroxine