Resumo

HERNANDEZ, A et al. RESPONSE TO STRESS BY HEAT IN THE TROPICAL MILKING CRIOLLO COW UNDER A DOUBLE PURPOSE SYSTEM IN MEXICO. Rev Salud Anim. [online]. 2007, vol.29, n.2, pp. 85-90. ISSN 0253-570X.

This research study presents the results of evaluating the response to stress by heat in a flock of one hundred tropical milking criollo cows (TMC), maintained under a system of double intention for more than forty years in the Mexican tropic. In addition, the behavior of the indicators: climate; ambient temperature (AT ºC) and relative humidity (RH %) were analyzed in the region under study during both years of experience and the previous forty years. The rectal temperature (RT ºC) and the respiratory frequency (RF) rpm were considered as physiological responses to stress. The climate of the region showed a great stability in the years of study and the previous forty as well as clear seasonality between the dry - rainy seasons, with a temperature and humidity index (THI) different among times and morning and evening readings (p< 0,001). Changes in the response variables to stress by heat of the (RF) of the cows allowed, by means the CLUSTER test, to establish three different types of response from stress by heat; 36 cows without stress, (> 36 rpm), 29 with soft and moderate stress (46 -60 rpm) and 35 with high stress (61 - 90 rpm). These data demonstrate that the cows belonging to the breed (TMC) are not a homogenous group against stress by heat, and allow to establish criteria for future selection programs.

Palavras-chave : thermoregulation; THI; double purpose bovine; criollo cattle.

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