Ask a Scientician

In response to Janelle’s question: “Have there been any significant changes to farm animals’ biology and behavior in the last few decades due to the increased use of growth hormones? Could our interference actually change or speed up their evolution as a species?”

Hormonal therapy has become an integral part of large animal veterinary medicine and livestock production. We use reproductive hormones to manipulate the estrus cycle of females to increase breeding efficiency, give hormones to animals to alter growth and development (largely for economic benefit) and remove naturally occurring hormones from animal to alter behavior and reproductive potential (spaying and neutering animals). So first some background information on growth hormones used in the animal industry:

Growth hormones have one basic principle, to increase production of an economically desirable commodity. Usually this is in the form of meat or milk. We’ll talk about these separately. First, to increase carcass yield from meat animals you must begin with an animal that has the potential to grow to your desired specifications, basically the genetic capability to grow, gain weight and convert feed to muscle mass. Hormonal additives are typically in the form of ear implants composed of estrogens, progesterones, synthetic anabolic steroids or a combination thereof. Estrogen and progesterone stimulate a natural release of growth hormone (GH) in higher levels than would naturally occur. This engages a cascade of metabolic processes that lower overall catabolism and increase the retention of amino acids by the animal’s body, thus leading to a significant gain in muscle mass over the animals life. Anabolic steroids are typically only used just prior to slaughter (if at all) for a short period of time as they create rapid growth but can sacrifice the quality of the meat if overused. So, growth hormone itself is not given, but rather other naturally occurring hormones are used to stimulate to animals body to produce its own GH.

When it comes to using hormonal therapy in increasing milk production the concept is similar. Recombinant bovine somatotropic hormone (rbST) has been widely publicized recently, especially in California where the dairy industry is huge. It is given to cows when they achieve peak lactation, typically about 10-12 weeks post-calving. It is a naturally occurring growth hormone that re-directs the use of absorbed nutrients to increase the milk yield over the course of an entire lactation. The cow does not produce more milk that she would naturally produce in a day at her peak lactation, but rather maintains that peak lactation for a longer period of time, thus increasing yield over the course of a lactation period (about 10 months). The cow isn’t over-milked or worn out as is often perceived, and no studies to this point have shown an increase in mastitis (udder infection) or other health issues.

These hormone supplements are perfectly safe for use and have all been approved by the FDA. When it comes to meat animal “enhancement” the animals receiving the treatment are not going to be used for breeding (and are usually steers or spayed heifers) and thus the effects are terminal and not propagated in the species. If these hormones are given to breeding animals they will usually have deleterious effects on reproductive capability and thus prevent any changes from entering the future gene pool. This is similar to men using anabolic steroids and having reduced testicular capacity due to negative feedback on hormonal regulation.  rbST in dairy cows also does not alter the genetics of the cow or her offspring but may make it more difficult for that particular cow to be re-bred with the efficiency desired on large dairies (studies vary on conception rates in cows using rbST). They still breed back, it can just take longer under normal management practices.

Our interference with these additives is not altering the evolution of these species, but our breeding selections and genetic manipulations are certainly placing pressure on all species to change. This is nothing new and has been ongoing since humans first domesticated animals and sought out desired characteristics. I am not aware of any effects these additives have on behavior but I will look into it. I’m sure Dave will have something to say about the biotechnology and genetic modification of animals lately…