Which factors predispose ruminants to urea toxicosis?

• A. Fasting, high roughage, high rumen pH, dehydration, hepatic insufficiency
• B. Low roughage, low rumen pH, high water intake
• C. Exercise and cold stress
• D. Vitamin C deficiency

Answer: (A)

Urea toxicosis in ruminants happens when ammonia produced from non-protein nitrogen builds up in the rumen and the animal cannot effectively convert or excrete it. The factors listed create a perfect setup for that: fasting lowers the energy available to rumen microbes, so they can’t efficiently convert ammonia into microbial protein; a high roughage diet provides nitrogen but not readily fermentable carbohydrate, further limiting microbial capture of ammonia and often keeping the rumen environment more alkaline; a high rumen pH shifts ammonia to the uncharged NH3 form, which crosses the rumen wall more readily and enters the bloodstream; dehydration concentrates rumen contents and reduces salivation and rumen turnover, increasing the duration and amount of ammonia exposure; and hepatic insufficiency impairs the liver’s ability to detoxify ammonia by converting it to urea, allowing toxic ammonia to accumulate systemically. Choices that involve low roughage with low pH, or non-etiologic stresses like exercise, cold exposure, or vitamin C deficiency, don’t create this same combination of reduced microbial ammonia utilization, increased NH3 absorption, and impaired hepatic detoxification, so they are not predispositions for urea toxicosis.