Tanks that are used to haul nitrogen-based fertilizer should not be used to transport drinking water for cattle as there is a risk of poisoning. Any nitrogen remaining in the tank can potentially cause nitrate or non-protein nitrogen (urea) toxicosis in ruminants (depending on form of fertilizer).
Nitrate toxicity
Nitrate toxicity occurs when high nitrate levels overwhelm the animal's digestive system to the extent that the rate of conversion of nitrate to nitrite is faster than the conversion of nitrite to ammonia. When this happens, nitrite accumulates and is absorbed into the bloodstream. There, it reacts with the oxygen-carrying hemoglobin, changing it into a form (methemoglobin) that cannot transport oxygen to the lungs and body tissues and the animal suffocates. Death from nitrate poisoning may occur within a few hours.
It takes about twice as much nitrate to kill a ruminant when nitrate is eaten in forage as when consuming contaminated water.
Urea toxicosis
Urea toxicosis occurs if more urea is consumed than the rumen organisms can use. The microbes convert urea to ammonia and excess ammonia is absorbed from the rumen into the blood. The ammonia is then converted back to urea in the liver, and is then excreted by the kidneys. However, this pathway can be overwhelmed, when excess ammonia circulates in the blood it can cause poisoning.
Poisoning can occur rapidly from a few minutes to a few hours after consumption. In the case of urea poisoning cattle are typically found dead close to the source.
Source: University of Nebraska