Low feed intake in the post-weaning period and consequent low growth can impair performance and negatively affect metabolism and health status in pigs.1,2 Although the effect of the number of pigs per feeder hole has been widely studied in production settings, few studies have evaluated its impact on growth performance, the onset of feed intake after weaning, and the incidence of ear and tail lesions. Wolter et al3 investigated the effects of feeder space and space allowance in wean-to-finish barns and observed a decrease in feed intake in pens with 18 pigs per feeder hole compared to those with 9 pigs per feeder hole up to 98 days post-weaning. However, no significant differences were observed in the overall growth rate and final body weight.

Lindemann et al4 suggested that when feed is provided ad libitum, pigs do not need to eat in groups or to have access to the feeder simultaneously, which would lessen the impact of the availability of feeder space. This phenomenon, however, has not been documented when ad libitum feed is provided during the nursery phase under high stocking density conditions. Preferable space allowances per pig, within the body weight ranges of 5 to 6 kg and 25 to 30 kg, are from 0.25 m² to 0.30 m², on slatted floors.5 With higher stocking densities, there is a competition for space and the likelihood of animals having a limited access to the feeder is increased. DeDecker et al6 evaluated the effect of stocking density on pig performance in a wean-to-finish production system and suggested that the feeder space may have a negative influence on the growth rate of animals under a higher stocking density. This supports our idea that higher stocking density without an increase in the number of feeder holes can impair the performance.

The competition for feed can not only reduce growth performance but also trigger aggressive interactions between the animals.7 Botermans et al7 observed that the most common interactions between pigs experiencing a feed competition during the growth phase included biting, pushing, and knocking of heads. The occurrence of tail biting indicates that some or all pigs within a pen are experiencing reduced welfare.8 However, very few studies have been published to evaluate the effects of number of pigs per feeder hole in pigs housed under commercial conditions and higher stocking density on ear and tail biting during the nursery phase.

Therefore, this study aimed to determine the effects of number of pigs per feeder hole and number of pigs per pen on the onset of feed intake post-weaning, growth performance, and the incidence of ear and tail lesions in nursery pigs under commercial conditions and with consistent space allowance.

Materials and methods

Institutional ethics committee approval

The Institutional Animal Care and Use Committee of the Federal University of Rio Grande do Sul approved the protocols used in this experiment according to the process PROPESQ-UFRGS 30556.

Animals, housing, and experimental design

The trial was conducted at a commercial research nursery facility in Videira, Santa Catarina, Brazil from May to July of 2016. A double curtain-sided nursery room with 28 identical pens, each with a total area of 6.84 m², was used for the experiment. All pens had solid concrete flooring along the entire length of the feeder, and slatted plastic flooring in the remaining area. The room temperature was maintained at 28°C to 30°C in the first and second week of the trial, and 25°C to 26°C thereafter. The temperature was monitored using three data loggers and two thermometers located at the center and extreme sides of the room.

A total of 630 intact males and gilts (PIC 337 × Camborough, Pig Improvement Company, Hendersonville, Tennessee), with initial mean (SD) body weight (BW) of 5.59 (0.9) kg and weaning age of 20.5 (0.9) days, were used in a 42-day study. The piglets’ needle teeth were ground after birth and one third of their tails were docked at 3 days of age. At weaning, pigs were individually weighed, ear-tagged, and assigned to pens to achieve balanced gender and weight across the pens. Pens of pigs were randomly allotted to one of four treatments in a completely randomized manner, with 7 replicate pens per treatment. Treatments consisted of 3.75, 5.00, 6.25, and 7.50 pigs per feeder hole, which were achieved by increasing group size with 15, 20, 25, and 30 pigs per pen, respectively. Adjustable pen gates were used to maintain a floor space allowance of 0.23 m² per pig across treatments. Therefore, the resulted pen dimensions were 4.28 × 1.6 m2, 3.56 × 1.6 m2, 2.85 × 1.6 m2, and 2.15 × 1.6 m2 for treatment groups with 3.75, 5.00, 6.25, and 7.50 pigs per feeder hole, respectively. Pen sizes were adjusted in the event of pig death or removal for poor health.

Each pen contained a semi-automatic feeder with a polypropylene reservoir and a stainless-steel tray with four feeding holes, each 16 cm wide by 14 cm deep (Veromix 40 C Premium Nursery Feeder, Magnani, Seara, Brazil), which provided a total space of 64 cm in length. Feeders were located at the front of the pens. Pigs had ad libitum access to a corn- and soybean-meal-based diet in a three-phase feeding program formulated according to the National Research Council.9 All diets were manufactured at the on-farm feed mill and were fed in meal form. The feed budget was 1 kg of Phase 1 diet fed per pig (3.6 Mcal/kg of metabolizable energy [ME], 21.9% crude protein [CP], and 1.46% standardized ileal digestible [SID] Lysine), 4 kg of Phase 2 diet fed per pig (3.6 Mcal/kg of ME, 21.4% CP, and 1.42% SID Lysine), followed by a Phase 3 diet (3.5 Mcal/kg of ME, 20.1% CP, and 1.30% SID Lysine) with approximately 17 kg/pig fed until the end of the trial. In addition, the drinkers were placed in the back third of the pen, on the side opposite to the feeder. Pens with 15 and 20 pigs had 2 nipple drinkers, and pens with 25 and 30 pigs had 3 drinkers to achieve a relatively similar number of pigs per nipple drinker in each pen (7.5, 10, 8.3, and 10, respectively).

Pigs presenting any health issues were treated according to the production company’s standard animal husbandry procedures.

Onset of feed intake

Four pens per treatment were randomly selected to receive the Phase 1 diet with the inclusion of 1% iron oxide as a red fecal marker. The onset of feed intake was evaluated for the first 66 hours post-weaning in each individual pig by rectal swab over seven time-intervals: 18, 26, 34, 42, 50, 58, and 66 hours. A red colored swab indicated the onset of feed consumption. This technique was followed as previously described by Bruininx et al10 and Sulabo et al11, who evaluated the percentage of pigs eating creep feed using fecal markers.

Ear and tail lesions

The presence of ear and tail lesions, which may be indicative of ear and tail biting, were recorded. Deeper lesions were considered, differentiating them from scratches. Observations were conducted by one veterinarian. Pigs with severe ear and tail lesions were removed from the pen but were included in the statistical analysis.

Growth performance

Individual pig weights on days 0 (weaning day), and 42 were used to determine the coefficient of variation (CV) within each pen. The total weight of pigs and feed disappearance of each pen were measured to determine average daily gain (ADG), average daily feed intake (ADFI), and feed efficiency (F:G). Growth performance data of the overall period were split into 3 periods of 14 days: period 1 (0 to 14 d); period 2 (15 to 28 d); and period 3 (29 to 42 d).

Pigs were removed from pens only upon death or if identified to be in a non-ambulatory condition and not responding to medical treatment. In such cases, adjustments were made at the pen level using adjustable gates to account for removed or deceased pigs and maintain the same floor space allowance of 0.23 m² per pig until the end of the trial.

Statistical analysis

Data were analyzed as completely randomized design using the GLIMMIX procedure of SAS software (Version 9.4, Institute Inc, Cary, North Carolina), considering pen as the experimental unit. Polynomial contrasts were implemented to evaluate the linear and quadratic effects of the dose-response (varying the number of pigs per feeder hole and pigs per pen) on average time of onset of feed intake, ADG, ADFI, BW, F:G, CV, mortality, and removal rate. For mortality and removal rate, a binomial distribution was fit to the data. The IML procedure of SAS was used to adjust linear and quadratic coefficients after accounting for unequally spaced treatments. The non-parametric NPAR1WAY procedure of SAS was used to analyze the percentages of ear and tail lesions, and groups were compared using the Kruskal-Wallis test. Results were considered significant at a P ≤ .05 and a trend at P ≤ .10.

Results

Throughout the experimental period, 20 pigs were removed, and 15 deaths were recorded; however, there was no effect (P > .05) of number of pigs per feeder hole on removals and mortality rate (Table 1).

Average time to onset of feed intake decreased as the number of pigs per feeder hole and the number of pigs per pen decreased (Linear, P < .001; Figure 1).

For period 1 (day 0 to 14), a decrease in the number of pigs per feeder hole by decreasing the number of pigs per pen resulted in a linear increase in ADFI (P = .02; Table 2). Variations in ADG and F:G and were not statistically affected by treatments (P > .05). For period 2 (day 15 to 28), ADG increased linearly (P = .01) as the number of pigs per feeder hole decreased to 3.75, or 15 pigs per pen. This was likely driven by a linear improvement in F:G as the number of pigs per feeder hole and pigs per pen decreased (P = .02). There was a tendency for a quadratic improvement in ADFI (P = .068) as the number of pigs per feeder hole decreased from 7.50 to 6.25 pigs per feeder hole (30 to 25 pigs per pen), with no improvement thereafter. During period 3 (day 29 to 42), none of the growth performance criteria were affected by treatments (P > .05). Overall (day 0 to 42), there was a tendency (P = .06) for a linear improvement in ADG as the number of pigs per feeder hole decreased from 7.50 (30 pigs per pen) to 3.75 (15 pigs per pen). There were no differences in ADFI, F:G, BW, and CV as the number of pigs per feeder hole and pigs per pen decreased (P > .05).

Percentages of ear and tail lesions are shown in Table 3. Pens with 3.75 pigs per feeder hole (15 pigs per pen) had no ear or tail lesions. Pigs in pens with 7.50 pigs per feeder hole (30 pigs per pen) had significantly more ear lesions than pens with fewer pigs per feeder hole (P < .05). Pigs in pens with 3.75 pigs per feeder hole (15 pigs per pen) had significantly fewer tail lesions when compared to the other 3 treatments (P < .05).

Discussion

In this study, the influence of variation in the number of pigs per feeder hole, by varying the number of pigs per pen while maintaining the same space allowance, on growth performance, the onset of feed intake, and incidence of ear and tail lesions was investigated.

A change in environment (eg, diet type, drinkers, cohort, etc) creates challenges to weaned pigs, especially relative to voluntary feed intake.12 Pens with 3.75 pigs per feeder hole, or 15 pigs per pen, had the lowest average time to the onset of feed intake after weaning and highest ADFI in the first 14 days post-weaning compared to the other treatments. These results agree with those from Weber et al13 who observed a tendency for improved feed intake as the number of pigs per feeder hole was reduced. A positive relationship between feed intake and villous height or villus to crypt ratio has been previously reported.14-16 Therefore, enhancing feed intake in the weaned pig is critical to overcome post-weaning challenges and prevent villous atrophy, reduce post-weaning diarrhea, and stimulate growth performance.2

In the present study, the difference between the two extreme treatments (3.75 and 7.5 pigs per feeder hole) was 3.75 pigs. In another study,13 in which the difference between the two extreme treatments (8.9 and 12.4 pigs per feeder hole) was similar to that used in the present study, no differences were observed in growth rate until the end of the 8th week after weaning, at a floor space of 0.26 m&sup2; per pig. It is possible that the small variation in the number of pigs per feeder hole between treatments explains the linear increase in ADG observed only during the second period as well as the linear trend of overall increasing ADG, as the number of pigs per feeder hole was decreased by reducing the number of pigs per pen. When a greater difference in the number of pigs per feeder hole between treatments was evaluated (9 and 18 pigs per feeder hole) in a wean-to-finish system, no differences were observed in growth performance in the first six weeks post-weaning.17 However, in the grower phase (7th and 8th weeks), the treatment of 9 pigs per feeder hole had higher ADG and BW compared with 18 pigs per feeder hole. Unlike the present work, these authors17 used a floor space of 0.30 m&sup2; per pig, which may explain that once the space allowance becomes a limiting factor for the animals (> 6th week), a greater feeder space becomes more determinant for improving growth performance.

Tail biting is a multi-factorial problem and factors that can induce frustration or psychological discomfort may trigger or intensify tail biting occurrence.18 Among these factors, limited space allowance,19 limited feed availability,20 increased number of pigs per feeder hole,21 and reduced feed space per pig22 are associated with aggressive interactions such as ear and tail biting. Delays in accessing feed are associated with stress and increased restlessness among pigs.23 Competition for food (such as access to the feeder) will increase the potential for some pigs to become frustrated, because they are not free to eat at desired times or to consume the desired amount of feed, hence leading to tail biting.21,24 It has been shown that five or more pigs per feeder hole are 2.7 times more likely to be subjected to or to perform ear and tail biting than pigs kept at a lower number of animals per feeder hole during the growing and finishing phases.19 In the present study, the treatment providing 3.75 pigs per feeder hole, or 15 pigs per pen, had no ear or tail lesions and was the only treatment with less than five pigs per feeder hole. Although more aggressive interactions among the pigs can be expected in large groups, the effect of group size on tail biting remains unclear.18,25 No effect of group size (22 vs 44 or 18 vs 108 pigs per pen) has been observed in docked pigs,18,26 but Kallio et al27 reported that groups with more than 9 long-tailed pigs were at higher risk of tail biting in finishing units. Although aggression and competition to access feed were not assessed in the present study, the dispute for feed access was shown to be more influenced by the availability of feeder space per pig than the total number of animals in the pen.28 We can speculate that the higher occurrence of ear and tail lesions in the treatment with 7.50 pigs per feeder hole was mainly associated with the increase in pigs per feeder hole since the space allowance was the same (0.23 m&sup2; per pig) for all pigs of the present study; however, a combined effect of the larger group size (30 pigs per pen) in this treatment cannot be discarded.

In large group sizes, social organization of the pigs may be altered, although the exact group size this occurs is unknown.29 Given that group size and number of pigs per feeder hole changed simultaneously across the treatments, it is not possible to separate the relative effects from each of these factors on the responses observed in this study. However, results from previous research provide some indication that may explain the dynamic of the effects on performance caused by these two factors. Spoolder et al 28 reported no effect on growth performance (from 36 to 85 kg BW) with group sizes of 20, 40, and 80 pigs per pen. Likewise, Wolter et al30 observed similar growth rates for groups of 25, 50, or 100 pigs in a wean-to-finish system. Evaluating the association between feeder space and group size, Turner et al31 reported that a reduction in feeder space from 4.25 to 3.25 cm per pig during the growing and finishing periods significantly decreased feed intake regardless of group size (20 vs 80 pigs per pen). The range of group sizes of the present study (15 to 30 pigs per pen) was relatively narrower than that used in the aforementioned studies in which the growth performance was not affected by the group size. It is important to note that, in the current study, although the number of pigs per pen changed along with the number of pigs per feeder hole, the stocking density was kept the same across all treatments using movable gates.

Reducing the number of pigs per feeder hole through a decrease in group size can be a valid strategy to improve growth performance and animal welfare when space allowance is restricted. Importantly, number of pigs per feeder hole and group size should be considered when planning the placement of pigs in nurseries. Further research evaluating the effects of number of nursery pigs per feeder hole under restricted space allowance while maintaining constant group size is warranted.

Implications

Under the conditions of the present study, decreasing the number of pigs per feeder hole through a reduction in group size while maintaining a consistent space allowance:

• Reduced the incidence of ear and tail lesions.
• Tended to increase growth rate during the overall period.
• Reduced the time to onset of feed intake and increased feed consumption in the initial phase of the nursery period.

Acknowledgments

Conflict of interest

None reported.

Disclaimer

Scientific manuscripts published in the Journal of Swine Health and Production are peer reviewed. However, information on medications, feed, and management techniques may be specific to the research or commercial situation presented in the manuscript. It is the responsibility of the reader to use information responsibly and in accordance with the rules and regulations governing research or the practice of veterinary medicine in their country or region.

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