In farming and ranching, the verb geld means to castrate — or remove the sexual organs of — a male animal. A farmer might geld a horse so that he will be calmer and easier to ride.
People who raise animals choose to geld them for various reasons. If a male animal is not going to be bred, or deliberately mated to produce offspring, it is often easier to manage him without his powerful hormones. When you geld an animal, he tends to be more mellow and well-behaved, making a gentler horse or donkey as a working animal or pet. The Old Norse root of geld is gelda, "castrate," from geldr, "barren."