Date of this Version
Profit from livestock production is affected by daily liveweight gain, cost of gain, and feed conversion efficiency. Young beef calves are recognized as suitable converters of forages and grains into wholesome red meat, but constraints to maximum production efficiency are recognized. While their growth and development are often discussed in light of genetic potential, nutritional requirements, and environmental constraints, administration of anabolic agents to maximize their gains is a common practice. Several implants are available to the cattle industry as growth promotants; some of these contain steroids of gonadal origin while others are nonsteroidal.
Steroids secreted by the testes are thought to be responsible for differences in growth rates and feedlot performance of bulls and steers. Testosterone is the most likely steroid conferring a growth advantage to the intact calf; however other steroids, including estradiol, possess growth-promoting activity. Estrogen and progesterone are effective ingredients in some implants but are often used in feedlot heifers to prevent cyclic ovarian activity; e.g., melengestrol acetate (MGA). Ovariectomy or spaying of heifers is popular in some feedlots, but initial weight loss and occasional death have kept this practice from being widespread in the U.S.
In view of the numerous implants available as growth promotants for cattle and the extensive application of these implants to growing-finishing steers, we have compared the efficacy of several implants singularly or in combination. For simple reference, the trade name, chemical ingredient, and structure of these anabolic agents are presented in Figure 1.