Tolerance to heat and atmospheric humidity of different breeds groups of cattle

Objective. The objective of the present study was to compare some physiological responses to heat and atmospheric humidity among Chinampos (Bos Taurus; Ch), ? Charolais ? Brahman (CB), Holstein (H) and Jersey (J) cattle. Materials and methods. The study was carried out in La Paz, Mexico; 12 H, 10 J, 15 Ch and 8 CB cows were used. During the summer the respiratory frequency (RF) and rectal temperature (RT) were measured each week (at 0600 and 1600 hours). The climatic variables were measured to calculate the temperature-humidity index (THI). Results. Throughout the summer, the RF was above the physiological values of cattle in the four groups, being higher in H and J than in Ch and CB (p<0.01). In all groups, the RT was within the physiological limits of cattle. The most affected group was H (p<0.05). When the physiological constants were analyzed within each of the three categories of THI (<72, ?72<78, and ?78), the H and J cows presented higher RF than that of Ch and CB (p<0.01). The RT increased as the THI rose, being more pronounced in J and H. Increase in the RF between the morning and the afternoon of each day of measurement was higher in H and J than in Ch and CB (p<0.05). The RT increased more in H than in Ch (p<0.05). Conclusions. The breed groups most tolerant to heat stress, based on RF and RT, were Ch and CB.