Autores
Alexander George Razook - Orientador: Walter R. Harvey
Resumo
The purpose of this study was to estimate environmental and genetic factors affecting weights and gain obtained during the performance test at the Estação Experimental de Zootecnia de Sertãozinho located in the State of São Paulo, Brazil. The study involved 779 Zebu bulls with 446 of the Nelore breed, 253 the Guzera breed and 80 of the Gir breed.
The bulls started the test with ages ranging from 8 to 12 months. The duration of the feeding period was 140 days and the ration was offered ad libidum. The traits measured during the period were: initial weight on test (INWT) which was an independent variable, 4 intermediate weights (WOT1, WOT2, WOT3, WOT4) taken at intervals of 28 days from INWT, Final weight on test (FNWT), adjusted final weight (FAWT) for age at 460 days and gain on test (GAINOT).
Preliminary analysis were completed in order to determine the final models used for the analyses for each breed. The final models included the effects of year, breeder, sires within breeder, INWT and initial age on test (IAGE). For the Nelore only linear regressions were fitted for each independent continuous variable. For the Guzera linear and quadratic regressions were fitted and for the Gir the linear and quadratic average regressions were fitted plus the deviations of these regressions for each year for initial weight. For initial age only the linear pooled regression was fitted with the Gir data.
The analyses of variance showed that year was a highly significant effect for all traits in the Guzera. For the Nelore WOT2, FNWT and GAINOT were significantly affected by years. The Gir shoed less significant differences due to years than the other two breeds.
The effects of breeders were significant in all traits in the Nelore breed. In the Guzera FNWT, FAWT and GAINOT were affected by breeder effects. For the Gir the analyses were of little value due to small number of degrees of freedom.
The effect of INWT was highly significant for all traits except for GAINOT in the three breeds. The linear partial regressions were around 1.00 for all traits except GAINOT. In the Guzera breed the quadratic partial regressions were also all significant including GAINOT. The regression values were: -0.0011, -0.0012, -0.0014, -0.0016, -0.0016, -0.0021 and -0.0016 for WOT1, WOT2, WOT3, WOT4, FNWT, FAWT and GAINOT, respectively.
The IAGE was also a highly significant source of variation for all traits measured. The partial linear regressions for the Nelore was 0.08, 0.10, 0.13, 0.15, 0.16, -0.54 and 0.17 for WOT1, WOT2, WOT3, WOT4, FNWT, FAWT and GAINOT, respectively. For the Guzera the linear partial regression for the different traits were: 0.04, 0.08, 0.11, 0.12, 0.12, -0.64 and 0.12. The quadratic partial regression coefficients were: 0.0013, 0.0018, 0.0027, 0.0029, 0.0025, 0.0032 and 0.0024. These coefficients were also significant.
The heritability estimates were obtained by paternal half-sib analyses obtained by use of a mixed model computer program. Two estimates for the Nelore and Guzera breeds were obtained. One set of estimates were obtained from the whole set of data for each breed and the other was obtained after excluding sires with single offspring. The heritability estimates for the Gir were of little value due to large standard errors. In the Nelore the two estimates were 0.41 and 0.051, 0.78 and 0.27, 0.63 and 0.37, 0.48 and 0.20, 0.88 and 0.58, 0.67 and 0.32 and finally 0.94 and 0.64, respectively, for WOT1, WOT2, WOT3, WOT4, FNWT, FAWT and GAINOT for the complete set of data and without the data on sires with single progeny.
For the Guzera the estimates were: 0.38 and 0.46, 1.05 and 1.12, 0.98 and 1.08, 0.84 and 0.91, 0.71 and 0.99, 0.71 and 0.99, 0.70 and 1.02 and finally 0.63 and 0.95 for the same traits.
The genetic and phenotypic correlations were medium to high in values (ranging from 0.50 to almost 1.00). They were higher for adjacent weights collected on test. The genetic and phenotypic correlations between final weight and gain were always high. Environmental correlations were negative in most cases especially betwenn adjacent periods.
The results of this study gave evidence that initial weight, initial age on test, years, breeders and sires are very important sources of variation for traits measured during the performance test. Therefore, proper adjustments for these factors must be made. It was also noted from this study that the gain during the test should be properly utilized to obtain adjusted final weight for better evaluation of zebu bulls for this characteristic.
Dissertação apresentada à Ohio State University, como parte das exigências para obtenção do Título de Mestre em Ciências, 1977, 99p.
