The response of group-housed sows to dietary inclusion of magnesium oxide and sugar beet pulp

Mixing unfamiliar sows during early gestation can often lead to injury and lameness as a result of inter-sow aggression which can be accentuated by an increased motivation to feed in systems where a restricted amount of feed is delivered once or twice daily. Nutritional satiety may be achieved in group-housed sows through the addition of fibre, such as sugar beet pulp (SBP; Danielsen and Vestergaard 2001). Magnesium oxide (MgO) has been suggested to play a role in insulin resistance (Barbagallo et al. 2003), resulting in stabilised insulin levels that in turn stabilise blood glucose levels, leading to satiation (Bo and Pisu 2008). It was hypothesised that the inclusion of SBP and (or) MgO in the diet of sows at mixing would lead to reduced inter-sow aggression.

Thirty-six multiparous sows (Landrace X Large White) were used across this study, re-randomised into four treatment groups (n = 6) for each of six replicates. Twenty-four sows were used in each replicate, with 12 sows off test, to allow for completely unfamiliar groups at each replicate. Each replicate ran for 7 days with sows being housed initially in individual stalls for the first 4 days and offered allocated treatment diet. At 0700 h on d 5, sows were shifted to their respective group pen. Daily data collection began on d 5 (day of mixing). Measures taken during each 3-day observation period included aggressive behaviour (push, chase, attack, bite and threat) and posture observations. This use of short-term assessment is suited due to the 1–2 days after mixing that are associated with dominance aggression at mixing (Arey and Edwards 1998), yet takes into account the extended length that sows recognise each other (Spoolder et al. 1996). All diets were formulated to be isoenergetic and isonitrogenous [12.9 MJ digestible energy (DE)/kg, 0.40 g standardised ileal digestible lysine/MJ DE] and were fed at 2.3 kg/d. The four diets offered over the 7 day period were: control diet; a diet including 20% SBP; a diet including 0.2% MgO; and a diet including both 20% SBP and 0.2% MgO. Data were analysed using the Univariate GLM procedure (Genstat, 15th Edition; UK) with the experimental unit being the pen.

The inclusion of SBP and (or) MgO in the diet had no significant effect on sow behaviour and no significant effect on aggressive behaviour (Table 1). However, a time effect was seen for some behavioural observations. Chase behaviour increased the day after mixing (d 6) before falling again the following day (P < 0.05), whilst threat behaviour increased over time. There was a trend (P < 0.10) for the time sows spent fighting to reduce after the first 24 hours of mixing (from d 5 to d 6). Salivary cortisol levels (data not shown) increased over time, which appears in conflict with the decline in fight time. Whilst these dietary interventions were not able to influence behaviour, this study did show that fighting behaviours are short-lived. The increase in threat behaviour over the period contrasting with the decreased fighting behaviour suggests a rapid establishment of hierarchal positions in the first days of mixing.

Table 1.  Time (min) sows spent engaged in behaviours 1 h after feeding, for diets containing 20% SBP and (or) 0.2% MgO and for all treatments over the experimental period