Efeito do polimorfismo do gene da beta-caseína na adaptabilidade de vacas sindi.

Abstract

The objective of this study was to evaluate the effect of betacasein genotypes submitted to different thermal comfort conditions on the adaptation and physiological responses of Sindi cows and to use them as a selection tool for thermotolerant animals. Twelve female Sindi cattle were subdivided into two groups with different genotypes for the production of betacasein (A1A2 and A2A2), conditioned in a semi-arid climate and arranged in three conditions of thermal comfort: before stress, shortly after the stress provoked by direct solar radiation and one hour after stress. The measured climatic variables were the ambient temperature (TA), dew point temperature (TPO), relative humidity (UR) and black globe temperature (TGN) in the shade (TGN SOM) and in the sun (TGN SOL). It was calculated the temperature of the black globe and humidity in the shade (ITGU SOM) and in the sun (ITGU SOL) and thermal load of radiant in the shade (CTR SOM) and in the sun (CTR SOL). The respiratory rate (FR), surface temperature (TS) and rectal temperature (TR) were evaluated. Heat tolerance index (ITC) and heat tolerance coefficient (CTC) were calculated for adaptability assessment. For the analysis of the physiological responses and the CTC, a completely randomized design (DIC) was used, with 6 replicates in a 2x3 factorial scheme, with two genotypes and three conditions of thermal comfort. For the ITC analysis, DIC was used with 6 replicates. The averages of TA were above the thermal comfort zone for cattle and presented averages of 35.29, 36.75 and 36.70 °C in the three conditions of thermal comfort, respectively. The UR decreased, whereas the TA increased with mean values of 37.70, 28.12 and 28.95%. The TGN SOM values (37.02, 37.90 and 37.44 °C) were lower than those of TGN SOL (50.35, 49.99 and 47.89 °C) for three thermal comfort conditions and TPO (17.18, 15.54 and 15.96 °C) remained stable under all conditions of thermal comfort. ITGU SOM presented values of 84.74, 84.99 and 84.68 whereas ITGU SOL values were higher (98.03, 97.09 and 95.13). The CTR SOM (602.96, 716.81 and 545.88 Wm-2) were lower than the values of CTR SOL (1002.51, 935.77 and 862.62 Wm-2), where the highest solar radiation found in the condition of thermal comfort before the stress provoked by the heat (13h). Physiological responses and thermotolerance tests did not show a significant effect (p>0.05) that differed between A1A2 and A2A2. However, with regard to the different thermal comfort conditions, there was an effect (p<0.05) for the parameters of TR, TS and thermal gradients, except the FR that did not show any significant alteration (p>0.05). The ITC and CTC had no significant effect (p>0.05) that differed from one genotype to the other. However, the results were potentially promising, where the A1A2 and A2A2 genotypes obtained CTC averages of 2.03 and 2.07 respectively and for ITC 9,78 and 9,69, demonstrating the adaptability and thermotolerance of the Sindhi race. Under the conditions of the present study it was verified that the different genotypes can not be used as a selection tool for thermotolerance.