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In cattle, the respiratory and gastrointestinal tracts are the main systems affected with disease. Bovine respiratory disease (BRD) contributes the majority of illness and death loss in the feedlot segment. Historically, 15-45% of feedlot cattle have been affected with BRD, with 1-5% of total cattle placed dying of BRD (Kelly 1986). Respiratory disease, alone, accounts for 44.1% of deaths in beef feedlot cattle (Vogel 1994). Apart from death loss, the Texas Ranch to Rail Program has suggested that clinical disease (most of which is BRD), even if treated successfully, results in treatment cost ($37.90/affected), decreased average daily gain (0.21 lb/d, for a 7.2 % decrease), decreased feed efficiency, and a decrease in quality grade (27% fewer choice). Together, these contribute an economic loss of approximately $88.00/ affected animal (Anonymous 1994).
Not restricted to the feedlot, respiratory disease accounts for 16% of the known causes of death in nursing calves, second in importance only to weather, and apparently increased from estimates made 5 years earlier (USDA/APHIS/VS 1994, 1997). Though the incidence of respiratory disease in unweaned calves is generally thought to be low, (1-2%), individual herds can have up to 65% of calves affected prior to weaning (Muggli-Cockett 1992). Unweaned calves affected with respiratory disease have a 36.3 lb reduction in weaning weight (Wittum 1994). This reduction, coupled with treatment cost, is the main economic loss from BRD in the cow-calf segment.
Disease of the gastrointestinal tract is the second most common cause of death in the feedlot (Vogel 1994). Acidosis is a common condition in feedlot cattle, and liver abscesses are a possible sequella. Slaughter surveys indicate that about 12% of beef cattle livers are condemned due to liver abscesses (Smith 1998).
Recently completed work indicates that productive losses associated with BRD are greater than previously thought. Liver abscesses continue to be a significant problem. With the beef industry desiring to maximize productive potential, minimize inputs, and potentially limit the use of antimicrobials, an understanding of the biology of disease is necessary to better control these problems. Understanding of the disease process, particularly respiratory disease, implicates events in early life as risk factors for subsequent illness.