Dietary deficiencies are the most common cause for a cow to lose weight and be weak. Nutritional intake in comparison to the protein and energy requirements of the cowherd can be evaluated. And enteric bacteria and parasites may be contributing factors. Instead of a herd situation, the problem may only be an individual cow problem where professional assistance is needed to identify a cause such as infections of body cavity linings (pleurisy, peritonitis) and abscesses and cancers of internal lymph glands and organs.
Sometimes beef cattle may swallow a small piece of metal that will have adverse effects on the rumen. This is called hardware disease, and cattle will lose weight.
Other potential issues to discuss with your veterinarian include Johne's disease (which will adversely affect the lining of the intestine) and bovine leukosis.
If cattle are in this condition, it is probably best to call your veterinarian to help sort out the problem.
