Potential for Toxic Nitrate Levels in Forages

Dr. Mark Sulc, OSU Extension Forage Specialist, The Ohio State University
(Previously published in the C.O.R.N. Newsletter 2020-13)

The recent cold and cloudy weather has raised the concern for higher nitrate levels in forages that could potentially be toxic to animals consuming those forages. It is true that any stress condition that slows plant growth and metabolism can increase the risk of higher plant nitrate levels. This article discusses factors to consider, especially given the recent cold weather we have been experiencing in Ohio and surrounding regions.

Plants readily take up nitrates from the soil, even under colder conditions, and especially since we have plentiful soil moisture to facilitate uptake. Once in the plant, nitrate is converted to nitrite, then ammonia, and finally into amino acids and plant protein. Any environmental stress that significantly slows down plant photosynthesis and metabolism can lead to excessive nitrate levels in the plant because the nitrate uptake from the soil will be faster than its metabolism into plant protein. Such stresses include frost, extended cold weather, cloudy conditions, hail damage, or drought. We have had all those conditions recently, except drought.

When ruminants consume excessive levels of nitrate in the diet, the nitrate is converted to nitrite by rumen microbes faster than it can be converted to ammonia, amino acids, and eventually to protein. Accumulated nitrite in the rumen is then absorbed into the bloodstream where it prevents oxygen transport, which leads to death. Livestock sensitivity to nitrates ranked from highest to lowest is: pigs > cattle > sheep > horses. Older or sick animals are generally more sensitive than young healthy animals. The fetus in pregnant animals is very sensitive to high nitrates ingested in the diet.

Below are factors to consider regarding the potential for high nitrate levels in forages, in the context of our situation this spring:

  • Forage growth has been significantly slowed due to extended cold nights, cloudy weather, several hard frost events, and even hail damage in some areas. All these stresses can lead to higher nitrate levels in plants. Warmer temperatures later this week will help reduce the plant nitrate levels as plants gain active growth again.
  • Nitrogen fertilizer or manure applications made to forages this spring definitely increase the risk for higher nitrate levels in plant tissue, especially where forage growth is slow.
  • Nitrate accumulation is possible in many forage species, including all cool-season perennial forage grasses, alfalfa, all cereal forages (oat, rye, triticale, wheat, barley, spelt, etc.), and brassicas (might be present in cover crop mixes). Nitrates can also accumulate in corn and sorghum species, but those are not an issue for harvest at this time in Ohio.
  • Several weed species are heavy nitrate accumulators, including lambsquarter, pigweed, dock, some mustard species, horse nettle, nightshade, quackgrass, and jimsonweed. Heavy infestations of those weeds when harvested with the forage will increase risk of nitrate toxicity.
  • Nitrate levels are generally higher in younger than more mature growth. Delaying cereal forage harvest to dough stage and other forages to flowering/heading stages can significantly reduce nitrate levels.
  • Nitrates accumulate in the lower one-third of plants more than in the upper two-thirds.
  • Plant nitrate concentrations are higher in the morning than later in the day (plant metabolism during daylight drives the conversion of nitrate to plant protein).
  • Risk of nitrate toxicity is highest with grazing, especially where nitrogen or manure applications were made this spring.
  • Making dry hay does not appreciably reduce nitrate levels in the forage.
  • The ensiling can reduce nitrate levels from 10 to 60% provided fermentation is good. But if the forage is initially very high in nitrates, the silage could yet contain toxic nitrate levels, so this is not an automatic fail-safe option.
  • Nitrate levels can vary across a field, so the harvested forage can be quite variable in nitrate concentration.

The bottom line is that if you suspect the forage could be high in nitrate levels, the safest thing to do is to sample the forage and have it tested before it is harvested, because if levels are high you can delay harvest to reduce the levels. You should certainly sample the stored forage before feeding it if you suspect higher levels! Call your forage lab and follow their guidelines closely for sampling the forage, packaging, and shipping the sample to them. It might be a good idea to delay harvest until we get warmer weather, more sunshine, and a little more maturity on those forages that are known to be nitrate accumulators, especially where several of the risk factors listed above are present.

For more details, see the following references:

Drewnoski et al., Nitrates in livestock feeding. University of Nebraska Extension NebGuide G1779. Available at http://extensionpublications.unl.edu/assets/pdf/g1779.pdf.

Adams et al., Prevention and control of nitrate toxicity in cattle. PennState Extension. Available at https://extension.psu.edu/prevention-and-control-of-nitrate-toxicity-in-cattle.

Glunk, E. Nitrate toxicity of Montana forages. Montana State University Extension MontGuide MT2002205AG. Available at http://www.animalrangeextension.montana.edu/forage/documents/Nitrate%20Toxicity%20MontGuide.pdf.