RESUMO
Dark spots in the fleece area are often associated with dark fibres in wool, which limits its competitiveness with other textile fibres. Field data from a sheep experiment in Uruguay revealed an excess number of zeros for dark spots. We compared the performance of four Poisson and zero-inflated Poisson (ZIP) models under four simulation scenarios. All models performed reasonably well under the same scenario for which the data were simulated. The deviance information criterion favoured a Poisson model with residual, while the ZIP model with a residual gave estimates closer to their true values under all simulation scenarios. Both Poisson and ZIP models with an error term at the regression level performed better than their counterparts without such an error. Field data from Corriedale sheep were analysed with Poisson and ZIP models with residuals. Parameter estimates were similar for both models. Although the posterior distribution of the sire variance was skewed due to a small number of rams in the dataset, the median of this variance suggested a scope for genetic selection. The main environmental factor was the age of the sheep at shearing. In summary, age related processes seem to drive the number of dark spots in this breed of sheep.
Assuntos
Envelhecimento/genética , Pigmentos Biológicos/genética , Carneiro Doméstico/crescimento & desenvolvimento , Carneiro Doméstico/genética , Lã/crescimento & desenvolvimento , Animais , Teorema de Bayes , Simulação por Computador , Funções Verossimilhança , Modelos Estatísticos , Distribuição de Poisson , Análise de Regressão , UruguaiRESUMO
O objetivo deste trabalho foi a avaliação da produção do leite e de seus componentes químicos de animais Corriedale e cruzas de Corriedale x Milchschaf. Foram utilizadas 33 ovelhas Corriedale, 43 cruzas Corriedale x Milchschaf (F1) e 23 cruzas F1 (Corriedale x Milchschaf) x Milchschaf (F2). Os animais foram ordenhados duas vezes ao dia, de setembro a dezembro de 2002. No total, foram realizados cinco controles leiteiros com intervalo de vinte e um dias. As amostras para composição química foram analisadas para determinação de gordura, proteína e lactose. Foi utilizado o delineamento completamente casualizado para avaliação do efeito do genótipo sobre a produção de leite corrigida para os 100 dias, e sobre a produção de gordura, proteína e lactose. No modelo matemático, a idade, o número de cordeiros ao parto e a condição corporal foram utilizadas como covariáveis. Houve diferença significativa (P<0,0001) entre os genótipos para a produção de leite (kg), gordura, proteína e lactose (g) corrigidas aos 100 dias de lactação, de modo que o genótipo F1 e F2 mostraram-se mais produtivos.
The objective of this study was to evaluate production and chemical components of milk from Corriedale sheep and Corriedale x Milchschaf breed. Thirty-three Corriedale, 43 Corriedale x Milchschaf (F1), and 23 F1 (Corriedale x Milchschaf) x Milchschaf (F2) were used. Animals were milked twice daily, from September to December 2002. A total of five milking evaluation periods with a 21-day interval was conducted. Fat, protein and lactose contents were determined. A completely randomized design was used to evaluate the effect of genotype in milk production, corrected for 100 days and yield of fat, protein and lactose. In the mathematical model, age, number of sheep born and body condition were used as co-factors. Significant difference (P<0.0001) was found between genotypes in terms of milk production (kg), fat, protein, and lactose (g) corrected for 100 days of lactation. Genotypes F1 and F2 were the most productive.
