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The objectives of this study were to assess the effect of some non-genetic factors, breed and heterosis on birth weight and weaning weight in crossbred dairy kids, and to estimate genetic parameters of these traits in a multibreed dairy goat farm. Non-genetic analysis revealed that sex, year and season of birth and litter size are important factors that affect birth and weaning weights. Estimates of heritabilities for direct genetic effects were moderate at birth and weaning (0.25 ± 0.06 and 0.24 ± 0.10, respectively). Heritability of maternal genetic effect for birth weight was 0.26 ± 0.03. The bivariate analysis also revealed that permanent maternal effects are significant for birth and weaning weights indicating that this component should be considered in management strategies. These results highlight the importance of considering non-genetic factors to design management strategies in combination with genetic selection strategies to improve the future performance of replacement of does. [ABSTRACT FROM AUTHOR]

Pelibuey and Katahdin are two the most extensively used hair sheep breeds in tropical regions characterized by high temperatures and humidity. Lactation influences profitability of sheep in dairy and non-dairy production systems. Therefore, the aim of this study was to evaluate the effect of breed and litter size (LS) on milk yield and milk composition of Pelibuey (n = 28) and Katahdin (n = 21) ewes reared under tropical conditions of Mexico. Milk, fat, protein and lactose yields, fat and energy-corrected milk, and fat, protein and lactose percentages were measured for seven weeks. Body weight (BW) of each ewe was measured weekly. Breed effects were significant (P < 0.001) favouring Katahdin breed on all yield traits. Ewes rearing two lambs had higher yields (P < 0.01) for all traits than ewes rearing one lamb. Fat percentage and BW were significantly (P < 0.05) affected by a triple interaction effect of Breed x LS x Week. So, important differences in milk yield and composition among breeds and LS were shown and might be considered in order to improve profitability of sheep production systems under tropical conditions. [ABSTRACT FROM AUTHOR]

A study was designed to develop a birth season (BS) classification methodology and assess its impact on live weight traits when compared to a traditional BS classification method. Using meteorological information, and aridity index was computed. The proposed and traditional BS were compared by including them into contemporary groups (CG= herd, sex, year and BS) to adjust genetic evaluation models of studied traits. The variance components and breeding values with accuracies were estimated and compared. The proposed BS explained more phenotypic variation than traditional (=9.8%). Genetic parameters showed important changes, more evident for weaning weight. According to the likelihood ratio test the compared models were statistically different (P<0.01). An improvement in CG structure was observed. Genetic correlations of breeding values showed important differences suggesting hierarchy changes. This method of BS classification might improve the statistical model fitting when meteorological information could be available. [ABSTRACT FROM AUTHOR]