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Entire Male Pig Production: Welfare Management Issues

17 August 2012, at 12:00am

A report investigating the existing and potential ethical impacts of rearing entire male pigs, and examining ways that welfare can be enhanced in the UK pig industry by Ruth Clements, MRCVS, of the Food Animal Initiative (FAI).


Ruth Clements, MRCVS
Food Animal Initiative (FAI)

1. Introduction

Overview

This review has been commissioned to investigate the existing and potential ethical impacts of rearing entire male pigs, and to examine ways welfare can be enhanced in the UK pig industry.

The document will first give an overview of relevant background information concerning UK pig production to allow the latter sections to be put in to context. Following this, Section 2 will consider normal piglet maturation and the development of pig behaviour with emphasis on social, aggressive and sexual behaviour in growing pigs.

Sections 3 and 4 will highlight how this developing behaviour manifests in terms of specific economic and ethical impacts.

UK pig production methods and husbandry

Pig production generally falls into three categories: producers who specialise in breeding (sow herds), those who specialise in rearing of piglets (weaner or finisher herds) or those who combine the two (farrow to finish herds.

Piglets are generally weaned at around 27 days of age (BPEX Pig Yearbook, 2011) and placed in weaner accommodation. From this point, various housing and production methods are used but most commonly weaners are transferred to finisher accommodation at a set weight or age. Pigs are either sex-segregated at a particular stage or left in mixed sex groups for the whole production cycle. After a fattening phase, pigs go direct to slaughter at around 100kg. In other EU countries, pigs are kept until much higher slaughter weights, typically between 110 to 120kg, although there are specific examples where they are reared up to 160kg.

A divergence in marketing, pork product development and consumer preference between countries, or even regions, has led to this difference in finishing weights. The UK’s lower average slaughter weight enabled the UK to move to a high welfare, non-castrating system with low taint. However, as UK slaughter weights increase for economic reasons, welfare and taint levels become a concern. In many other European countries or regions they continue to castrate, enabling producers there to finish pigs at heavier weights, typically 100 to 160kg.

Although there are extensive national differences in housing of pigs in the weaning and finishing phases, the majority of pigs in the EU are kept on fully or partially slatted flooring systems and fed commercial concentrate diets. In the UK, this is still the case, although there is an increased interest from consumers and retailers in pigs reared in enriched environments such as straw yards.

In the EU, the abnormal behaviour of tail biting remains a significant welfare problem in pig production and thus the majority of producers carry out tail docking in piglets under seven days of age in an attempt to reduce the incidence. It is common practice to carry out castration alongside tail docking, both procedures being carried out legally without the use of anaesthetic or analgesic in piglets of this age. Both these procedures are considered mutilations and, as such, their necessity has been the basis of extensive review by legislative, producer and lobbying organisations (EFSA 2004). Focus is currently on the sow stall ban coming in to effect in January 2013, and compliance with existing legislation on provision of manipulable materials. Also under consideration is the proposed total ban on castration by 2018.

There is no doubt that surgical procedures such as castration, when performed without anaesthesia, are the cause of considerable pain regardless of the age at which they are carried out (Prunier et al., 2005). However,there are concerns that raising entire male pigs in some of the systems currently in existence may in itself constitute a significant welfare problem.

2. Normal Pig Maturation and Behaviour


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"Where normal behaviour is carried out at an increased frequency and becomes problematic, this is often a result of increased social and body contact with other pigs, a lack of environmental enrichment such as manipulable materials and high competition for resources such as food and water."

Piglet maturation

Domestic pigs have inherited extensive social and hierarchical traits from their ancestors. This can first be observed on the day after birth among litter-mates as a more or less stable ‘teat order’ develops, representing a social dominance order between piglets (McBride, 1964). The strongest piglets gain access to teats with the most milk, with weaker litter-mates relegated to teats with lower milk yield. During feeding bouts, piglets will return to the same teats once this dominance order has been established (McBride, 1964).

In domestic pigs, the prepubertal period can be considered to be between three and five months of age with sexual maturity occurring between sevenand nine months; considerable variation to these figures has been observed due to influence of genetic, social and environmental factors (Kirkwood 1984).

Social (including aggressive and sexual) behaviours change during sexual maturation (puberty) and are particularly apparent in entire males compared to females and castrates (Fredriksen et al., 2004).

Social behaviour

Pigs in semi-natural environments form matriarchal social groups consisting of closely related sows and their piglets (Stolba and Wood-Gush, 1989), while males tend to lead a more solitary existence, generally entering groups only for the purpose of mating.

Commercial pig systems can place pigs in challenging social situations and lead to a range of abnormal, or normal but unwanted behaviours.

Where normal behaviour is carried out at an increased frequency and becomes problematic, this is often a result of increased social and body contact with other pigs, a lack of environmental enrichment such as manipulable materials and high competition for resources such as food and water.

Aggressive behaviour

Aggressive behaviours can be observed in pigs of any age, beginning in the neonatal stage when teat order is established. In general, aggression can be observed in two scenarios – firstly, during the mixing of unfamiliar pigs when there can be a period of intensive fighting as a social dominance is established, and secondly, there can be a longer term competition over feed or other resources (Fraser, 1984).

The first type of aggression is infrequently observed in small established social groups – up to 25 pigs – while some studies have reported that it is more difficult for pigs to maintain a social structure in very large rearing groups, when pigs are frequently mixed or when weight difference between pigs are small (Andersen et al., 2000).

Social hierarchy in a group of pigs predetermines access to resources such as food, water and favourable lying areas, through use of threat, submission and avoidance (Giersing, 1996).

In general, aggressive behaviours are increased in entire male pigs during and after puberty (Cronin et al., 2003), and are also increased during mixing and moving of these animals (Fredriksen et al., 2004). Various studies have indicated that this increased aggression is stimulated by testicular steroid hormones (Ford, 1990), and that these behaviours themselves stimulate an increase in plasma testosterone, forming a positive feedback system between hormone levels and aggressive and sexual behaviour (Fredriksen et al., 2008).

Some studies have proposed a relationship between aggressive behaviour and lean growth (Schinkel et al., 2003.) This suggests that the extensive selection that has occurred in favour of rapid and lean growth may favour the production of aggressive pigs, and that these individuals may be less suitable for rearing as entire males.

Sexual behaviour

During the first three weeks after birth, serum testosterone and oestrogen concentrations are elevated in boar piglets, after which they show an increase in prepubertal mounting activity and greater social interactions than do gilts or boars castrated at birth (Berry, 1984). This behaviour peaks at around two months of age and then declines to a low frequency in prepubertal pigs (Ford, 1990).

The pubertal rise in testicular steroid secretion begins by three months of age continuing through six months of age, and is associated with de-feminisation of sexual behaviours in boars (Ford, 1989). There is little indication of a masculinisation of sexual behaviours in pigs. This hormonal process correlates with an increase in aggressive and sexual behaviours seen in entire males during and after puberty (Cronin, 2003.)

Sexual behaviour such as mounting is part of pigs’ normal behavioural repertoire (Hemsworth and Tilbrook, 2007) but can be problematic when in the context of many commercial housing systems, which place pigs in close contact, give them minimal enrichment and frequently house them in large groups.

Entire male pigs have been shown to spend an increased amount of time engaging in social, aggressive and sexual behaviours and less time resting and feeding when compared to females and castrates (Fabrega et al., 2010; Rydhmer et al., 2010). Fredriksen et al. (2004) observed almost no sexual behaviour (mounting) among females and castrates during the last few weeks before slaughter.

Sexual behaviour has been shown to occur in some studies at a greater frequency when pigs are housed in single sex pens (Salmon and Edward, 2006) and in others at an equal frequency when housed in single sex or mixed sexed pens (Andersson, 2005). Male pigs may be as likely to mount other male pigs as they are to mount female pigs (Rhydmer, 2006).

3. Economic Implications of Commercial Entire Male Pig Production

Growth rate and feed conversion ratio (FCR)

Growth rates and feeding behaviour of entire male pigs individually housed are well understood. In general, they exhibit faster live weight gain (LWG) and more efficient, leaner growth than castrated males (Campbell & Taverner., 1988). In particular, the endocrine system is thought to play a major role in the expression of the growing pigs’ potential for protein growth. Castration reduces the ceiling for protein deposition by 30 per cent (Campbell & Taverner, 1988).

Commercial group-housed pigs (of any gender or genotype) tend not to reach their physiological or genetic potential achieved by individually housed animals due to confounding factors such as space allowance, social dynamics and group size.

Group-housed entire males tend to exhibit more efficient growth pre-puberty than castrates, however, the above confounding factors become particularly important as they progress through puberty. Entire males become less interested in feed and more interested in social and sexual behaviour (Velarde, 2007).

Male behaviours in entire pigs during and after puberty have a negative effect on feed intake and hence growth rates. Entire males may be more easily distracted from feeding behaviour, have a reduced feed intake and spend more time in social behaviour including aggressive and mounting behaviour (Cronin, 2003). Males performing much increased sexual behaviour may grow more slowly as a result of their higher energy requirements or their lower feed intake (Rydhmer, 2006). Subordinate pigs that are frequently displaced from feeding by aggressive males, may have reduced feed intake and growth (Schmidt, 2011).

4. Ethical Implications of Commercial Entire Male Pig Production

Behaviours

As a result of changes in behaviour seen in growing entire male pigs, other individuals in groups can be exposed to or be on the receiving end of agonistic behaviour.

Sexual behaviour may disturb not only the one being mounted but all pigs in a pen (Rydhmer, 2006).

In addition to aggressive and sexual behaviours, entire male pigs have been shown to perform more non-violent social interactions and spend less time resting than castrates (Rydhmer, 2010). This has been proposed to contribute to an increased level of general disturbance in a pen.

Several studies have shown that pigs show less aggressive behaviour, walked less and slept more with increasing age. Cronin (2003) observed a lower activity level in entire males at 21 weeks than 17 weeks but no difference in frequency of aggressive behaviour.

Husbandry

Several studies have considered the impact of mixed- or single-sex groups on behaviour of entire males. Salmon and Edwards (2006) found that entire males performed more sexual behaviour in single-sex pens than in mixed pens, while Kaminder (1991) observed the opposite in an outdoor production model. Boyle and Bjorklund (2007) claim that a higher rate of agonistic interactions in single-sex groups compromised the welfare of males in these groups while in contrast, Rhydmer (2006) doubts that rearing pigs in mixed-sex groups could reduce aggression in males, concluding that males probably mount other pigs regardless of sex and weight.

Pigs have been shown to exhibit higher levels of aggressive behaviour when they are uniform in terms of size or weight (Andersen et al., 2000). Removing some pigs – for instance, split marketing of heaviest pigs to go to slaughter – can increase aggressive interactions in a pen as the animals re-establish a dominance order (Fredriken, 2008). This process may be accentuated in groups containing entire males (Rydhmer, 2006).

It is also likely that limiting factors in the rearing environment, which create competitive conditions, become more critical due to higher boar aggression (Velarde, 2007). Cronin (2002) suggests that housing should be adjusted for the rearing of entire males by increasing feeder places or feed flow rate, or using visual barriers in a pen to reduce the opportunity for social interactions.

Fredriksen (2007) investigated rearing entire male pigs in sibling groups (in farrow-to-finish pens) and found that this strategy could reduce the frequency of skin wounds to levels similar to those of castrates and may be an appropriate management strategy.

Oliver (2008) found that enriching the environment can improve welfare in all classes of pigs by reducing skin lesions and probably levels of chronic stress. Averos (2010) found that time engaged in negative social behaviours decreased in the presence of point source objects, such as chewable rods, and bedding substrates.

Injury

Some consequences of sexual and aggressive behaviour on welfare include an increased risk of leg problems (from mounting behaviour) and skin lesions (Ryhdmer, 2006). In Ryhdmer’s study, pigs mounting and being mounted had more skin lesions than those not involved in mounting. The incidence of leg problems caused by persistent mounting is inconsistent among studies, with some reporting problems (Rhydmer, 2006) and others not identifying this as an issue (Fredriksen, 2008).

Pregnancy

Rydhmer (unpublished results) examined the ovaries of female pigs at slaughter, and found 29 per cent to be sexually mature. Rearing entire males with sexually mature females raises the possibility of pregnancy in females pigs intended for slaughter (Andersson, 1999).

Worker welfare

There is no scientific documentation to show that entire males at a normal slaughter weight (90 to 120kg) are more aggressive towards humans and thus pose any increased risk to handlers either on farm or at the slaughter house (Velarde, 2007).

Transport, lairage and meat quality

Entire males have been shown to have an increased number of skin lesions at the slaughter house than castrated males (Fabrega, 2010) and females (Velarde, 2007). Mixing of unfamiliar pigs at the slaughter house causes more skin lesions in entire males than in females (Andersson, 2005).

Pigs selected for high lean tissue growth were shown to be more aggressive during transport than pigs selected for low lean tissue growth rate (Shea-Moore, 1999).

In general, fighting can lead to deterioration of meat quality measured by increase in the incidence of dark, firm and dry (DFD) meat (Wariss, 2000). Fighting sometimes results in only superficial skin blemishes but in other cases, it can cause major carcass bruising and consequently financial losses to the meat industry (Velarde, 2007). Pigs with skin damage due to fighting have progressively higher levels of cortisol, CPK and lactate in blood at slaughter (Faucitano, 2001), affecting the quality of meat.

D’Eath et al. (2009) concluded that mixing pigs at the slaughter house with an above-average propensity towards aggression could be an important determinant of increased stress and inferior meat quality. Chemical changes in the carcasses of animals having undergone stress can be a cause of poor meat quality in terms of PSE meat (pale, soft, exudative) or DFD meat.

5. Summary

  • It is well established that castration of male piglets causes pain, and is therefore a welfare issue in the UK. Rearing of entire male pigs brings its own challenges in terms of economics and welfare.
  • Entire male pigs have a physiological capability for higher and leaner growth compared to castrated males or females. When reared under commercial conditions, this potential is rarely realised due to increased aggressive, sexual and social behaviour and reduced feeding behaviour.
  • Modern commercial pig systems put pigs in challenging social situations and can result in range of abnormal, or normal but unwanted behaviours, leading to welfare concerns. Rearing entire male pigs in these systems increases the challenges in terms of levels of aggressive, sexual and social behaviour for all pigs in the pens.
  • Normal social and sexual behaviours observed in entire male pigs become problematic, particularly when these animals are reared in high density environments, with little enrichment and high competition for resources.
  • Aggressive and sexual behaviour leads to injury of pigs both on farm and at the slaughter house, including skin lesions, bruising, locomotor problems and stress.
  • Rearing entire male pigs alongside gilts has the potential to lead pregnancy and the subsequent slaughter of pregnant gilts.
  • Rearing and handling entire male pigs poses little additional threat in terms of worker welfare.
  • High levels of stress and fighting during transport and in slaughter house lairage, have the potential to affect meat quality in terms of direct damage to the carcass and via detrimental chemical changes within the meat.

6. Conclusions

UK pig producers have made significant progress on various welfare and production related issues over the past decade but problems associated with rearing entire male pigs have received little focus.

There are clearly a range of potential adverse behaviours and consequences associated with the rearing of entire male pigs:

  • mounting of both males and females
  • fighting and tail biting
  • skin wounds and associated pain, and
  • stress.

The occurrence and severity of these problems is likely to be associated with factors such as stocking density, genetics associated with high lean growth, environmental enrichment and provision of manipulable materials.

A range of potential solutions can be suggested which could allow for possible advantage in terms of welfare of piglets (no castration), and the potential for better growth characteristics seen in entire males over castrates:

  • selection of genetic lines with reduced aggressive tendencies
  • modification of housing environment to reduce the level of social contact
  • provision of manipulable materials which allow natural rooting behaviour, and
  • temporary reduction of testosterone in entire male pigs during puberty through an Improvac® welfare management programme.

Producers may be able to achieve significant benefit both in terms of welfare and productivity by considering a combination of these solutions. An appreciation of individual circumstances would be required to ascertain the usefulness and practicality of any strategy.

References

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The report was sponsored by Pfizer Animal Health.

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August 2012