Adaptabilidade ao calor de vacas da raça Girolando, em duas estações do ano

Abstract

Two experiments were conducted in the city of Paudalho,Zona da Mata of Pernambuco, in the summer and the winter season in order to evaluate the physiological thermoregulation of dairy cows from three genetic groups Girolando: 1/2, 5/8 and 3/4 Holstein / Gir (HG) and compare genetic groups which have greater adaptability to heat. The animals were distributed in a completely randomized split plot with five repetitions (genetic group x days of data collection). In each experiment used 15 animals with five cows from each breed group. Physiological parameters, sweating rate (SR), respiratory rate (RR), skin temperature (ST) and hair coat surface temperature (HST) were evaluated once a week in the afternoon during 09 weeks in the summer and 10 weeks during the winter. The estimation of critical levels of thermal comfort index was based on rectal temperature (RT) and respiratory rate (RR) obtained from animals during the winter, twice a week, at morning and afternoon. In summer, the group of animals was kept in confinement, while in the winter were subjected to grazing on pasture grass tifton. The atmosphere was monitored continuously, in both periods, using automatic weather station data and environmental variables recorded were used to determine the level of comfort. There were significant correlations between environmental variables and indices of comfort with the rate of sweating and other physiological parameters in both experiments. During the summer, animals 1/2 HG showed higher SR and lowest RR, ST and HST of the three genetic groups, while 3/4 HG cows were found the lowest grades of SR and higher ST and HST. For 5/8 HG cows fell in the intermediate to the values of SR, ST and HST. The mean respiratory rate 5/8 and 3/4 HG did not differ. In the winter period, the 1/2 HG dairy cows showed the higher SR and the lower RR, while in 3/4 HG cows the RR was the highest and the lowest SR of the three genetic groups. For 5/8 cows the SR and RR were intermediate. The ST was lower for 1/2 HG cows did not differ between 5/8 and 3/4 HG animals. The HST did not differ among the three genetic groups. Based on RT the estimated upper critical levels of WBGT for 1/2, 5/8 and 3/4 HG dairy cows were 84.7, 83.5 and 83.1, respectively. Based on the RR was estimated critical value of WBGT of 86 for animals 3/4 HG and THI (Temperature-humidity Index) equal to 78.1 and 78.3 for the genetic group 5/8 and 3/4 HG, respectively. It was concluded that 1/2 HG dairy cows display great heat tolerance than 5/8 and 3/4 HG cows.