In Europe, many citizens are concerned about the impact of intensive production conditions of farm animals on animal welfare and the environment. A critical evaluation of the aspects which cause public disapproval is necessary and sustainable improvements have to be introduced, where negative conditions can be avoided. A main problem is that conflicting aims may occur which must be balanced with different market needs as well as stakeholder requirements. Such a situation currently applies in Europe in the debate about castration of male piglets . Surgical castration is painful and hurts the animals’ integrity, it is therefore a major welfare issue . For centuries, male piglets designated for pork production have been surgically castrated in Europe to improve behavior and product quality. The fattening of boars has advantages such as requiring fewer resources to produce the same amount of meat due to more efficient feed conversion ratio, reduced nitrogen excretion, and a higher protein accretion compared to barrows and gilts. However, disadvantages in pork production with boars remain, including boar taint and welfare problems due to increased aggression and mounting behavior. Those may limit the acceptance of pork production with boars by farmers, the meat industry and consumers. In boars, a sex-specific off-odor of the carcass may develop in some growing boars during puberty, which can be mainly ascribed to two substances, androstenone and skatole. Androstenone is a male pheromone which is formed in the Leydig cells of the testes and has a urine-like smell. Skatole is a metabolite of the amino acid tryptophan with a fecal odor and is synthesized in the colon by microbial degradation]. Boars may accumulate more skatole than barrows or gilts in adipose tissue because the hepatic degradation of skatole is reduced, due to lower activities of CYP2E1 and CYP2A enzymes if concentrations of androstenone or testosterone are high. A cross-national European study by Walstra and co-authors showed that while 29% of the carcasses reveal high androstenone concentrations, only 11% show elevated skatole concentrations and that slaughter weight and genotype modify this percentage considerably (high androstenone level: range 18–42%, high skatole level: range 5–23%). Androstenone and skatole are perceived differently by consumers depending on individual sensitivity. Whereas most consumers are sensitive to skatole, Lunde and co-authors described a specific anosmia for androstenone, which explains the variable percentage of consumers perceiving androstenone. Both compounds share the disadvantage that consumers who are sensitive to the substances rate them as very unpleasant and may therefore reject pork from boars more often. In addition, aggressive and sexual behavior of boars may lead to animal welfare problems such as leg weakness or penile injuries.

Growth is the result of a predominance of anabolic over catabolic metabolic processes. In pigs, it is orchestrated by the activity of sex-independent anabolic hormones such as growth hormone and IGF-I, and of catabolic hormones such as glucocorticoids. Gonadal hormones in boars already interact during the fattening period, with the endocrine regulation of growth by decreasing catabolic processes (e.g., androgens) or increasing anabolic processes (e.g., estrogens via stimulatory action on IGF-I secretion). This leads to more efficient growth, increased nitrogen retention and higher protein accretion rate when compared to barrows . Both androgens and estrogens also decrease the voluntary feed intake and improve the feed conversion ratio, explaining the known differences between barrows, gilts, and boars. Taken together, all those factors lead to higher muscle and lower fat deposition. Thus, boars have a higher lean meat content than barrows. Moreover, boars have a higher concentration of polyunsaturated fatty acids (PUFA) in adipose tissue, which may be healthier for human consumption but is regarded as a problem for processed meat products. Therefore, boars are more efficient in the fattening period but create problems in terms of product quality and animal welfare.

Surgical castration is effective in preventing those problems, but also removes the anabolic advantage of boars. In most European countries, surgical castration of male piglets without anesthesia or analgesia is still permitted within the first seven days of life. Usually, farmers castrate male piglets without any pain-relieving methods. Already in October 2010, representatives of major stakeholders committed themselves to a roadmap to voluntarily end surgical castration of male pigs in Europe by 1 January 2018. Despite this commitment, about 63% of all male piglets in Europe were still surgically castrated in 2017, most of them without any pain relieving methods or anesthesia even though alternatives exist. Today, these figures have not changed considerably in Europe.

Immunocastration is an active immunization against GnRH and could be a sustainable alternative to solve the above-mentioned problems of pork production with boars and surgical castrates, and thus could make European pork production more competitive. In contrast to other parts of the world such as Australia and Brazil, immunocastration in Europe is not frequently used, with a low percentage of 2.8% in 2017. Little practical experience of stakeholders and no targeted communication about the consequences of immunocastration of European pig genotypes for management, feed requirements, and product quality exist in the market. These knowledge gaps may explain why a method that might have economic, ecological, and societal advantages still has no market relevance at the moment. Market acceptance could be increased, if the sustainability of immunocastration is scientifically demonstrated. In order to evaluate immunocastration from a sustainability point of view, this review examines factors which are part of the three pillars of sustainability (society, economy, and environment) and how their interactions can lead to both synergic and conflicting relationships within the three pillars.

Immunocastration is an active immunization against GnRH, a key hormone of the endocrine cascade regulating reproductive functions. In consequence, the pig’s immune system starts the production of antibodies against the hypothalamic hormone GnRH and postpones the pubertal development by suppressing the hypothalamic pituitary gonadal axis . The treatment consists of at least two injections of the vaccine Improvac^® during the fattening period. The manufacturer recommends vaccinations at an age of about 12 weeks and again at 4–6 weeks before slaughter. Within a time interval of at least 4 weeks between the first and second vaccination. After the first application of Improvac^®, some GnRH antibodies are already formed but their concentration is not sufficient to limit gonadal axis activity. Within one week after the second vaccination, the production of GnRH antibodies increases markedly and suppresses testicular steroid synthesis and in consequence spermatogenesis. The drop in testosterone and estradiol concentrations occurs within a week, followed by a reduction in IGF-I secretion. Due to the lack of testosterone in the hormonal feedback system, both immunocastration and surgical castration initially increase the release of GnRH by the hypothalamus but lead to a reduced GnRH synthesis in the hypothalamus further on ]. In Europe, only one product (Improvac^®) is available to date for immunocastration of male pigs. The vaccine is manufactured by Zoetis (formerly Pfizer Ltd., formerly CSL Limited, Parkville, Victoria, Australia) and has been approved by the European Commission in May 2009 for use in pigs within the European Union. Due to the strong interlinked regulation of boar taint compounds and testicular hormones, immunocastration always affects their formation in a similar way. Thus, the only way to maintain the anabolic advantage of boars is the appropriate timing of the antibody formation leading to a tailored cessation of the testicular steroid synthesis. This avoids the accumulation of boar taint till slaughter, although the anabolic effects of testicular hormones are still maintained during the main part of the fattening period. Active immunization against GnRH was already discussed in the 1970s as a potential means by which the reproductive system of mammals might be shut down for various practical and clinical reasons . In 1998, a patent (International application number: PCT/AU1998/000532) has been submitted and was published under the international publication number WO 1999/002180 (21 January 1999; Pfizer Inc. New York, NY, USA). As GnRH itself has no immunogenic effect and does not stimulate antibody production, a proprietary strategy must be used to deceive the immune system and recognize GnRH as an antigen. This strategy includes the use of GnRH or a modified GnRH (truncated or repeated, with or without amino acid substitution) as antigenic target linked to a carrier substance . Antigens which are conjugated through its C terminus seem to produce a higher specific antibody response than constructs in which GnRH is conjugated through its N terminus. In case of Improvac^®, the antigenic part of the construct is the C-terminal fragment of GnRH (AS 2–10) conjugated to a diphtheria toxoid and adsorbed to DEAE-dextran (Patent US 8741.303 B2; 3 June 2014; McNamara).