Cintia Righetti Marcondes

Structure and genetic variability in Nellore (Bos indicus) cattle by pedigree analysis

Parameters based on the probability of gene origin were used to describe the genetic variability in strains of Nellore and polled Nellore (Bos indicus) cattle that participated in the Program for Genetic Improvement of the Nellore Breed (Programa de Melhoramento Genetico da Raca Nelore). The effective number of founders was 87.2 for Nellore and 107.9 for polled Nellore, while the number of ancestors was 59.8 for Nellore and 61.5 for polled Nellore and the remaining genomes were 39.4 for Nellore and 34.5 for polled Nellore cattle. The results indicate an intense use (by artificial insemination) of some sires and the absence of subdivisions in the population. The family structure in the two breeds was mainly caused by the genetic contribution of the same sires and only mating preferences for descendants of some founders are recorded in either breed. The results suggest that genetic variability needs monitoring in order to avoid the compromise of genetic improvement in economically important traits in the breeding program.

Estudo de definição alternativa da probabilidade de permanência no rebanho para a raça Nelore

Uma definicao alternativa de permanencia no rebanho foi estudada em comparacao a definicao usual. Na definicao usual, foram consideradas vacas com pelo menos tres partos ate os 76 meses de idade como valor 1 e vacas com menos de 3 partos com valor zero. A definicao alternativa considerou os valores 0, 1, 2 e 3, respectivamente, para vacas com menos de tres, com tres, quatro ou cinco partos ate os 76 meses de idade. Foram obtidas estimativas de herdabilidade e preditas as DEPs (Diferencas Esperadas na Progenie) para probabilidade de permanencia no rebanho de 4.180 touros com filhas na base de dados do Programa de Melhoramento Genetico da Raca Nelore (PMGRN-USP). Utilizaram-se o modelo linear unicarater de touro avo-materno e a analise Bayesiana, por meio do software MTGSAM_threshold (Multiple-Trait Gibbs Sampler for Animal Models). Considerou-se tamanho de cadeia de Gibbs de 225 mil, periodo de descarte amostral de 25 mil e tomada de amostra a cada 1.000 rodadas. A correlacao entre classificacoes foi calculada por meio do SAS. As estimativas de herdabilidade foram de menor magnitude para a caracteristica padrao (0,07 vs 0,08). A correlacao de posto foi de 87,5%, ou seja, houve consideravel alteracao na classificacao dos touros avaliados. Dados os valores das estimativas de variância de touro, de herdabilidade e de variacoes nas DEPs preditas, concluiu-se que a caracteristica alternativa de permanencia no rebanho possui maior capacidade de deteccao da variabilidade genetica que a caracteristica padrao, sob modelo linear.

Genetic trends in the expected progeny difference of the asymptotic weight of Nelore females

There are few studies on weight covering the full life cycle of Zebu cattle, and there is no entire growth description or mean growth pattern for animals belonging to this breed. In order to provide such data, 1,158 Nelore females born between 1985 and 1995 were weighed 14,563 times from birth to full growth maturity, in ten herds spread over seven Brazilian states. The Von Bertalanffy, Brody, logistic and Gompertz non-linear models were used to obtain the asymptotic weights (A) and the maturation rates (K). The (co)variance and breeding value components for A and K were obtained by using the multiple trait derivative free restricted maximum likelihood method under the animal model. Genetic trends were calculated in function of the mean expected progeny differences (EPD) for the trait (A or K) divided by the number of animals according to their year of birth. The genetic trends of the expected progeny difference with reference to the date of birth of the cows were, on average, -6.5g y-1 for A and 2.0g y-1 for K, close to zero as confirmed by the low (0.0023 to 0.003) coefficient of regression values. The curve parameters are recommended as a selection criterion to reach precocity and avoid adult weight increase in the female herd.