The stress of isolation from the individual husbandry of stallions can result in behavioral problems, aggression, and diseases of the respiratory, nutritional, and musculoskeletal systems. Several examples from practice show that the keeping of stallions in groups is possible in principle. It only poses a risk for injuries if the necessary requirements for this type of husbandry are not taken into account. If the size and design of the exercise area/pasture, the group constellations, and the characters of the stallions are considered, keeping stallions in groups represents the most species-appropriate form of husbandry for them. This takes into account animal welfare aspects and complies best with the requirements of modern horse husbandry. However, the integration of the stallion into an existing group should only be carried out by qualified, experienced horse owners, who must proceed professionally and step by step. Consequently, stress, disadvantages, and the potential for injury can be reduced to a minimum or, ideally, avoided altogether, and the wellbeing and mental and physical health can be supported in the best possible way.
The demand for keeping horses in groups is growing due to an increased social awareness of animal welfare . Owners are also aware of the importance of keeping horses in a species-appropriate manner for their wellbeing by keeping them as close to nature as possible, i.e., living in a herd and the natural environment. Horses are very social animals that live in groups in the wild, and husbandry methods in recent years have increasingly aimed at species-appropriate management in groups . By contrast, however, stallions are still kept in the traditional way  and, according to a survey from a province in Germany (Baden-Württemberg), in which 20 farms keeping breeding stallions were examined, 74% of the stallions are still kept individually in indoor stalls .
Problems occur due to this solitary confinement regarding the behavior and handling of the stallion , thereby influencing its wellbeing  and health . The guidelines for the assessment of horse husbandry under animal welfare aspects (Federal Ministry of Food, Agriculture, and Consumer Protection (BMELV) and the German Animal Welfare Act (TSchG) ) state that basically all horses are suitable for group husbandry regardless of their age, breed, sex, and type of use .
Furthermore, the ethical principles require that the husbandry of a horse should take into account its natural needs . This is still opposed by the outdated attitude that stallions have always been kept singly and the fear of injury when kept in a group due to the assumption of the dangerous nature of stallions among many stud owners . However, visual, auditory, and olfactory contact between animals is a minimum requirement in horse husbandry . Nevertheless, the issue of keeping stallions in a species-appropriate manner is currently of utmost relevance, as more and more stallions are being kept privately , for example, for baroque riding . Optimization of the type of husbandry is, therefore, the order of the day for the farms .
The results of this literature review facilitate the answering of the underlying questions well and in detail. It is indicated uniformly that the group husbandry of stallions is possible in principle and has already been practiced, even if so far only sporadically, in increasing instances in Germany and Switzerland. The study of Pollmann et al. already argued in 2003 for the possibility of keeping stallions in different group constellations. In 2020, Pinto and Hirata found out that some feral horse populations form multi-male groups independent of the size of the stallions . However, not only stallions of small horse breeds are currently kept in groups. In 2008, Briefer-Ferymond integrated four stallions as a pilot phase at the Agroscope (Schweizer Nationalgestüt, Switzerland), and there was also a warmblood horse in the group. For their point of view, it is not a question of size (personal communication, Briefer Freymond). The idea of many stallion owners that pony breeds are especially more compatible and less aggressive than large horses may have had an influence here. Schmelzer (1998)  also assumes that the nature of ponies is different from that of large horses and that pony stallions have been commonly kept in groups for much longer. By contrast, Irrgang and Gerken recorded in 2010 that almost one-third of all Arabians are kept in groups .
The authors pointed out that Arabians are kept in groups at a much younger age. Steiner , however, pointed out that this is traditionally done only in the summer months but not in winter. Here, the author continued by stating that they are also housed alone in individual stalls because of their assumed sensitivity. Winther Christensen et al.  studied social behavior in domesticated groups of 2 year old stallions in comparison to a mixed age group of Przewalski stallions. They found out that social interactions and agonistic behaviors were very similar . The nondomestic relatives showed even more social grooming than the homogeneous domestic group . In principle, it must also be possible to keep stallions of warmblood horse breeds in groups. Steiner  determined that just under 12% of stallions of various breeds are kept in groups in Lower Saxony. Nevertheless, Zilow  also stated that the keeping of stallions of different breeds is practiced in Bavaria for just under 23% of cases.
Although the prevailing assumption that stallions cannot be kept in groups may seem antiquated, it is, nevertheless, in the course of the implementation of animal welfare, linked to certain preconditions. Thus, group husbandry can only be successful if the stress and discrimination of individual horses are avoided as much as possible, and it must be based on the premise of preventing injuries. The individuality of the respective stallion and that of the other group members, as well as the conditions on the respective farm, must be considered. This includes, above all, sufficient space. This varies particularly strongly in practice . Different amounts of space are required depending on the group constellation and the varying need for individual distance of each stallion. Nevertheless, the design of the space available is also important. Dividing the space into functional areas prevents a concentration of horses and facilitates simultaneous care of all horses, which also minimizes the risk of injury. Dead-end situations must be avoided, and retreats for lower-ranking horses must be available . A sufficient number of separate feeding and drinking places, as well as lying and resting areas, must be available in order to avoid stress and injuries . This represents the needs of all horses regardless of their breed . However, the individuality of stallions should not be disregarded. Not every stallion is suitable for group housing. Thus, social shortcomings and a high level of aggression have the effect of preventing integration . By contrast, current breeding use is by no means an obstacle to group housing, as studies by Pollmann et al. , Irrgang and Gerken , Zilow , and Steiner  have shown. Lastly, this was also confirmed by the trial at the Swiss National Stud, where breeding stallions could be successfully integrated into a group , while no serious injuries could be detected. In addition, the aggression of the horses decreased significantly within 4 days. In principle, the authors assumed that an integration into a group is possible, since it is known from observations of wild herds that stallions there also live in the group, regardless of whether they are currently breeding or not. Furthermore, the researchers found that the individual age of the stallion is not decisive for the success of integration. Again, this result is not surprising when compared to wild horse herds.
Stallions, to which the opportunities for social interactions are given, may even increase reproductive efficiency . It could be beneficial if grouping took place outside the breeding season and mare contact was avoided during the grouping time .
Much more crucial, however, is the expertise of a farm’s manager and management . This must take the social structure and the compatibility of the individual horses into account  and carry out the integration gradually. Farms with a low turnover, in which the social structure can be kept constant and stable, therefore, have a favorable influence . Independently of the presence of a stallion, group stability prevents aggressive social interactions between members of the herd . Management plays a particularly decisive role in determining whether stress arises in the group and, thus, whether injuries are promoted or avoided.
If, in spite of everything, group husbandry is not possible, work must be done to ensure that the stallion is kept as species-appropriate as possible . It would be conceivable to keep the stallion with a conspecific in a common run during the day and a box at night . If this is also not possible, at least visual, auditory, and olfactory contact with conspecifics should be provided . Integration boxes in which the stallions stand side by side with the possibility of physical contact and withdrawal are also suitable alternatives . Paddock boxes with social contact would, nevertheless, be a species-appropriate accommodation option . Alternatively, an open stall in an open stable provides a separate independent unit for the stallion with an adjacent individual paddock and pasture a few meters away from the group paddock and pasture. Incompatible stallions can also be kept at a distance from the neighboring herd with a pasture. The so-called stallion star, on the other hand, facilitates more or less possibility of contact in its indoor stalls .
The group husbandry of stallions can function in the most different constellations, as proven in the respective studies of Irrgang and Gerken , Zilow , and, for example, Steiner . This is not surprising when observing wild horse herds and drawing comparisons from them. The work of Steiner  found that, when horses are kept in groups in stable systems, just over 63% of stallions are kept with one or more male horses and just under 37% of stallions are kept with mares in family groups. By contrast, when kept in groups on pasture, stallions are more often kept with mares and foals than other stallions and geldings. It is a better alternative to place high-ranking, dominant stallions together with calm geldings . Rank fights and competitive behavior with subsequent injuries can then be avoided as far as possible. It is also better to avoid contact with mares . Stallions can be kept with mares if the latter are in foal to the same stallion, are to be covered by the latter, or are not in estrus. They must always be separated at the onset of estrus . However, one must be aware that there is still a residual risk of unwanted pregnancies. It is also possible to keep stallions together if they are of different rank. Nature already provides this model as well .
The group size also does not play a role in whether the stallion husbandry is successful or not. However, it is known that the social memory of horses holds about 20 members at most. In other words, horses can easily interact with up to 20 herd members and are able to assign traits and characteristics to them . If this size is exceeded, the herd tends to divide, possibly causing space problems. Therefore, group sizes of 10 to 12 horses are ideal .
If a horse is unable to meet its natural needs, undesirable behavior is more likely to occur . Spending daily free time in a paddock has a positive impact on the horses’ welfare by lessening stereotypic behaviors and increasing oxytocin levels in their blood . This is independent of gender. Irrgang and Gerken  and Zilow  found in their work that about half of the stallions in solitary confinement showed behavioral problems. Boredom and stress were given as possible causes for this. It is also known from a study by Bachmann  that behavioral disorders occur significantly more frequently in solitary confinement. Nevertheless, diseases of the respiratory, digestive, and musculoskeletal systems are more noticeable in individual husbandry , which can also be due to a higher stress load, poor stable climate, and lack of exercise. It is, therefore, not surprising that group housing had a particularly favorable effect on the healing of diseases of the respiratory and musculoskeletal systems in a study by Szivacz .
In an experiment on the effects of social environment and training on the human–animal relationship, group-housed horses responded clearly better to training than singly housed horses . Additionally, singly housed horses bit the trainer more frequently than did group-housed horses . This also demonstrates the benefits of raising young horses in groups for human’s safety.
Weaknesses of the present work are due to the fact that only one scientifically constructed integration trial of stallions in the off-season was available for review. However, some good-quality studies could be found, which independently came to the same conclusion that keeping stallions in groups is possible in principle. Therefore, these positive tendencies should be scientifically investigated by future work and, thus, be proven. For this purpose, surveys of stallion owners to assess the success of group husbandry could be examined. It is also important to differentiate the effect of different group compositions of stallions (i.e., age, breed, and range) to determine advantages and disadvantages and, thus, develop recommendations for species-appropriate stallion husbandry.