Minimizing aggression during mixing of gestating sows with supplementation of a tryptophan-enriched diet

• Feeding a tryptophan (TRP)-enriched diet increased blood concentration of TRP in gestating sows.
• The TRP-enriched diet reduced sow aggressiveness at mixing.
• The diet increased the sows' activity levels, especially exploratory behaviors.
• The TRP-enriched diet reduced sham-chewing in older sows housed in stall.
• Feeding a TRP-enriched diet to gestating sows may facilitate group formation.

Gestation stalls are criticized for its negative physical and psycho-physiological effects on sow welfare. Group housing benefits sow well-being and when planned properly can minimize aggression during mixing. This study aimed to evaluate the effect of short-term feeding of a TRP-enriched diet at a concentration of 220% the control (CTL) diet, on aggressiveness at mixing of sows at 4 weeks of gestation. Treatment diets were fed for 7 consecutive days; from days 1 to 5 sows were housed in stalls, early in the morning on day 6 sows were grouped by parity and assessed until day 7. Eighteen pens with 4 sows each (n = 72) of similar parity were assigned to CTL and TRP treatments. Sows' behaviors were recorded daily for 12 h, from days 1 to 7. Inactive and active behaviors (alert, walking (pen), rooting, feeding, drinking, eliminating), stereotypic behaviors (bar biting and sham-chewing), and postures (standing, sitting, lying) were assessed by 10-minute scan sampling. Occurrence of agonistic interactions, number of actions such as bites, head knocks and pursuits and their sum per interaction were recorded for each pen using 2-h continuous behavioral observation, at days 6 and 7. Skin lesion scores were assessed from each sow at day 5 and at 48 h post-mixing, using a sow body map subdivided into anterior, central and posterior body regions. A linear mixed model with day as repeated measure, stall or pen as experimental unit, tested the fixed effects of treatment, day, period within day, their interactions, and block by treatment interaction; stall (trt) or pen (trt) as appropriate was used as random effect. Blood concentration of TRP was higher on the mixing day in TRP-fed sows compared to baseline (76%) and CLT-fed sows at mixing (79%; P < 0.05), while serotonin concentration did not differ between treatments (P > 0.05). The TRP-enriched diet was effective in reducing sham-chewing in stall housed sows of parity 5–9 (P < 0.05). In pens, TRP-fed sows spent more time rooting (TRP = 28.0 vs. CTL = 20.7 ± 1.0%; P < 0.05) and consequently less time lying down than CTL-fed sows (TRP = 56.1 vs. CTL = 65.1 ± 2.0%; P < 0.05). The total number of offensive actions per interaction was greater in the morning than afternoon for both days (P < 0.05), but this was less evident in TRP-fed compared to CTL sows mainly on the morning following mixing (3.4 vs. 7.2 ± 1.0, respectively; Trt ∗ period (day) = P < 0.05). The average lesion score was lower in the anterior body region of TRP-fed compared to CTL sows (2.1 vs. 2.5 ± 0.2; P < 0.05), the most affected area during fights. The TRP-enriched diet reduced sow aggression while increasing behavioral activity, as evidenced by more time rooting and standing while sows had fewer offensive actions per agonistic interaction and lower skin lesion score 48 h post-mixing. A TRP enriched diet provided to gestating sows for a short period prior to social mixing and continued for a short time after is an effective means of reducing aggression and improving the welfare of sows during group formation.