How to assess litter uniformity in pigs: comparing metrics and investigating associations with sow and litter factors

Variation in piglet BW may affect animal welfare and the environmental and economic aspects of pig production. Higher within-litter weight uniformity is often associated with improved preweaning survival. Conversely, lower within-litter uniformity is often linked to a higher prevalence of lightweight piglets that encounter various challenges during the neonatal phase. To date, there is no consensus on how to quantify within-litter uniformity in literature due to the use of various metrics. The objectives of this study were: (1) to examine metrics for assessing litter uniformity in pigs (i.e., CV, SD, range in weights, average z-score and the percentage of lightweight piglets (< 0.8 kg and < 1 kg)), (2) to investigate possible alternatives to weighing all piglets at birth in order to compute the CV, (3) to assess the repeatability of the above metrics per sow and (4) to explore the associations between the different uniformity metrics and factors related to sow and litter. This study used data from 732 litters of 190 hybrid sows mated with 42 Piétrain boars. In total, 11 574 piglets were weighed at birth to assess the within-litter uniformity. The CV, SD and range in weights (r = 0.81-0.91) could be used interchangeably to a certain extent. Additionally, assessing the percentage of lightweight piglets per litter might offer pig farmers a valid method to evaluate within-litter uniformity with reduced labour intensity (r = 0.33-0.70). Our results indicate that it is possible to compute the CV of birth weight of a litter without including the weight of the four lightest piglets (r = 0.80-0.95) and that 8-63% of the within-litter variation is linked to the features of each individual sow. This points to the importance of individual sow characteristics to enhance within-litter uniformity.