Answer:
Bulls are able to grow faster and leaner muscle than steers are with natural steriods involving testosterone. Steers with implants are not as able to grow as quick as bulls are, but with a boost from the steriods, they are able to turn feed into muscle (in a manner of speaking) relatively quickly. Steers are not implanted with testosterone to make them grow, but a man-made estrogen steriod. This, believe it or not, helps them gain marbling as well as muscle. Estrogen-implanted steers are often used for collecting semen on bulls to avoid any mistakes in case the bull being collected inseminates the heifer or cow being used instead.