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A field study involving 7734 Holstein calves from eight large intensive dairy herds in northern Mexico (26 degrees N; 24.2 degrees C mean annual temperature; same location) was conducted to determine factors associated with early postnatal (one to 21-d of age) dairy calf mortality. The effects of season of birth, size of dairy operation, birth type (single or twin), sex of calves, type of feeding (milk or milk replacer), herd and temperature-humidity index (THI) shortly before, during or shortly after calving were analyzed by the GENMOD procedure of SAS. The total mortality rate was 14% (range among dairy operations 7.0-29.1%). Mortality of calves was associated with the THI during birth, with a decreased (P < 0.05) mortality rate from 16 to 12% with THI above 81 units. Mortality rates were 4 percentage point higher (P < 0.05) in winter than all other seasons. Single-born calves had fewer deaths (14%) than twin-born calves (19%). The mortality rate was higher (P < 0.05) in male than in female calves (17% vs. 12%). Calves that were born in dairy operations > 1800 cows were more likely to die than calves born in smaller dairies. Both respiration rate (74.4 +/- 15.6 vs. 67.8 +/- 16.3) and heart rate (119.2 +/- 19.6 vs. 113.9 +/- 17.0) were higher (P < 0.05) in calves born in winter than in summer. Rectal temperature was not altered by high ambient temperature and panting was not observed in calves during days with high ambient temperature. It was concluded that in this particular zone characterized by an intense heat load for most of the year, calf mortality was not linked to high ambient temperature around calving, rather, winter weather negatively affected survival of calves. Thus, efforts to protect calves from cold weather would likely alleviate calf losses. (C) 2013 Elsevier B.V. All rights reserved.