"When cattle are fed grain, productivity is increased, but fiber-deficient rations can disrupt physiological mechanisms," said James B. Russell of the Agricultural Research Service, U.S. Department of Agriculture in Ithaca, New York.

Based on information from 53 different scholarly investigations, the Science article provides peer-reviewed confirmation of findings on the health impacts of high-grain cattle feed.

Diets containing 50- to 90-percent grain "are more digestible and ferment faster, so the animal is provided with nutrients at a faster rate, and they can grow faster," explained Russell, lead author of the Science paper, with Jennifer L. Rychlik, a graduate student at Cornell University.

Low-fiber rations "can be very stressful for the animal," however, because they allow fermentation acids to accumulate within a digestive compartment called the "rumen," Russell added. In ruminant animals, he noted, fiber digestion depends on fiber-degrading microorganisms, which supply the cattle with useful protein, vitamins and short-chain organic acids. Without fiber, these acids are not absorbed as efficiently, as the animal's physiological mechanisms can be disturbed.

Acid buildup can cause ulcers in animals consuming too much grain: "Then what happens is that infectious bacteria come from the rumen through the ulcers, into blood, and finally into the liver, where they cause abscesses," Russell said. Feed additives such as antibiotics can counteract such ailments, but they further alter the ruminal microbial ecosystem, he added.

Grains can accumulate in an animal's intestines because they lack starch-digesting enzymes. Thus, a high-grain diet can promote an overgrowth of Clostridium perfringens, a bacterium associated with sudden death in feedlot cattle, Russell's article suggests.

Finally, grain-based diets can promote Escherichia coli (E. coli) within the digestive tract of cattle, and these E. coli are more likely to survive acid shocks that mimic the human gastric stomach. This discovery, first reported by Russell and colleagues in 1998 (Science, 11 September), has now been confirmed. Other USDA scientists have likewise shown that cattle switched from grain-based diets to hay were less likely to shed harmful E. coli 0157:H7 in feces.

Note: If no author is given, the source is cited instead.

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