High Protein Level in Hay - an Indicator of High Nitrates?

by Patricia Woodbury Kuvik

Until recently, nitrite toxicity from high nitrates in hay was not viewed as an ongoing problem for horses.  Little research has been done to actually determine levels affecting equines and nitrate accumulation seemed to be restricted to specific forage species and stressed growing conditions. Hays known to be nitrate accumulators include small grains (oat, barley, wheat, rye), sorghum hybrids and some perennial grasses and weeds.

While testing hay intended for broodmares for nitrates is common, little thought has been given to the possibility that healthy adult horses might be affected by nitrates in “average” hay.  However, when a student in Dr. Eleanor Kellon’s nutrition program noted a correlation between vague symptoms including depression, inappetence, tripping and unusually high protein in the grass hay they were fed, she became suspicious and began testing all her hay for nitrate levels. Other horse owners began checking their hay, finding a high incidence of excessive crude protein (CP) and nitrate levels, primarily in grass hays from the Imperial Valley in California but also in hay grown in other Southwest areas. Species included Bermuda grass, Timothy and Orchard grass hays.

After recently working with a horse owner whose hay tested high in both crude protein (18%) and nitrates (0.98%), and whose horse had been having vague symptoms of anxiety, work intolerance and depression her veterinarian was unable to account for over a long period, I am now looking more closely at “inappropriate” CP levels in grass hays that I balance. In addition to there being little research in general on nitrite toxicity in horses, even less is know about the effect of ingesting chronic “less than toxic” levels of nitrate over a period of time. Nitrate competes with the uptake of iodide and can affect thyroid function. We immediately removed the high nitrate hay, substituting a tested low nitrate Timothy pellet, and began supplementing iodine at double the NRC requirement. As is often the case when symptoms have been vague, any subjective signs of “improvement” will be inconclusive.

North Carolina State University has adopted the same guidelines suggested as safe limits for cattle for equines. “Excessive intake of nitrates decreases the blood’s oxygen-carrying ability, which causes anxiety, increased respiration rate, and breathing difficulty. Severe cases result in loss of coordination, muscle twitching, and death. Excess levels of nitrates can occur where more fertilizer nitrogen is applied than needed and from drought stress on plants."  NC State University Fact Sheet.  They suggest the levels can be made safer by “diluting” high nitrate hays with grain, which does not contain nitrate.

This is not feasible for many horses, especially those with metabolic issues that require a “low carbohydrate” diet.

I suspect that there may be several factors involved in seeing higher levels of crude protein (CP) and nitrate in more grass hays coming to market:

  • More intense use of dairy sludge fertilization in industrial farming
  • General increase in fertilizer use attempting to increase yield
  • Use of the minimal amounts of water needed for irrigation to maintain yield
  • Drought conditions in the Southwest
  • Emphasis on yield over quality
  • A tendency to view higher CP hay as better quality - a hold over from production animal farming but not appropriate for horses 
  • Sampling error - coring bales from low lying area instead of randomly addressing a field

I am now sending the following note to my clients when their hay analysis results in an “inappropriately high” level of CP.

“Your grass hay has a crude protein level that is atypical for grass hay.  Some mixed grass/alfalfa hays will have high protein levels but will also have a corresponding high level of calcium. A higher than "expected" protein level in grass hay is sometimes the first indication we have that a hay might have high Nitrate levels.  I'm now requesting all my clients having  grass hay (or borderline mixed hay) that tests with protein levels above the expected range (14%) have Dairy One or Equi-Analytical run "Nitrates".  This test is an additional $6.00 per sample. If you call to add the test to your sample, they should be able to get results to you within a few days.  

The maximum acceptable Nitrate level in forage for adult horses is 0.5%. Higher levels may be "diluted" with very low nitrate hay (or diluted with grain which isn't an option for most of us) to bring the Nitrate level for total forage fed below 0.5%. Hay for breeding mares and growing horses should not exceed 0.20% Nitrate.

You may be advised by a lab that Nitrate is not an issue for horses but there simply is insufficient research on this. UC Davis suggests lower levels for cattle than Dairy One Forage Lab. The inexpensive test is “cheap” insurance. If you suspect high Nitrate and your horse is showing any signs of Nitrate toxicity (lethargy, work intolerance, ataxia) stop feeding that hay and substitute another forage source.

The Nitrates test should be requested for:  any grass hay with protein level >14%, mixed hay with protein level above 14% and calcium <0.9%, “known” nitrate accumulators (small grain hays),any hay grown in drought conditions and any hay to be fed to pregnant mares.”

These are the “expected” ranges for CP for grass and mixed hays:


% Crude Protein





Grass Hay





Bermudagrass Hay





Mixed Mostly Grass





Mixed Mostly Legume





Legume (Alfalfa) Hay





Dairy One Forage Lab - Feed Composition Library    

Further information about nitrate/nitrite toxicity, symptoms and treatment can be found at these websites:

Nitrate Poisoning

by T.L. Stanton and J. Whittier1 (3/06)





Nitrate Toxicity



Alejandro R. Castillo & John H. Kirk University of California, Davis. Cooperative Extension


Horse Feeding Management: 

Feed Sampling and Analysis

NC State University Fact Sheet


Effect of Dietary Nitrate on Thyroid Function

  1. Richard A. Bloomfield, Clifford W. Welsch, George B. Garner and Merle E. Muhrer