HEAT TOLERANCE OF ZEBU ANIMALS AND ITS CROSSES IN FATTENING GRAZING SYSTEMS, IN THE HUMID TROPICAL FOREST. GUALACA, PANAMA. 1999-2004.

Abstract

The present study was conducted at the Agricultural Experimental Station of Gualaca (IDIAP, 1999 a 2004) located in the Lowland Gualaca Ecosystem (humid tropical forest), with the purpose of determining heat tolerance (TAC) of five racial groups used in grazing systems. The racial groups (GR) were: Criollo Guaymi (CRG); Brahman (BRH); 1/2 Simmental (MSM); 1/2 Charolais (MCH) y >1/2 Holstein (MHL). Aspects of interest were: environment: environmental temperature, average relative humidity and Temperature-Humidity Index (THI); information of animal: rectal temperature (TR), breathing frequency (FR), cardiac frequency (FC), long of the hair (LP), surface body temperature (T_s), emission temperature (T_5cm), skin thickness (GP) and body surface (A). Termorregulation traits were calculated: conduction (Q_k), conviction (Q_c), radiation (Q_r) and emission (Q_e). Data were analyzed by a fixed lineal model, cross classified with unequal observation per subclass and with a covariance (THI). Independent variables were: season of the year (EA, EBP = season of low precipitation vs EAP = season of high precipitation), racial group (GR) and its interaction. Simple and polynomial regression techniques from the origin were applied. THI was significant (P<0.05) for FC and TR. Difference between EA was significant (P<0.01) on FC, FR, TR and LP, while GR was significant for FR and TR (<0.05), and LP (P<0.01). EA and GR did not interact (P>0.05). MHL had the highest FC (78.97 bits/min), TR (39.93°C) and LP (10.01 mm). Low values of FC, FR, TR and LP were found on EBP (P<0.05) for all GR. THI was significant for Q_k and Q_c (P<0.05) and Q_r (P<0.01), but not for Q_e (P>0.05). Difference between EA was not significant (P>0.05) on Q_k, but was on Q_e (P<0.05) and Q_c and Q_r (P<0.01). Differences among GR were P<0.01 on all termorregulation variables, but EA and GR did not interact (P>0.05). BRH uses more Q_k, while MCH uses more Q_c. MSM utilized more Q_r, however, CRG utilized more Q_r and MHL, the Q_k. All GR showed differences on Q_e (P<0.05). FC, FR, TR, LP, Q_k and Q_e were adjusted to lineal functions (P<0.01) on THI, but Q_k and Q_r were adjusted to quadratic functions (P<0.01). Correlation of TR with FC and Q_k were medium associated (P<0.05), but with FR, LP and Q_c were low (P<0.05). FR was associated only with Q_r and Q_c. It was concluded that exists important differences in TAC among GR and that was influenced by EA. Future programs of crossbreeding should base on heat tolerant breeds and selection on those outstanding animals in this quality.