Many years ago, my partner was called out to a farm to see why two horses had suddenly died. What he found was heartbreaking—neighbors had dumped a copious amount of grass clippings over the fence and the horses happily indulged to the point where the grass fermented enough to rupture their stomachs, which resulted in death.
There are many good reasons why neighbors and strangers should not feed your horses. They might not have any knowledge of horse physiology, so they won't know what horses need to eat and what should be avoided.
Some horses are on strict dietary restrictions due to endocrine and obesity issues and laminitis risk. Oddball offerings pose a risk for colic or choke. A horse might be aggressive in how it takes an offering, which could result in a person getting bitten and causing potential liability concerns for you. Horses also can get into skirmishes as they jockey for position around a person offering handouts over the fence, leading to horse or human injury.
So, how do you protect your horses from unwanted consequences of well-intentioned people?
First, talk to your neighbors and educate them how feeding horses can endanger the animals' health. Use the scenario above from a veterinarian to support your case. If there is something you allow neighbors to feed the horses (i.e., peppermints or specific horse treats), make sure they don't overdo.
The primary strategy for preventing strangers from feeding your horses relies on appropriate and well-positioned signage. If you have road frontage, post several signs so they are visible not only all along that fenceline, but can't be ignored. Cyclists and tourists often want to stop and "pet the pretty horses," and they will unfortunately feed them whatever is in the car or on their person.
Electrifying the fence line (with appropriate signage) keeps people a distance away. In extreme cases, you might need to double fence horses away from road frontage so people can’t reach the animals directly. But that doesn’t stop people from throwing things over the fence.
If you suspect a person is violating your requests to leave your horses alone, you might have to monitor the fence (in person or with cameras) and confront the individual directly. It can help to place a sign or twothat states that there are cameras recording them.
"No trespassing” signs might help ward off less-motivated individuals. Also, posting "please don't feed the horses" signs can help people understand that it's not just trespassing you are forbidding, but the interaction between their food and your horses.
In difficult cases, you might have to contact the sheriff for advice and help in preventing trespassing and potential harm to horses or humans.