Nitrate will accumulate in a plant as long as it is taking up more nitrate than it can convert to protein. Soil nitrogen levels greatly influence plant nitrate content. Large applications of nitrogen fertilizer or manure increase soil nitrate and thus the nitrate available to the plant. Herbicide application may disrupt or interfere with normal plant function, such as photosynthesis or nutrient movement within the plant. This may result in nitrate accumulation.
Ruminants have different capacities to convert nitrate into nitrite and finally to ammonia.
Sheep have the highest tolerance to nitrates, because they have the greatest capability of all ruminants to convert methemoglobin back to hemoglobin. Cattle have the lowest capability and therefore are at greatest risk. The ability of individual animals to tolerate nitrates in feed is variable, which complicates the determination of a safe level of nitrate in a feed. An animal in good body condition, receiving a diet that meets daily nutrient requirements is able to convert nitrate to nitrite and finally to ammonia more efficiently than an animal that is inadequately fed or in poor condition. Animals in poor condition, even if well fed, have more trouble converting nitrite to ammonia.