Factors that alter rumen microbial ecology

Ruminant animals and ruminal microorganisms have a symbiotic relation-ship that facilitates Þber digestion, but domestic ruminants in developed countries are often fed an abundance of grain and little Þber. When ruminants are fed Þber-deÞcient rations, physiological mechanisms of homeostasis are disrupted, ruminal pH declines, microbial ecology is altered, and the animal becomes more susceptible to metabolic disorders and, in some cases, infectious disease. Some disorders can be counteracted by feed additives (for example, antibiotics and buffers), but these additives can alter the composition of the ruminal ecosystem even further. Ruminants and humans have shared a long his-tory. Prehistoric, and often nomadic, hunters realized that they could exploit the photosyn-thetic potential of grasslands by domesticating ruminants, and subsequent increases in food supply allowed them to form stable social com-munities. The Bible recognized the value of ruminants as livestock, and the Old Testament states, in Leviticus 11:3, “Whatsoever parteth the hoof, and is clovenfooted, and cheweth the cud among the beasts, that shall ye eat. ” (1). Much of our meat and virtually all of our milk is still produced by domestic ruminants, but hu-mans have modified the type of feed that these animals consume. The success of wild ruminants can be largely explained by their ability to digest fibrous plant materials (1). Ruminants themselves do not pro-duce fiber-degrading enzymes, but they harbor bacteria, fungi, and protozoa that can. The host provides the microorganisms with a suitable habitat for growth, and the microbes supply protein, vitamins, and short-chain organic acids for the animal. Microbial protein accounts for as much as 90 % of the amino acids reaching the small intestine, and energy from short-chain or-ganic acids (primarily acetic, propionic, and butyric acids) drives animal metabolism (2). Ruminal microorganisms can also ferment starch and sugars, and these nonfibrous ma-terials increase fermentation rate and animal productivity (2). However, when ruminants are fed fiber-deficient rations, homeostatic mechanisms of digesta flow, gas removal, and pH regulation are disrupted, and the an-imal can be severely affected (3, 4)