Normally nitrate is absorbed by a plant's roots and transported up the stem to its leaves where the nitrate is converted to proteins and other substances through the process of photosynthesis.  Under optimal conditions photosynthesis occurs almost as fast as the nitrate is absorbed from the soil.  During less than optimal periods, the photosynthesis slows or stops and nitrates accumulate.   Nitrate poisoning occurs when an animal consumes too much nitrate in its diet.  Microorganisms in the rumen cause nitrate to be converted to nitrite.  If too much nitrate is consumed there will be a build up of nitrite which is then absorbed into the bloodstream where it converts the hemoglobin into methemoglobin.  Methemoglobin is incapable of carrying oxygen, therefore death by asphyxiation can occur quite quickly.

SYMPTOMS  

The symptoms of nitrate poisoning are somewhat similar to those of prussic acid poisoning including a quickening heart rate, shallow rapid breathing, muscle tremors and staggering.  In fatal cases the animal will fall the to ground, convulse and die due to
asphyxiation.  The one striking difference is that the blood of the animal suffering from nitrate poisoning turns a chocolate brown color instead of the bright "cherry red" associated with prussic acid poisoning.   Nitrate poisoning is not always fatal.  Lower "subclinical" levels can cause a number of vague symptoms including failure to thrive, reduced weight gains, vitamin A deficiency, reduced milk production and abortion.

TREATMENT  

Treatment is available, but in fatal doses death occurs so quickly that you almost have to be in the field with remedy in hand in order to save any livestock.  Consequently, prevention is once again worth a lot more than a pound of cure.  So if there is any question in your mind about the safety of a forage I suggest that you let one animal
graze for awhile before risking your entire herd.  Just don't use your prized herd sire as the guinea pig!   The recommended treatment for nitrate poisoning is methylene blue and it must be administered immediately upon observation of symptoms.  Methylene blue is not sold over the counter so talk to your veterinarian about it.

CONDITIONS EFFECTING NITRATE LEVELS IN FORAGE

Anything that slows the photosynthesis process will increase the accumulation of nitrate.  The most common causes of high nitrate content in forage are as follows:
Species.  All plants are at risk of accumulating nitrate, but some species are known to be especially prone to accumulating dangerously high levels at certain times.  Grasses that you want to be especially careful with include barley, bromegrass, corn, fescue, Johnsongrass, oats, rescuegrass, rye, sorghum, sudangrass, wheat, and pearl millet.
Some forbs to watch out for include horsenettle, kochia, lambsquarter, morningglory, pigweeds, goathead (a.k.a. puncturevines), Russian thistle, and sunflowers.  Varieties may differ in their tendency to accumulate nitrate, so talk with your County Extension Agent about the best varieties for your area before planting.  
Growth stages.  Stems (or stalks) are the highest in nitrate content, followed by leaves and then seeds.  However, young immature plants have a greater potential for nitrate accumulation than older plants because they are more actively absorbing nitrogen (in the form of nitrate) from the soil.
Fertilizer.  High applications of nitrogen fertilizer can aggravate the problem.  Two applications of half the required nitrogen in lieu of one application not only helps reduce the risk of nitrate poisoning but also helps to prevent some of the nitrogen from being washed away from the root zone by subsequent heavy rains.  Acidic soils and phosphorus deficiencies tend to cause nitrate accumulation. I always recommend that
soil tests be taken and fields fertilized accordingly.
Weather.  Cool, cloudy days result in a slow down of photosynthesis but it only takes 2 to 4 days to deplete the accumulated nitrate after the sun begins shining again.  Drought is probably the most dangerous time because nitrate tends to accumulate in the stem while photosynthesis eases all together.
Damage to plant tissue.  Defoliation from grazing the grass too short, herbicide treatments, or from weather events such as hail will all effect the rate of photosynthesis and hence the level of nitrate accumulation.

MANAGING NITRATE POISONING RISK 

The good news is that the risk of nitrate poisoning can be managed.  I know of several "old timers" in my neck of the woods who have had livestock for decades and claim to have never lost an animal to nitrate poisoning.
Supplemental Feeding.  Risk of nitrate poisoning can be reduced by supplementing grazing with grain and other high energy feed (such as molasses).  This appears to help the rumenal microorganisms utilize more of the nitrite, leaving less of it to be absorbed into the blood stream.  Just keep in mind that while this may help, it doesn't eliminate the risk of poisoning.
Grazing.  Avoid grazing  at high risk times - during drought, immediately after a drought breaking rain, during and immediately after a series of cool, cloudy days, or during any period of rapid growth.  It's not always possible to convince deer to leave a lovely green pasture just because it might be at risk so, in a pinch, you can opt for
offering a low-nitrate hay to help reduce the overall levels of nitrate and provide high energy supplements.  Also, don't let livestock graze a pasture too short as they will be eating the higher ratio of stems which contain the highest amounts of nitrate.
Hay.  Unlike prussic acid, nitrate levels do not dissipate after cutting.  If the live plant is high in nitrate, so will be the hay made from it.  Remember that stems tend to be higher in nitrate content than leaves, so raising the cutter bar may reduce the overall levels in the resulting hay.  However, beware that bales from the same field will not
all necessarily have the same nitrate levels.  It is very common for there to be "hot spots" in a field which received more nitrogen fertilizer than other spots.   Your County Extension Agent will be happy to test samples from several randomly picked bales if there is any question.
Silage.  The microbial action that takes place during the fermentation of silage reduces the amount of nitrate, but only up to 25 to 50%.  Again, test for nitrate levels before incurring the cost of ensiling excess forage.
Water.  Water can become polluted with nitrates and pose a risk, especially if your animals are eating forage high in nitrate.  Ask you Extension Agent to test your water if you are concerned.