HEAT TOLERANCE IN TULI-SIRED, SENEPOL-SIRED, AND BRAHMAN-SIRED F-1 ANGUS HEIFERS IN FLORIDA

We investigated heat tolerance and growth rate in two trials under amb ient conditions in central Florida. Trial 1(1994) involved 38 Brahman (B), 21 Senepol(S), 19 B x Angus (A), 20 S x A, and 20 Tuli (T) x A he ifers. Trial 2 (1995) involved 13 A, 35 B, 30 S, 23 B x A, 17 S x A, a nd 28 T x A heifers. Measurements were made on three consecutive weeks during the hotter and cooler seasons of each year and included rectal temperature (RT, degrees C), respiration rate (RR, bpm), temperament score (TS; 1 = very docile, 5 very aggressive), blood packed-cell volu me (PCV), and plasma cortisol concentration (CORT). Data for RT were t ransformed (log(10) [RT - 37]) before analysis. On the hottest date in Trial 1, log(10) RT was not different between B (.39 +/- .011) and B x A(.37 +/- .016) or between T x A (.35 +/- .015) and B x A, but log(1 0) RT was lower (P < .05) in S x A (.30 +/- .015) than in either S (.3 5 +/- .015) or T x A. On all dates in Trial 1, RR was lower (P < .05 t o .001) and PCV was higher (P < .05 to .001) in B than in B x A. There were few differences in TS except on two dates when B scored higher ( P < .01 to .001) than B x A, and these differences were associated wit h higher (P < .05) CORT in B than in B x A. Using initial BW as a cova riate, adjusted ADG (kg) of T x A (.52 +/- .023) was not different fro m adjusted ADG of B x A (.57 +/- .024) or S x A (.54 +/- .023). On the hottest date in Trial 2, log(10) RT and RR were higher (P +/- .001) i n A (.59 +/- .017, 74 +/- 2.7) than in B (.47 +/- .010, 39 +/- 1.6), S (.42 +/- .011, 50 +/- 1.8), and crossbred heifers (.47 +/- .011, 60 /- 1.8; .43 +/- .014, 55 +/- 2.4; and .50 +/- .012, 48 +/- 2.0 for T x A, S x A and B x A, respectively), and RR was higher (P < .001) in B x A than in B. On the coolest date in Trial 2, RR was slightly lower i n B (32 +/- .5) than in A (34 +/- .7, P < .01) and B x A(36 +/- .6, P < .001) and was associated with higher PCV in B than in A. On both dat es, TS and CORT were higher (P < .01) in B than in A. In Trial 2, adju sted ADG (kg) was higher (P < .01) in B (.43 +/- .017) than in A (.32 +/- .033), higher(P < .001) in S (.46 +/- .019) than in A, and higher (P < .001) in crossbreds (B x A [.53 +/- .023] + S x A [.44 +/- .025] + T x A [.46 +/- .019]) than in A. These data indicate that heat toler ance in F-1 crosses of tropically adapted breeds (Tuli, Senepol, Brahm an) with a temperate breed (Angus) is similar to heat tolerance displa yed by purebred tropical breeds (Senepol, Brahman).