Let's take a momentary break from animal subjects to talk about plants.
Let's pretend you're an alfalfa plant in the middle of a central South Dakota field. Dry conditions have made it tough on you and your brethren lately.
The first part of the growing season was going well. Your buds were shooting up from your crown and you were developing stalks and leaves: the result of your deep roots taking up moisture and nutrients.
Of those nutrients, one of the most important is nitrogen. With help from the moisture in the subsoil, your roots take in nitrogen in the form of nitrate salts. These nitrates are sucked up into your developing stems and leaves. This is a good thing, because your value as an alfalfa plant is based on how much protein you contain. Nitrates are necessary building blocks for amino acids, which are tied together to form proteins. Your physiology is a thing of beauty when moisture and nitrogen are plentiful around your roots. Like a refinery where crude oil comes in and gasoline comes out, you are a factory: a factory making protein, which will nourish a hungry cow or calf this fall.
But then your world changed. Just when this factory was working at full capacity, the soil around your roots became drier and drier. Soon, there wasn't enough moisture to keep this protein factory working. The raw materials (the nitrates) were still coming in, but they weren't being used up to make proteins anymore. These nitrates were accumulating like crude oil in a refinery holding tank, in especially high concentrations in your lower stalks and stems.
This is a big deal. Because now you have been transformed from a valuable source of bovine nourishment into a potential killer. You look and taste just the same, but you have many times more nitrate molecules in your stems and leaves than normal.
Interestingly, ruminants use nitrates for the same reason you do. They use them to form ammonia, then amino acids that are stitched together into proteins. This process is carried out by the bacteria and other microbes that normally live in the rumen. Before nitrate is made into ammonia, though, it's first converted to a form called a nitrite. When there's too much nitrate, this nitrite builds up in the rumen and is absorbed into the bloodstream.
Now this is an even bigger deal. The nitrite in the cow's bloodstream disturbingly likes to react with iron molecules, which happen to be a necessary part of hemoglobin. Hemoglobin is the molecule found in red blood cells that carries oxygen to all the cells of the body. When nitrite reacts with iron molecules in the hemoglobin, the hemoglobin is no longer able to carry oxygen. When that happens, the cow's cells don't have oxygen to function anymore. Soon, if there's enough nitrite in the bloodstream to choke off the oxygen from the red blood cells, the cow starts to suffocate from within. If a person is around to witness it, they might notice the cow becoming anxious, breathing faster and faster, with the heart beating faster and faster in a futile attempt to deliver oxygen to the brain or the heart or anywhere. In acute nitrate poisonings, this often happens quickly enough that the person just finds the animal dead. A blood sample taken from affected animals is dark chocolate brown, devoid of the red tint of healthy oxygen-carrying hemoglobin found in a normal animal.
As an alfalfa plant, you are not alone in culpability for the possibility of nitrate poisoning. Corn, sorghum, oats, wheat, and a long list of crops and weeds can accumulate nitrates and do the same thing to a ruminant.
With this year's dry conditions, cattle producers need to think about high nitrates in forage crops this summer. SDSU Extension centers have the ability to perform quick rapid tests for nitrates on forage material in many cases, or can direct you to have a more accurate analysis performed. Nitrate testing is a very inexpensive insurance policy that should be done this year before grazing or haying any forage that may have accumulated nitrates due to dry conditions.