Tail status at both individual and pen level affects tail biting victimisation in pigs over the finishing period

Tail biting is a significant welfare problem in commercial pig production. Individual tail health was assessed in non-docked pigs (n = 1 579) on five farms (180-441 pigs and 16-30 pens per farm) at introduction to the finishing unit (W0) and 9 weeks later (W9) to present longitudinal descriptive data, establish risk factors for being tail bitten and investigate possibilities to infer information about W0 tail health based on W9 tail appearance. Tail health was described in terms of posture (hanging), shortening (loss of distal flattening), lesion type (TLT; 0 = no lesion, 1 = minor lesion on intact skin; or with skin penetration 2 = < 0.5 cm, 3 = 0.5-2 cm, or 4 = > 2 cm), length (iL) and change therein from W0 to W9 (ΔL). ΔL may include cumulating information on received tail biting given that the variation was large and negative values were present. The average intact tail measured 22 ± 2.1 and 29 ± 2.3 cm at W0 and W9, respectively, with no effect of sex. Tail lesion prevalence decreased unexpectedly from W0 to W9, possibly due to a docking-like effect in the present population showing a relatively high injury prevalence at W0. Mixed models predicting tail health at W9 were built with pig (n = 1 323-1 492) as statistical unit and pen (n = 94-124) nested within farm as random effects. Pen-level tail appearance at W0 was included as eligible predictors. No model predicted TLT 2,3 or 4 at W9, suggesting that factors other than early tail appearance determined late victimisation. A larger ΔL was predicted by the individual-level factors barrow (vs female), a larger iL, no tail shortening and any other TLT as compared to 4 in the individual at W0; as well as smaller percentage of hanging tails in the pen, a larger average iL in the pen (px̅L) and an interaction between iL and px̅L. According to the interaction effect, two different scenarios predicted a smaller ΔL: smaller iL and px̅L, or especially larger iL and px̅L. The scenarios may be re-targeting of previous victims by individuals accustomed to an environment with tail biting and two-stage tail-biting where the longest tails were the most attractive objects for exploration. We conclude that pigs with better tail health at introduction to the finishing unit receive less injurious biting during 9 weeks. Pen-level tail health has a profound effect that is very different at extremes of tail length. One-fourth of early skin-penetrating tail injuries may not be detectable shortly before slaughter.