Genetic Selection on IGE for Growth (IGEg)
Background information on IGEs, and the estimation of IGEs for growth during the finishing phase (from 25 to 110 kg) for the current trial, here denoted as IGEg, has been given in detail in Camerlink et al. (2013). Briefly, sows (64 Topigs-20 sows: sow line of Great Yorkshire × Dutch Landrace) and boars (24 Tempo boars: commercial synthetic boar line with Great Yorkshire genetic background) were selected based on their estimated breeding value for IGEg. Sires and dams with the most extreme high and low IGEg of the available population were mated within their IGEg group (high vs. low), while the direct breeding value was kept equal between groups. This resulted in a contrast of 2.8 g ADG (average daily gain) between high and low IGEg offspring (40 high IGEg litters and 40 low IGEg litters). With 6 pigs per pen this results in a total contrast of 14 g ADG, i.e. (6−1) × 2.8 = 14. Hence, high IGEg offspring would increase the growth of their pen mates, whereas low IGEg offspring would decrease the growth of their pen mates (effects on growth have been reported in Camerlink et al. 2014). Offspring were studied over five batches of 96 pigs each (n = 480), between September 2010 and February 2012.
Animals and Housing
Piglets were born in conventional farrowing pens with farrowing crates (TOPIGS experimental farm, Beilen, The Netherlands). Tails and teeth were kept intact. Male piglets were castrated (at 3 days of age) because IGEg have currently been estimated on gilts and castrated males. Cross fostering was applied only if litter sizes exceeded 14 piglets, and always within the same IGEg group. At ~14 days of age, piglets were subjected to the backtest to assess their coping style (Hessing et al. 1993). Classification of piglets based on their response in the backtest, for which no relationship with IGEg was found (Reimert et al. 2013), was used to standardize group composition with regard to coping style. Piglets were weaned at 26 days of age, whereby maximum eight piglets per sow were selected. Selection was based on good health, sex (1:1), and backtest response (to the ratio of the tested population). Selected piglets (n = 480 in total) were transported to experimental farm De Haar (Wageningen, The Netherlands).
From weaning to slaughter (4–23 week of age), a 2 × 2 experimental arrangement was applied with IGEg (low vs. high) and housing conditions (barren vs. enriched) as factors at the pen level. Pigs were housed with six per pen, leading to 80 pens in total. Group composition was balanced for sex (1:1) and backtest classification (at least two of each classification). Half of the pigs from each IGEg group, and half of the selected piglets from each sow, were allocated to barren pens and the other half to enriched pens. Pens were composed of pigs which were unfamiliar to each other, i.e. no littermates or sibs were kept together.
Barren pens had a floor which was half solid concrete and half slatted. Enriched pens had a solid floor with a bedding of 12 kg of wood shavings and 1.5 kg of straw. Fresh wood shavings (3 kg/pen) and straw (0.25–1.5 kg/pen depending on age) were added to enriched pens daily. Pen dimensions were either 1.90 × 3.20 m or 2.25 × 3.25 m (1–1.2 m2/pig), depending on batch, and were within batch equal between barren and enriched pens. All pens had a metal chain with ball attached to the pen wall as toy. Dry pelleted commercial feed was offered ad libitum from a single space feeder. Feed was provided according to commercial practice, with a total of four feed changes whereby on the first day the old and new feed types were mixed to create a gradual transition between feed types. Water was continuously available from a single nipple drinker per pen. Temperature was until 10 days after weaning set at a minimum of 25 °C, and was hereafter set at 22 °C for 3 weeks, followed by 20 °C until slaughter. Lights and a radio were on from 7:00 till 19:00 h. To reduce damaging tail biting behaviour, i.e. chewing on the tail of a conspecific which can lead to injury and in extreme cases even to mortality of the bitten animal, all pens received a handful of wood shavings per day from week 6 onwards and a jute sack was attached to the wall from week 8 onwards. Pigs were housed in these pens from weaning until slaughter. Due to diverse health reasons including tail biting, 18 high IGEg and 11 low IGEg pigs were removed from the experiment.
Behaviours of individual pigs were recorded at 4, 5, 8, 12, 16, and 21 weeks of age. Each pig was identified by a spray marked number on the back, which was refreshed before behavioural observations. Behaviour, as described in Table 3 (Appendix), was scored during live observations using 2-min instantaneous scan sampling for 6 h during the active period of the day, consisting of six 1 h blocks from 8:00 to 11.30 h and from 14.00 to 17:30 h with after each hour a 15 min break. This procedure resulted in 180 observations per pig per observation day, with one observation day in each of the weeks mentioned. The Observer 5.0 software package (Noldus Information Technology B.V., Wageningen, The Netherlands) installed on a hand-held computer was used for behaviour recordings. Observations were carried out by observers who were unaware of the IGEg of the pigs.
Tail Damage Scores
Tail damage scores can serve as an indicator for the amount of tail biting behaviour in a pen. Scores were obtained using an adapted procedure from Zonderland et al. (2008). Scores ranged from 1 to 4, with score 1 being no visible tail damage; score 2 for hair removed from the tail; score 3 for bite marks; and score 4 for a clearly visible wound. Tail damage was scored each week on each individual pig, leading up to 20 observations per pig. When a pig had to be removed from the trial due to being bitten severely its score was set to 4 for the remaining period till slaughter. When a tail biter had to be removed from the pen it kept its last score before being removed from the pen. Scores were obtained by multiple observers who were trained to score in the same way, and who were unaware of the IGEg of the pigs.
Interventions to Limit Damage Due to Tail Biting
Oral manipulation amongst pigs is the repeatedly biting on the tail, ear or paw of a group member, and may result in injury, impaired health or mortality of the bitten animal. Oral manipulation such as tail biting may start harmlessly, but when no measures are taken many animals may be severely damaged (Statham et al. 2009). During the trial, measures were taken to reduce tail biting to an acceptable level to prevent the loss of animals and to guarantee a certain level of animal welfare. Tail biting wounds became significant from 6 weeks of age. To reduce the amount of damaging tail biting behaviour, a handful of wood shavings was provided to each pen from week 6 onward and from week 8 a jute sack was attached to the pen wall as material to chew on. The jute sack was a commercially available sack of approximately 60 × 105 cm, which was over the width attached to the pen wall and was replaced when there was less than 1/3 of the sack left (Fig. 1). When the sack was replaced, the remainders were approximated in cm2. The amount of jute sack that was ‘consumed’ was noted by pen. To reduce tail biting, the tails of bitten pigs were alternating between days covered with the aversive P.B.H. spray (Kommer Biopharm B.V.) or Stockholm tar (Rapide®). Pigs were removed from the pen when they had a reduction in tail length, irrespective of the amount of reduction. Six high IGEg pigs and three low IGE pigs, from eight different pens in total, were removed from the trial due to reduced tail length. One tail biter (low IGEg) was removed to limit further tail damage of its five pen mates.
Statistical analyses were performed using SAS (SAS 9.2, Institute Inc.). Data were analysed and presented by production phase as applied in commercial pig farming to facilitate comparison between animal behaviour studies. The nursery phase is from 4 to 8 weeks of age, whereas the finishing phase is generally from 8 weeks of age till slaughter (here at 23 weeks of age).
Behavioural scans were analysed on pen level (n = 80) and averaged over production phase (nursery phase: observations weeks 4, 5, and 8; finishing phase: observations weeks 11, 12, 16 and 21). Hereto the behaviours of pigs were averaged by pen (6 pigs/pen). Residuals of the response variables were checked for normality, and if needed, behaviours were arcsine square root transformed. Behaviours by pen and production phase were analysed in a general linear model (GLM procedure), and included IGEg group, housing condition, the interaction between IGEg group and housing condition, and batch as fixed class effects.
The weekly tail damage scores were averaged into two scores per pig, one for the nursery phase (weeks 4–7) and one for the finishing phase (weeks 8–23). Scores were analysed at individual animal level (n = 480) in a generalized linear mixed model (MIXED procedure) with IGEg group, housing condition, the interaction between IGEg group and housing condition, sex, and batch as fixed class effects, and as random factor pen nested within IGEg group, housing condition and batch.
The total cm2 of ‘consumed’ jute sacks per pen (from weeks 8–23) was analysed at pen level (n = 80) in a general linear model (GLM procedure) with IGE group, housing condition, the interaction between IGE group and housing condition, and batch as fixed class effects. To facilitate the interpretation of consumed bags in cm2, results are presented in number of jute sacks consumed [total cm2/(60 × 105)]. The amount of jute sacks per pen was correlated to the average tail damage scores per pen by Pearson correlation.
In the results, average trait values for the treatments are reported as (untransformed) LSmeans ± SEM. P values below 0.05 are considered significant.