Pseudopregnancy and aseasonal breeding in dairy goats - genetic basis of fertility and impact on lifetime productivity
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Until recently, the main selection focus in UK dairy goats has been on milk yield. To develop a selection index suitably weighted for a variety of traits, it is important to understand the genetic relationships between production, health, and fertility traits. This study focused on 3 aspects of reproduction that are of interest to goat breeders. 1) Out of season kidding ability (OOS): goats are highly seasonal breeders so achieving consistent, year-round dairy production presents a challenge. It may be possible to select for extended or shifted breeding cycles, however there are no published studies on the genetic basis of seasonal kidding ability, and a genetic correlation with milk production in dairy goats; 2) Age at first kidding (AFK): a reduced age at first kidding offers the opportunity for more rapid genetic improvement, as well as reducing the amount of time and resources required to raise the animals to producing age; 3) Pseudopregnancy (PPG): as it is difficult to diagnose pregnancy within 30 days of mating, high herd levels of pseudopregnancy could add a significant delay in breeding replacement animals, or commencing a new lactation. Using records from 9546 goats, the objective of this study was to investigate the genetic relationships between the reproductive traits described above, and the production traits 520 day milk yield (MY520), lifetime milk yield (MYLife), and lifetime number of days in milk (DIMLife). The ‘out of season’ phenotype was defined as week of kidding relative to the four weeks of the year where the highest average number of births occur. Incidences of pseudopregnancy that occurred during the first lactation were used as cases, while goats with none were assigned as controls. Relevant fixed and random effects were fitted in the models. In line with other reproduction traits, heritability estimates were low ranging from 0.08 to 0.11. A negative genetic correlation was found between AFK and MY520 (-0.22 ± 0.10), while a positive genetic correlation was found between PPG and DIMLife (0.58 ± 0.11). Pseudopregnancy and OOS were positively genetically correlated (0.36 ± 0.15). All other genetic correlations were very low. The results of this study indicate that selection for the reproductive traits analysed is feasible, without adversely affecting lifetime milk yield.
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