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Improving Tropical Beef Cattle by Simultaneous Selection for Weight and Heat Tolerance. Heritabilities and Correlations of the Traits¹

Department of Genetics, Faculdade de Medicina, Universidade de Sao Paulo, 14100 Ribeirão Prêto, São Paulo, Brazil

Abstract

The possibility of selection of beef cattle simultaneously for weight gain and heat tolerance traits is examined. A group of 192 bullocks and heifers were exposed for about 7 hr. to direct sun in a pen. Temperature and respiratory rate were taken before and after heat treatment, the haemoglobin level being recorded also, together with average daily weight gain from weaning to 18 months of age. The data were analysed by least squares methods, considering as effects on the model: sex, haemoglobin type, coat color, day of observation, age, and sires. The results include heritability estimates for the initial rectal temperature (.11±.16), increase in rectal temperature (.44±.27), initial respiratory rate (.59±.28), logarithm of the increase in respiratory rate (.44±.2 7), haemoglobin level (.55±.27), and rate of daily weight gain after weaning (.46±.25). Genetic, phenotypic, and environmental correlations of the six traits were estimated. The results indicated no relationship of the haemoglobin type to heat tolerance. The possibility of joint selection for weight and heat tolerance traits is suggested with the recommendations of omitting haemoglobin level.

¹ Based on a MS dissertation. The author received a fellowship from Fundaço de Amparo à Pesquisa do Estado de So Paulo.

² Present address: Department of Animal Science, Faculdade de Medicina Veterinária e Agronomia de Jaboticabal, 14870 Jaboticabal, So Paulo, Brasil. The author is indebted to Dr. W. E. Kerr, Department of Genetics, Faculdade de Medicina de Ribeiro Prêto, for his advice and valuable suggestions; to Mr. M. Santiago, who permitted the use of animals and facilities at So Carlos Experimental Station; to Mr. L. A. F. Bezerra, for his advice in the programming and computation of the data: to the Data Processing Center of Escola de Engenharia de So Carlos, for the permission of use of its computer systems.

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