VARIAÇÕES FISIOLÓGICAS FACE À ACLIMATAÇÃO SAZONAL – ESTUDO EM VACAS LEITEIRAS COM DIFERENTE POTENCIAL LEITEIRO

Abstract

This study aimed to evaluate the acclimatisation process of cows with different milk yield potential during summer and winter periods. 13 Holstein Friesian cows were chosen from a dairy farm in Alentejo, 7 of those with high milk yield potential (HMP) and 6 with low milk yield potential (LMP). All cows were evaluated during summer (P1: heat stress) and winter (P2: thermoneutrality) in respiratory frequency (RF), and rectal temperature (RT) as well as milk and blood parameters. RF and RT values were significantly higher in P1 (64.13±12.78 mov./min.; 38.82±0.68 °C) than in P2 (36.13±7.67 mov./min.; 38.06±0.52 °C), without differences between HMP and LMP. Haematocrit and triiodothyronine levels were significantly lower in P1 (23.80±9.39 %; 142.00±13.77 ng/dL) than in P2 (30.70±5.00 %; 170.69±17.78 ng/dL) for both groups. However, in P1, HMP cows presented triiodothyronine blood concentrations significantly lower than the LMP. Milk urea levels, in P1, were significantly higher in the HMP (293.62±35.97 mg/kg) than in LMP (253.69±33.81 mg/kg). These results showed that although HMP cows did not differed significantly in the first responses to heat (RF and RT) from LMP cows, with the acclimatisation process, they showed higher physiological modifications, decreasing the metabolism and changing nitrogen pathways.