HEAT TOLERANCE IN 2 TROPICALLY ADAPTED BOS-TAURUS BREEDS, SENEPOL ANDROMOSINUANO, COMPARED WITH BRAHMAN, ANGUS, AND HEREFORD CATTLE IN FLORIDA

Two trials were conducted with heifers to determine heat tolerance amo ng temperate Bos taurus (Angus, Hereford), Bos indicus (Brahman), trop ical Bos taurus (Senepol, Romosinuano), and the reciprocal crosses of Hereford and Senepol. Differences among breeds in temperament score, c irculating concentrations of cortisol, and blood packed cell volume we re also investigated. Trial 1 used 43 Angus, 28 Brahman, 12 Hereford, 23 Romosinuano, 16 Senepol, 5 Hereford x Senepol (H x S), and 5 Senepo l x Hereford (S x H) heifers. Trial 2 used 36 Angus, 31 Brahman, 9 Her eford, 14 Senepol, 19 H x S, and 10 S x H heifers. On the hottest summ er date in Trial 1, rectal temperature of Angus was greater (P < .001) than that of Brahman, Senepol, or Romosinuano. Rectal temperature and plasma cortisol were significantly less in Senepol than in Brahman, s uggesting that the differences in rectal temperature between these bre eds may be due to differences in stress response possibly related to d ifferences in temperament. Reciprocal crosses of Hereford and Senepol had rectal temperatures nearly as low as that of Senepol and displayed substantial heterosis (-9.4%, P < .05) in log(10) rectal temperature on the hottest summer date. On both the hottest and coolest dates in T rial 1, Angus heifers had significantly faster respiration rates than Brahman, Romosinuano, or Senepol heifers, and Brahman had significantl y slower respiration rates than Romosinuano or Senepol. On the hottest summer date in Trial 2, rectal temperature in Angus heifers was great er (P < .001) than in Brahman or Senepol heifers. Reciprocal crosses o f Hereford and Senepol had rectal temperatures similar to that of Sene pol, and heterosis for log(10) rectal temperature was similar to that in Trial 1 (-9.8%, P < .05). Considering rank order among breeds, Brah man always had the slowest respiration rate and greatest packed cell v olume. Brahman had significantly greater temperament scores and plasma cortisol concentrations than Angus or Senepol, except that plasma cor tisol was not different between Brahman and Senepol on the hottest sum mer date. On this date, rectal temperature did not differ between Brah man and Senepol, which supports the hypothesis that there is a relatio nship between response to stress and rectal temperature that helps exp lain differences in rectal temperature between Brahman and Senepol. Th e results of these trials demonstrate heat tolerance of the Senepol an d Romosinuano, two Bos taurus breeds. Furthermore, the results suggest a substantial level of dominance of the Senepol's ability to maintain constant body temperature in a hot environment as measured by rectal temperature in crosses with a nonadapted breed.