MÊS DE COLETA DE SANGUE, ORDEM E ESTÁDIO DE LACTAÇÃO E HORMÔNIOS TIREOIDIANOS EM VACAS GUZERÁ

Abstract

Thyroid hormones are important metabolic modulators, influence the growth and development of the mammary gland and participate in the regulation of milk production. The objective was to investigate the effects of month of blood collection, parity and stage of lactation on thyroxine (T4) and 3, 5, 3' triiodothyronine (T3) concentration in 31 Guzerat cows, in Uberlândia, MG, Brazil. The hormonal dosages were made from blood samples collected monthly, during eleven months. Cows in first third lactation had T4 concentration lower than the non-lactating and cows in middle and final thirds of lactation. The month of blood collection influenced the T4 concentration; the highest concentration was detected in April and lowest in July. In animals at the 1st to 6th parity, T3 concentration did not vary according to the stage of lactation, while at the 7th and 8th parity, the T3 concentration were higher at final third of lactation, and lower at initial third of lactation and in non-lactating cows. Nonlactating cows and cows at the initial and middle thirds of lactation presented the highest T3 concentration in March. Non-lactating cows and cows at the initial third of lactation had lower T3 concentration in January and April to December. Cows in middle thirds lactation had lower T3 values in June, October and December. The lowest concentration of T4 occurs in Guzerat cows, in the first third of lactation, which reflects the great demand of the mammary gland. The order of lactation did not influence the values of T4 and T3. The month of collection influence the values of T4 and T3 for the month of collection is dependent on the stage of lactation