You can’t see it or smell it but hay can harbor toxic levels of nitrate.
Nitrate is a simple molecule – one nitrogen and three oxygen. It is present naturally in soil minerals, is produced by bacterial breakdown of protein and also produced from nitrogen in the air when there is lightning.
Plants require nitrate and use it to produce amino acids/protein. The roots take it up from soil and it is converted to amino acids in the leaves but things that interfere with that conversion can lead to toxic nitrate levels in the plant.
Among the factors which increase the risk of high nitrates are: excessive application of nitrogen or manure, drought, prolonged low sunlight, freezing stress, being raised in an irrigation system.
Nitrate must be converted to the more toxic nitrite to cause problems. This is done by the bacteria in the digestive tract, starting in the mouth. Rapid, efficient conversion occurs in the rumen of cows and other ruminants but monogastrics like the horse and human are also at risk.
Full blown toxicity causes interference with oxygen carrying capacity of the blood to the point it turns blue from formation of methemoglobin. This is rapidly fatal to young animals and in rare cases can kill adults. Lower levels cause abortion, impaired exercise tolerance and lethargy. Hoof pain has been reported.
No work has been done with horses but in people chronic exposure to nitrate at levels below the threshold for acute methemoglobin toxicity has hormonal effects including low thyroid function by interfering with iodine uptake and development of metabolic syndrome.
Estimates of toxic levels for horses range from as low as 6000 ppm to 10,000 ppm (equivalent to 0.6 to 1%), roughly double the acutely toxic level in cattle. Also of concern is the potential for lower levels to cause hoof pain or hormonal effects.
On the Equine Cushing’s and Insulin Resistance Group we regularly see elevated nitrate levels in irrigated hays from the Western half of the country. The last one I saw, which inspired this topic, was just below the 10,000 ppm mark in a California Orchardgrass. If nitrate is detected over the 2500 ppm mark, iodine supplementation at 7 to 10 mg/day is used to protect the thyroid. With higher levels, soaking can reduce the nitrate but you need to retest a soaked then dried sample to make sure it is safe.
Obviously more research is needed but there’s no reason to take risks with high nitrate hay. This is something you can control.
Eleanor Kellon, VMD
Dr. K's Horse Sense 
I came back from vacation only to find my mare foaming at the mouth, she had foam spit in her stall and slobbered while being ridden (bit less bridle). She is IR so I soak her hay, don’t know if she was getting soaked hay all the time I was on vacation. I washed all her hay bags, rinsed her mouth several times a day, she still was spitting foam all over, I almost thought she had rabies, but she didn’t have any fever or looked ill in any other way. Tried feeding her dry hay, no difference. Sent in the orchard hay for analysis, protein came back at 17% (and also pretty high sodium). Called Equi-Analytical and added nitrates to the test and it came back at 2000 ppm. As far as I know none of the other horses at the barn (25+ horses) had any symptoms. Took three days of feeding a soaked mature timothy hay for her to stop foaming. Then I gave her less than 1/2 lb of the orchard hay and 3 hours later when I walked her she was spitting foam again… So I ended up having to buy my own timothy hay (very expensive in California) plus paying full board.
Needless to say I always add nitrates to the hay analysis.
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I can not prove it was the nitrates that made her foam at the mouth, it could have been something else in that particular orchard hay, although she has eaten other high protein orchard hay for several years without problems.
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Intravenous methylene blue is the treatment for acute symptoms of nitrate poisoning but if you know the water and hay do not have high levels what you are seeing is not related to nitrate. Nitrite is cleared rapidly from the body.
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Nitrate levels do not show up on hay analysis from Equi-Analytical, How do you know?
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You have to request it as an add-on test.
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what or is there a test to see if a horse has ingested high levels???
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Very high levels cause high respiratory rate and pulse, staggering, collapse, chocolate brown blood. For lower level exposure, blood can be tested for methemoglobin.
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AND IS THERE A TREATMENT FOR HIGH LEVELS????? HAVE HORSE WITH SYMPTOMS EVEN AFTER 3 COMPLETE CHANGES OF HAY
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