A tiny pellet inserted under the skin of a calf's ear may increase weight gains as much as 15 to 20 percent. This same result would take years to accomplish through breeding and selection. These tiny pellets are growth stimulants. They are made of hormones that are constructed to slowly release minute amounts into the blood stream that stimulate the animal to produce natural body hormones. One of these hormones is a growth hormone. It regulates the rate of growth of the animal. Increasing the rate of growth will almost always improve feed efficiency and reduce maintenance costs. These pellets are called implants. They don't necessarily increase mature size, but rather stimulate early growth causing the calf to reach market weight more quickly and at less expense to the producer.
Matthews, Nyle J., "Growth Stimulants" (1989). Archived Agriculture Publications. Paper 3.