Effects of month, order and lactation stage on the thyroid hormones of heifers and cows of the Holstein breed

Abstract

The aim of the present research work was to investigate the effects of thyroid hormones, thyroxine (T4) and 3,5,3’ triiodothyronine (T3), on the mechanisms of lactation ando f thermal regulation and anergy balance, in regard to the month, the order and lactation stage from 18 heifers and 30 cows of the Holstein breed. Blood samples were withdrawed monthly for the hormones assay by the method of immunoenzymatic fluorometry. The month of blood sampling displayed na influence on the T3 levels, disclosing the higher levels in July and the lowest ones in March. Results on seric concentration of T4 showed significant effect on the lactation order (P<0.05) with lower values in cows from 2nd and 3rd lactation and higher values (P<0.005) in heifers, with no apparent effect in regard to the T3 concentrations. Heifers and dry cows T4 serum levels were the highest in January and the lowest in May, while cows in the middle and final third lactation displayed the heigher serum levels of T4 in September and the lower in November and May, respectively. The lactation stage had no influence on the T3 serum levels