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Introducing a New Horse to the Herd

Here's how to reduce the risks of introducing new horses into established herds.

Bringing a new horse into your pasture is sure to disrupt the herd.

If you've ever watched an unfamiliar horse try to gain entry into an established herd, you will probably never forget it. The violence of the herd members as they pursue the newcomer can be frightening. And yet this is all completely natural behavior that, if you heed the whys and whats of herd dynamics in managing the introductions, needn't cause more than passing disruption.

In Search of Stability
As commodities and competitors, horses have always been subject to relocation. Now, in our increasingly mobile society, we move from place to place, dragging our horses along with us and expecting them to adjust to new surroundings with ease. But horses place considerable trust in their herds and their "home territories." Imagine how it feels for a horse to be uprooted from a place of security and plentiful food, of preferred associates, as horse friends are called, and a well-defined place in the hierarchy, to be dropped somewhere completely unknown to him, where at worst a hungry carnivore could already be stalking him and at best a herd of venom-spitting horses stands just across the fence.

The outsider is not the only one affected by the change: During the several weeks following the introduction of a new herd member, the other horses have to redefine their hierarchy to make a place for the stranger. This momentary uncertainty in the social rankings may be the perfect moment for an ambitious young horse to challenge the system, or it may be such an unsettling time that usually docile horses battle fiercely to protect their long-held social rank. Not only does the risk of injury skyrocket, but the turmoil, in general, can be quite stressful.


As with people, stress can debilitate horses and make them vulnerable to infectious illnesses. The risk of viral and bacterial diseases greatly increases in mobile populations of horses. The newcomer might bring in a disease the other horses have no immunity to or inoculation for, or the herd members and environment might harbor infectious organisms against which the new horse has no protection.

The Gender Factor
One of the most obvious results of domestication is the introduction of a new gender of horse: the gelding. Yet, says Katherine Houpt, VMD, PhD, domestic herds containing a mix of geldings and mares may faithfully enact the instinctive sexual displays and behaviors seen in truly wild herds. "Sex isn't a big issue in the day-to-day life of feral horses," explains Houpt. "In the wild, all horses are rendered neutral by the fact that most of the time the majority of mares are pregnant."

But unbred domestic mares spend a good part of the year cycling in and out of estrus, and in mixed-gender herds, sexual pressures may heighten an established group's hostility toward outsiders. "So many geldings don't realize that they are geldings," Houpt says. "The main problem is not that they are showing sexual behavior but that they are being aggressive to other males. If a new gelding shows stallion-like behavior when he's introduced to a herd, that may cause him to be rejected."

And then there's the not insignificant concern about the physical hazards to battlers and bystanders posed by these stallion-like fights.

For the good of the entire herd, a sexually aggressive gelding is best kept only with other geldings and away from mares. "The gelding's sexual behavior is both innate and learned," says Cynthia McCall, PhD. "If a gelding was gelded a little late, say after four or five years of age, he might herd mares, fight with other geldings and mount mares."

Aggression may be present in single-gender herds. Mares may threaten each other to establish dominance but usually stay relatively calm. Geldings will play rough, even when kept apart from the mares but they usually aren't a serious danger to each other. If necessary, one quiet gelding can be kept with a herd of mares without causing a problem, and of course, a stallion can be kept among mares, with the obvious consequence.

The Cost of Confinement
Although their wild ancestors had unlimited acreage to roam, domestic horses are confined to much smaller areas. Their nutritional need to roam may be reduced because food is there for the taking, but the mental need remains. Freedom of movement is essential to horses' physical and mental well-being, and entrapment in small spaces is a naturally fearful situation for them, as you may have discovered when a cornered horse decided to escape the threat by running around or over you.

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