For hundreds of years, farmers around the world used draft animals for transportation and field work. These typically included horses, mules, or oxen, as depicted in this postcard printed in Germany.

Draft animals reduced the need for human labor, and when teamed up -- like the pair of mules shown here -- they could haul more and work faster. But these animals required care. They needed food and water, exercise and rest, regular grooming, and clean, comfortable lodging.

Draft animals had to be fitted with special gear that, among other things, converted their forward, pushing motion into pulling power. Farmers typically used harnesses for horses, mules, and sometimes donkeys. Oxen were often fitted with yokes--wooden devices, such as this one, placed on and around the animals' necks.

As agricultural practices changed in the 1800s, some farmers began combining teams of draft animals -- like the two teams of horses hitched to the plow in this photograph -- to increase their hauling power. This helped meet growing demand for agricultural production. It was also easier on the individual animals.

Combining many teams of draft animals further increased their power. This enabled specialized production on an industrial scale. It took at least two dozen mules to haul this early combine on large “bonanza” farms in the American Northwest.

In addition to hauling agricultural equipment across fields, draft animals moved vehicles carrying goods, people, and anything else that needed to be transported to, from, or around farms. This team, or "yoke," of oxen hauled a heaping wagon of hay on a Massachusetts farm.

This engraving depicts animals hauling crops and powering agricultural machinery. On the right, a yoke of oxen arrives with a cart of apples. The horse at center drives a crusher to prepare apples for pressing into cider.

Animals could directly power a range of agricultural machinery. Through rotational motion, a team of horses drove this grinder on a farm in the American Southwest.

Farmers began to use stationary power sources in the nineteenth century as they mechanized barn or farmyard work like threshing, winnowing, corn shelling and corn grinding. Some invested in devices called "powers," which used the motion of animals to drive agricultural machinery. This advertisement depicts treadmill-type powers designed to be run by horses.

Treadmill-type powers, also called "tread powers" or "treadmills," may no longer be in common use, but the form is familiar. Animals walked on an "endless belt," a device similar to those on modern exercise treadmills. A wheel and belt converted the animals' movement to drive a variety of agricultural machinery.

Most treadmill-type powers were designed for horses, but other animals could use them too. A treadmill run by two oxen drove machinery in this Connecticut family's barn.

Scaled down treadmills, like this one, could be used by dogs, goats, or sheep. They drove small machines, such as butter churns or cream separators on dairy farms.

People introduced variations on the treadmill-type animal power. This "turntable" version, patented in 1882, employed a circular platform to drive a butter churn.

Sweep-type powers, also called "sweeps," used rotational motion to run heavy machines, such as saws or grain threshers. As many as 16 horses could be hitched to a sweep's arms. They walked in a circle to generate energy.

Sweeps were cumbersome and worked horses hard. It was impractical for most farmers to own one, so they might hire someone to bring in and operate a sweep for special jobs.

A number of enterprising nineteenth-century manufacturers specialized in producing and selling both animal powers and the machines they were designed to drive.

Engines eventually replaced animals as the main stationary power source for agricultural machinery. The transition began with large portable steam engines like the one visible just outside the barn door in this image. Gasoline engines, introduced in the first decade of the twentieth century, ultimately supplanted small animal treadmills.

Trucks and tractors driven by internal combustion engines offered farmers more horsepower than horses (or any other animal) could. These vehicles eventually displaced draft animals on most American farms during the early twentieth century.

The shift was gradual, and working animals retained active roles on many farms long after internal combustion engines became available. Today, working animals provide power in a variety of ways across America and around the world, especially on organic farms and farms operated among certain cultural groups.