Montana State University Extension   |   Updated: August 30, 2011

The main cause of illness in freshly weaned calves is the tremendous exposure to infectious agents and stress associated with weaning, commingling, and transportation. When compared to other ages and classes of cattle, newly weaned beef calves and stocker calves have the highest levels of morbidity (sickness) and mortality (death). Bovine respiratory disease (BRD) accounts for a significant portion of cattle/calf losses in the beef industry. In one study, over 30% of these death losses were attributable to BRD. Although mortality (death) is often the most visible problem, morbidity (sickness) accounts for most production losses. Estimates of the cost of clinical diseases for backgrounded or feedlot cattle are quite variable, but one occurrence of respiratory disease in a feedlot animal costs almost $90 per head.

Strategies to Keep Calves Healthy
There are three strategies designed to prevent disease from entering or occurring in a backgrounding yard or feedlot:

• Prevent or limit the introduction of infected cattle. Buy calves from verified sources with a proven record of healthy animals.

• Minimize exposure to infectious disease. Maintain records of order buyers and transportation companies that promptly fill and ship orders. Require trucks to be cleaned and disinfected. Keep facilities clean and free of contamination from manure, rodents and other disease vectors.

• Raise overall level of the animal's resistance to infectious disease. Develop sound vaccination programs at the farm or ranch of origin. Reduce environmental stress by providing proper shelter and ensuring that pens and lots are free of mud. Use balanced starter rations which ensure good feed intake and minimize stress during initial handling and processing procedures.

Bacterial and Viral Agents Which Cause Bovine Respiratory Disease
The agents responsible for producing respiratory disease in beef cattle are both viral and bacterial. Viruses rely on the animal's own cells to produce more virus whereas bacteria have all the cellular functions necessary to reproduce without the aid of an animal's cell. Antibiotics can be used for fighting bacterial infections but are ineffective at fighting viral infections. The viral and bacterial agents most commonly associated with BRD are shown in the chart below.

Of all the viral agents, IBR, BVD, BRSV, and PI3 are the only viruses which cause acute respiratory disease by themselves. All the other viruses require significant interaction with other pathogens. Of the bacterial pathogens, neither Mycoplasmaspp. nor Chlamydiaspp. are considered primary pathogens in weaned or yearling cattle.

A pathogen causes disease. Rarely is only one pathogen responsible for BRD. Two or more pathogens work in concert with each other to bring about morbidity and mortality. Respiratory viruses compromise the animal's respiratory defense mechanisms to allow bacterial pathogens access to the lower respiratory tract. This is what causes the pneumonia commonly associated with BRD.

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