Dying for uniformity: is pre-weaning mortality the major cause of the decrease in variation?

Uniformity of fattening pigs may increase the efficiency and sustainability of meat production and reduce economic, environmental and social costs. This research aimed to evaluate a change in uniformity in the first 6 weeks after birth and to see if this is mostly due to pre-weaning mortality. Data was collected in one farm on 791 crossbred progeny born in 46 litters, sired by 4 Piétrain and 4 MaxiMus boars. From the 696 live-born piglets, 623 (89.5%) survived until the age of 6 weeks. To evaluate the uniformity, the coefficient of variation (CV% = (standard deviation/mean) ×100) of body weight was analysed at birth, at weaning (24-29 days), 1 week after weaning, and 2 weeks after weaning (after post-weaning dip). The average within litter CV% at birth of all live-born piglets (including piglets dying before 2 weeks after weaning) was 20.6%. In the first 6 weeks, the variation within litter slightly decreased (P=0.10) to 18.5%. However, the average CV% at birth of the surviving piglets only is 17.8%, which is significantly lower (P=0.028) than the CV% when including all live-born piglets. This indicates that the pre-weaning mortality causes a decrease in within litter weight variation. When considering only the surviving piglets, CV% did not change significantly from birth to 6 weeks (17.8% to 18.5%, (P=0.55)). Our findings suggest that pre-weaning mortality should be accounted for when investigating ‘uniformity’ of litters of piglets. However, additional data need to be collected to confirm our results and to study other effects on the CV% of body weight