PS10-2. Using the Number of Victimized Pigs to Evaluate Tail-biting Outbreaks.

Abstract Tail-biting outbreaks (TBO) can lead to significant consequences for animal welfare and economic losses for pork producers. While many studies have focused on predicting or minimizing TBO, the term has been defined inconsistently across experiments, making cross-study comparisons difficult. The objective of this study was to explore defining TBO among pigs housed in two group sizes based on the number of victimized pigs and the frequency of these events, aiming to develop a framework for consistent application across group sizes. Pigs (n = 432) with intact tails were housed in small (SG; 9 pigs/pen, 24 pens) or large (LG; 18 pigs/pen, 12 pens) groups in a barn with fully slatted floors for 14 weeks (22.0 ± 3.7 kg to 120.7 ± 11.8 kg BW). Floor space (0.76 m2/pig) and feeder space (4 spaces/9 pigs) allowance were identical across group sizes, and pigs were stratified by initial weight and sex across treatment pens. Tail injury was assessed weekly using a scale of 0 to 4 (0 = no damage, 4 = loss of the tail). Pigs with a tail score ≥ 2 (visible blood or infection) were classified as victimized. Each pen was scored 16 times during the study period, and the number of victimized pigs was recorded at each time point. Data were analyzed using the PROC FREQ procedure with the Chi-square tests in SAS. To evaluate TBO, four separate analyses were conducted, defining an event as having at least one, two, three, or four victimized pigs within a pen at a single time point. The frequency of these events was categorized into 0, 1, 2 to 4, or > 4 times over the study period. A total of 117 and 123 victimized pigs were observed in SG and LG, respectively. At the ≥ 1 victim threshold, 92% of SG pens and 100% of LG pens met the criteria (Table 1), suggesting this threshold lacks specificity. Conversely, 75% of both SG and LG pens had ≥ 2 victimized pigs, indicating this criterion is applicable across both group sizes. At the ≥ 3 victim threshold, 67% of SG pens and 75% of LG pens met the criteria. As the threshold increased to ≥ 4 victims, only 37% of SG pens met the criteria compared to 67% of LG pens, suggesting this higher threshold underestimates TBO in SG pens. Event frequency helps evaluate threshold utility. Multiple occurrences may demonstrate that a threshold captures meaningful TBO rather than an isolated incident. Based on these results, we suggest that a baseline of 2-3 and 2-4 victimized pigs per pen be used to define TBO for SG and LG, respectively, to account for group size differences. Further validation in varied housing conditions is needed. For image description, please refer to the figure legend and surrounding text.