29 September 2023
Why male piglets are castrated: boar taint (androstenone and skatole)
What is androstenone and how does it cause boar taint?
In pork, odour can be adversely affected by accumulation of high levels of androstenone and/or skatole. Androstenone is a testicular steroid (with no anabolic effects) and is described as having urine or sweat-like odour, although only some humans (from 30% to 60% depending on the country) are able to smell it. It is produced by testicular Leydig cells of sexually mature males, so their levels depend fundamentally on the state of sexual maturity of the animal at the time of slaughter and genetics variations. Once in the blood, the androstenone can visit three places, the salivary glands, the liver and adipose tissue. When the androstenone arrives to the salivary glands, becomes 3-alfa-androstenol and 3-beta-androstenol, being both pheromones that can also be found in humans (this proximity is the reason why some people smell the precursor). Three-beta-androstenol for instance contributes to axillary odour in humans and derivates of androstenol are found in black truffles, what is used as explanation of how pigs locate them deep in the ground.
As mentioned above, the androstenone can also finish in the liver, where is destroyed, or alternatively, can be storage in the adipose tissue due to its lipophilic character. In fact, it can be found in higher concentrations in fat than other steroid hormones. This is interesting for pigs, because a reserve to produce pheromones without a continuous androstenone testicular production is very useful. The problem is then for humans consuming this product if they are sensible to the odour, as specially when the fat gets warm, the intensity of the smell can be unbearable and the consumer can be lost forever.
What is skatole and how is it linked to androstenone?
Skatole, for its part, comes from the degradation of the amino acid tryptophan in the distal part of the large intestine, and its presence depends to a large extent on the food received during fattening and the environmental conditions of temperature and hygiene in the pen. As dirtier of faeces the pig, for instance, higher the risk of abortion of skatole through the skin. Skatole, if present, is detected by practically all consumers, and is associated with faecal or, to a lesser extent, naphthalene odour. Functions of skatole are not clear and it is probably related with gut and mental health, as tryptophan is in the gut precursor of important neurotransmitters and compounds that contribute to the health and welfare of the animal. When absorbed in the blood (through the gut or skin), should be metabolised in the liver, but its hepatic metabolism is inhibited by steroid hormones (including androstenone). Therefore, it is clear that it has a role in the sexual communication between pigs. In fact, as previously, due to its lipophilic nature, accumulates in the adipose tissue. So an increased concentration of androstenone in fat will be correlated with high levels of skatole.
Consumers detect boar taint in ranges from 0.5 to 1.0 ppm for androstenone and 0.2–0.25 ppm for skatole in fat. As mentioned previously, the castration of pigs seeks to eliminate the presence of androstenone and other steroid hormones that favour their metabolism in the liver, in the case of skatole, being the main reason why is performed.
Current legislation in Europe
How have animal welfare expectations influenced piglet castration practices?
Castration has been strongly contested by animal welfare organisations in several European countries. With some variation between studies, 40 to 50% of the consumers indicates awareness of castration of male pigs, while only 14 to 21% indicates that they are well aware of this practice. In fact, most of the citizens take for granted that when performed, the castration does not differ dramatically from the one performed in other domestic species, such as dogs (where anaesthesia and analgesia is ensured at all ages). Citizens and pork production chain as well as other societal stakeholders have demanded a ban on surgical castration without pain relief, and the European Commission set a deadline of 2018. However, while alternatives are applied in a number of countries, others have not adhered to this deadline. Finally, for some citizens of the EU, and this movement increases every year, castration and other so called "mutilations", such as tail docking and teeth clipping, are seen as violations of the animals' integrity and "naturalness".
What does Directive 2001/93/CE allow?
The current legislation on pig welfare in the EU was drafted at the end of nineteenth century and definitively approved at the beginning of two thousand years. At this time, there were different studies highlighting how piglets of less than 10 days old vocalised in front of a painful situation less than older piglets. One of the hypothesis considered was that in younger animals, as less tissue was affected in comparison to older ones, it was logical that the painful stimulus was less and for this reason the vocalisations (correlated with this less pain) would be also less frequent and of lower intensity. Accordingly, the European legislation (Directive 2001/93/CE) allows surgical castration without anaesthesia and analgesia in males before seven days of life, but not for older animals, where anaesthesia and analgesia are mandatory. Few years later of being published this legislation some studies demonstrated that the problem with piglets was not a different capacity of feeling pain but a different capacity of express it through vocalisations, as a piglet of 7 days old never will vocalises at the same level than one piglet of 15 days because this ability evolves during the first weeks of life. Therefore, nowadays exist a scientific consensus that an animal of 3 days old suffer at the same level than one of 15 days old when castrated, so both should be subjected to a proper anaesthesia and analgesia. Accordingly, some countries of the EU have already banned the surgical castration without anaesthesia and analgesia to any age of the piglet, as it is the case of Germany (2021) or France (2022). The future legislation on pig welfare in the EU, that is nowadays being discussed, will take the same direction.
How do castration practices and alternatives vary across Europe?
Application of analgesia is demanded by several quality assurance programs, e.g., in Belgium or Netherlands. Some countries have a longer tradition of using anaesthesia during castration, such as Norway (by the veterinarian), Sweden (by the farmer), and Switzerland (farmer/vet), while this practice was recently introduced in Denmark (2020, by the farmer) and Germany (2021, farmer). Production of boars is common in Spain, Portugal, Ireland, and United Kingdom, and since 2010, introduced in countries like the Netherlands, Belgium, and France.
Animal welfare considerations in surgical castration
What does research show about pain associated with piglet castration?
The European Food Safety Authority (EFSA) reported, based on numerous scientific references, that surgical castration is a painful practice, even if it is done before the animal is seven days old. The following indicators of pain have been described in association with castration:
- an increase in high-frequency vocalisations
- an increase in the hormones ACTH and cortisol, indicators of stress
- a decrease in play behaviour and activity
- less visits to the mammary gland
Therefore, it is a practice objectively considered as painful for piglets.
What are the limits of anaesthesia and analgesia?
It exists the alternative of providing anaesthesia and analgesia. However, castration of male piglets is predominantly done without analgesia and/or anaesthesia in the EU. Furthermore, effectiveness of pain mitigation interventions has been questioned for all methods of anaesthesia if not combined with analgesia. In the same line, analgesics given alone do not fulfil this requirement as they are mainly effective to mitigate pain post-surgically. It seems that some practices, such as local anaesthesia and inhalation anaesthesia with isoflurane, both combined with analgesic preventive treatment, could be considered as the best practices, as long lasting pain reducing drugs that could be effective during and after castration are not available for the use on piglets.
Of course, the use of anaesthesia and analgesia increases:
- the costs (some approaches talk about 2 euros per piglet)
- handling needs (personnel and time)
- depending on the type of anaesthesia, the risk of hypothermia (when is general) and to be crushed by the sow (specially if no time enough is given to the piglet for recovery, if they are very young or when long lasting drugs are used)
What health and recovery factors should be considered after surgical castration?
Apart of the effects of the anaesthesia, that can be also local (in this case some pain should be considered during the application of the injection in the testicular area), we should consider the effects of the castration by itself, even when is performed properly in the absence of pain. In this case, we need to remember that castration is a surgical procedure commonly performed in non-optimal hygiene conditions in very young animals (immune system in development) and results in open wounds. In fact, the mortality of castrated piglets is higher than non-castrated because of complications of the surgery or infections that can arrive to other tissues, such as arthritis or meningitis. Another effect that has been described is a lower antibody response to an immune challenge in castrated piglets compared to intact piglets due to the stress effect of castration, so this need to be considered in the vaccination plan of the farm. This effect on the immune system could last several weeks in some animals and the reduction in activity and feeding behaviour around the surgical procedure, especially when an efficient pain killer strategy is not followed, will produce an impact in the growing rhythm of the animals that never will be recovered if compared with an intact pig. Therefore, even when pain is treated, we need to take into account that a surgery is a surgery.
Alternatives to piglet castration: immunocastration, GnRH vaccine and entire males
Theoretically, surgical castration could be replaced by:
- chemical castration
- sperm selection
- intact male production
- vaccination against the GnRF (vaccination against boar taint)
Chemical castration: why is it not a serious alternative?
It is important to highlight the difference between vaccination against boar taint and chemical castration, as sometimes there is a misunderstanding in the use of these terms. Chemical castration consists in the application of chemicals in the testes, such as zinc or silver salts. This is painful and long lasting, so it should never be considered a serious alternative.
Sperm selection: why is it unlikely today?
Sperm selection consists of the sexing of the spermatozoa with the aim of producing only females. Flow cytometry is the methodology used to separate spermatozoa with X chromosome from those with Y. This method is based on the difference in the size of the DNA between the two chromosomes, which gives them a different electrical charge. Currently, few laboratories in the world are equipped with this system, which allows processing between 15 and 20 million sperm per hour. Therefore, a significant amount of time would have to be invested to produce a seminal dose with adequate sperm content. In addition, the sexing process can produce a decrease in sperm life expectancy and intrauterine instead of intravaginal insemination is needed. In consequence, today it is an unlikely alternative. In other words, the real alternatives are just the production of intact males and vaccination against boar taint.
Rearing intact males: management and welfare considerations
When we are talking about intact males, various problems that can occur under commercial conventional housing and management conditions are usually underestimated in their importance. Due to the increased formation of male hormones during puberty, aggressive confrontations are more frequent in intact males than in castrates, that may lead to more skin lesions. Particularly under husbandry conditions that are highly competitive due to a small number of feeding places or very limited access to feed or resting, increased aggression among intact males can be observed easily, which is more pronounced than in castrated males and females. A reduced frequency of aggression is observed when the litter siblings remain together in a stable group until slaughter (farrow-to-finish pens).
Another aspect of intact males is that in the course of puberty these pigs exhibit an increased sexually motivated mounting behaviour. These mating attempts are often accompanied by intense vocalisations of the pen mate being mounted, that sometimes can increase the risk of lameness in these animals. Another problem are penile injuries, that ranges from 3% to 82% in farms with intact males and is not seen in castrated animals, as they are not able to extrude their penis. In some animals very serious penile injuries with suppurations and necroses can be found. In this case, mixed housing together with females and a higher age at slaughter increases the risk of penis injuries. Penis injuries have been described in wild boars in the mating season. However, the occurrence of such injurious behaviour under natural conditions does not mean that it is also acceptable in farm animal housing, as these penis injuries are presumably painful.
It is important to remember that in natural conditions when males reach puberty, they leave the group (they've been preparing for this all their lives) and form in bachelor groups (groups of young males). Within these groups they will test themselves against others. If the conditions allow to do it, the more subordinate will show submission over the more dominant and when the mating season will arrive more extreme aggressions, such as penis bites will occur until males will broke the bachelor group to look for a couple. All this process if full of aggressions and damaging behaviour between males, and if the management and handling of these animals don't consider this in a production system and look for a way to reduce their consequences should not be considered an animal friendly alternative. In other words, the alternative to castration in pigs is not to do the same but with intact males, because the result will be far away from animal welfare for these males, even if the pain related with the surgical castration is avoided.
Immunocastration: how does vaccination against GnRF work?
Vaccination against boar taint is a practice that uses a vaccine against gonadotrophin-releasing factor (GnRF). It uses the natural immune system of the pig to form specific antibodies that bind and neutralise GnRF; thus hypothalamic-pituitary-gonadal axis is blocked and sexual steroids synthesis is effectively inhibited. In this way, the result is the animal becomes sexually inactive (if the vaccination arrives after sexual maturation) or even better, remains immature (if the vaccination arrives before the sexual maturation). In biological terms is just to stop the sexual maturation of the animal affecting the cascade programmed to begin the production of hormones. It becomes effective after the second vaccine injection, and is technically feasible in heavy pigs. It prevents most of the disadvantages associated with intact males, avoiding them to arrive to the puberty and all the damaging behaviour related with this.
There are however a number of remaining issues that should be solved:
- the security of the operators during the vaccination procedures
- the practical feasibility of the interventions on pigs that are raised in free ranging systems
- the practical feasibility of the interventions on pigs with heavy weights that might require a third vaccination
Vaccination against boar taint is applied to 5 up to 10% of the male piglets in a number of countries and as a routine in Latin America, and there is being held back in Europe mainly because consumer acceptance of this practice is being questioned by stakeholders in the pork chain. In general, immunocastrated male pigs exhibit similar meat quality to surgically castrated males and provide the possibility to have the feed efficiency of a intact pig during part of the growing period.
Impact on growth and performance
A particularly attractive aspect from the point of view of resource and environmental efficiency is that intact males have a stronger protein-anabolic metabolic state due to testicular hormone production and show a reduced ad libitum feed intake. Compared with castrates:
- Their carcasses contain more lean meat and less fat than carcasses of castrates.
- Feed conversion is about 10–15% better.
- Nitrogen excretion is reduced.
These effects are due to the fact that in entire males, in addition to androgens, oestrogens are also produced in the Leydig cells of the testes, which both reduce protein breakdown and promote protein build-up. Entire males are, therefore, superior in performance in terms of growth rate and feed efficiency compared to castrates and females. However, there are some products in which the presence of more fat is needed. In this cases, some form of castration is needed and is probably where the vaccination against boar taint is more useful, because allows the manager to have all the advantages of an entire male until the moment when it is needed to become the animal a castrated one.
Closing thoughts
Castration of pigs has been used traditionally to avoid boar taint, that have a high detrimental effect in the quality perception of consumers when they detect this smell in a product. In the same way, castration is considered a painful procedure by the scientific community, even in young animal. For different reasons, anaesthesia and analgesia is not very used in the EU, and only in few countries and in the last years, is mandatory and, in any case, the animals are not free of problems. At the end, with or without pain, animals are subjected to a surgery, that increases the risk of disease, immunodepression and mortality. The future for pig farmers in the EU will be to use a proper anaesthesia and analgesia protocol at all ages of animals or look for alternatives to surgical castration.
The two more feasible alternatives are rearing intact males and vaccination against boar taint. Rearing intact males are not free of problems, as animals become more aggressive around puberty, increasing the number of lesions, mounting behaviour, penis bites and stress associated with dominance-subordination in animals learning to be adults. To be considered an animal welfare friendly alternative, rearing intact males will need to manage all this kind of problems trying to reduce them to the minimal. Finally, vaccination against boar taint provides really a suitable solution, as the maturation of males, and their puberty, is avoided just with two injections that affects the sexual cascade of the animal. In addition, in terms of performance, the farmer can decide until when the animal can be an intact male and at which moment the behaviour of the male begins to be damaging for the co-specifics and should be immunised against the GnRF. However, this will not be a real animal welfare friendly solution without a good communication strategy to the consumer.
Key Takeaways
- Boar taint is the main reason for castration: Male piglets are castrated to avoid boar taint, caused by androstenone and skatole accumulating in fat, which consumers detect at 0.5–1.0 ppm and 0.2–0.25 ppm respectively.
- Pain management is essential in surgical castration: Research shows that piglets experience pain during castration, including at a young age. Anaesthesia and analgesia have complementary roles in managing pain during and after the procedure.
- Surgical castration also requires attention to health and recovery: Alongside pain management, hygiene, handling and post-procedure care are important considerations because surgery carries risks such as infection and other complications.
- Rearing intact males requires appropriate management: This approach avoids surgical castration, but housing and management need to account for behaviours such as aggression and mounting, as well as the risk of boar taint.
- Immunocastration offers an alternative for managing boar taint: Vaccination against GnRF temporarily suppresses testicular activity and can help manage boar taint and behaviours associated with sexual maturity. Practical considerations include vaccination timing, handling and operator safety.
