Mecanismos homeostáticos e homeorréticos decorrentes da aclimatização sazonal em bovinos leiteiros de diferente capacidade produtiva

Abstract

The aim of this study was to compare the physiological responses of dairy cows, Holstein-Frisian breed, with different milk yield potential regarding the seasonal acclimatisation process. Using milk production values adjusted to 305 days in milk, 6 and 7 multiparous cows with a milk yield lower than 9000 kg (BP) and equal or higher that 9000 kg (AP), respectively, were selected. All cows were evaluated during 5 days in summer (P1: heat stress) and winter (P2: thermoneutrality). Black globe temperature (BGT) at sun and shade, respiratory frequency (RF), sweat rate (SR) and rectal temperature (RT) were measured daily. Milk yield was recorded daily, and samples were taken for milk chemistry and somatic cell count determination. In each period a blood sample was taken from every cow for hematologic analysis and triiodothyronine (T3) determination. Period affected BGT, RF, SR, RT, milk composition, haematological parameters, and metabolism (T3). AP had increased milk urea levels and decreased T3 levels in P1 when compared to BP. These results showed that although AP and BP cows had similar first responses to heat (RF and RT), with the acclimatisation process, AP showed higher physiological modifications, decreasing the metabolism, and changing nitrogen pathways.