Nitrate Poisoning in Livestock — Summary and Management Guide

High nitrate concentrations can develop in forages whenever normal plant growth is disrupted by hail, drought, frost, cloudy weather, heavy fetilization or spray drift. Although acute nitrate poisoning is relatively uncommon, when it occurs the losses can be sudden and severe. With proper testing and ration management, high‑nitrate feeds can often be used safely.

 

What Are Nitrates and How Poisoning Occurs

  • Plants absorb nitrogen primarily as nitrate (NO₃), which is normally converted to ammonia and used to form plant protein.
  • Stress conditions interrupt this process, causing nitrate to accumulate, particularly in stems and lower leaves.
  • In the rumen, nitrates convert to nitrite, then to ammonia for microbial use.
  • Toxicity occurs when nitrate → nitrite conversion exceeds nitrite → ammonia conversion, causing nitrite to enter the bloodstream.
  • Nitrite binds to hemoglobin, forming methemoglobin, which severely reduces the blood’s oxygen-carrying capacity, leading to suffocation.
 

Factors Increasing Nitrate Accumulation

1. Forage Species

  • High-risk: cereals (oats, barley, wheat), annual grasses, and many weeds (e.g., pigweed).
  • Lower risk: legumes and perennial grasses (though issues can occur under extreme stress).

2. Soil Nitrogen Levels

  • High nitrogen fertilizer, heavy manure application, legume plowdown, or summerfallow can elevate plant nitrate concentrations.

3. Stage of Growth

  • Early stages (boot–milk) tend to have higher nitrate levels.
  • Levels typically decline as crops mature (dough stage).

4. Light and Temperature

  • Shaded conditions and high temperatures increase nitrate accumulation.
  • Nitrate builds up overnight and is normally reduced by bright sunlight.

5. Frost

  • Early frost often hits just before sunrise, when nitrate levels are at their peak.
  • Damaged top leaves shade surviving leaves, limiting nitrate reduction.

6. Hail

  • Nitrate is initially low immediately after a storm (following sunny days), but rapidly accumulates overnight.
  • Loss of leaf area restricts the plant’s ability to convert nitrates to protein.

7. Drought

  • Once growth resumes, only top sun‑exposed leaves reduce nitrates.
  • Lower shaded tissues may still contain dangerous concentrations.
 

Changes After Harvest

  • Hay:
    • Slow drying or rain in windrows can reduce nitrates.
    • Rapid drying results in little change.
    • Nitrate levels in baled hay remain stable over time.
  • Silage:
    • Fermentation reduces nitrates by 40–60%, depending on moisture and packing quality.
 

Testing for Nitrates

  • Nitrates can be detected only by chemical analysis.
  • A quick field test indicates presence; laboratory analysis provides actual concentration.
  • Always test suspect feeds before feeding.
 

Feeding High-Nitrate Forages

  • Do not feed forages containing >0.5% nitrate (NO₃) on a dry matter basis.
  • High-nitrate feed must be thoroughly mixed with low-nitrate feed—not fed side-by-side as separate bales.
  • Ensiling can reduce nitrates by 40–60%
  • Adapt cattle gradually over 1–2 weeks.
  • Maintain adequate energy and vitamin A in the ration.
  • Avoid NPN sources (e.g., urea), which worsen toxicity risk.
  • Provide unlimited clean water.

Risk Levels (Dry Matter Basis)

% Nitrate (NO₃)

Risk Level

0–0.3%

Safe

0.3–0.6%

Moderately safe; limit to 50% of ration if animals are stressed

0.6–0.9%

Potentially toxic; do not feed as sole forage

>0.9%

Dangerous; often lethal
 

Symptoms of Nitrate Poisoning

Acute (Lethal)

  • Labored breathing
  • Frothing
  • Rapid pulse
  • Weakness, incoordination
  • Frequent urination
  • Diarrhea
  • Convulsions
  • Sudden death (3–4 hours)
  • Chocolate-brown blood on postmortem

Treatment: Methylene blue (veterinary administration only).

Subclinical

  • Reduced appetite
  • Lower milk production
  • Poor growth
  • Abortions
 

Adapting Cattle to High-Nitrate Feeds

  • Gradually increase inclusion over 1–2 weeks.
  • Frequent small feedings help rumen microbes adapt.
  • If cattle are removed from high‑nitrate feed for several days, they must be re‑adapted.
  • Supplement 2–5 lb grain/head/day when feeding high-nitrate forage to:
    • Dilute total nitrate intake
    • Supply energy for microbes to convert nitrite to ammonia
 

Conversion Table for Lab Reports

 

Unit

Chemical Form

Multiply by → Nitrate (NO₃)

Nitrate

NO₃

1.0

Nitrate-N

NO₃–N

4.4

Potassium nitrate

KNO₃

0.6

Sodium nitrate

NaNO₃

0.7