Stephen B. Smith
Department of Animal Science, Meat Science Section, Texas A&M University 相关阅读

In the U.S. and in many developed countries, beef cattle are fed in feedlots until they are processed. Finishing cattle on grain-based diets increases their rate of gain, provides better flavor and texture of their beef, and increases the amount and healthfulness of the marbling within their beef. However, finishing cattle in feedlots also exposes them to diseases they might not encounter if raised on pasture. Economically, the most important disease affecting feedlot cattle in the U.S. is bovine respiratory disease (BRD), which accounts for 75% of feedlot morbidity, and which averages about 8% of the total U.S. beef cattle herd. Lost performance due to BRD results in a loss of nearly $100 U.S. per animal. Healthy steers have higher daily gains and over 10% more USDA Choice carcasses than cattle identified as sick. Cattle showing signs of BRD can have a 5 kg decrease in hot carcass weight, a 15% reduction in 12th rib fat thickness, and a 7% decrease in USDA yield grade, indicating that sick cattle are smaller and leaner than healthy cattle. In an evaluation of over 7,000 cattle in the U.S., 39% graded USDA Choice or better if healthy, but only 27% graded USDA Choice if they were treated once for BRD. In another study, virtually no cattle graded USDA Choice if they had been treated more than once for BRD. However, beef from BRD-infected cattle is not tougher than beef from healthy cattle, indicating that factors that decrease quality grade in BRD-infected cattle are unrelated to those factors that regulate tenderness. It is likely that pro-inflammatory cytokines promote skeletal muscle protein degradation, resulting in lesser muscle mass in BRD-infected cattle. In summary, it is important from an economic standpoint as well as from a product quality standpoint that cattle are kept healthy during all phases of production.
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