A Tragedy In Winter

by Dr. Carl S. Hoveland

A Georgia cattleman drives into one of his pastures early one frosty January morning and finds ten cows lying dead. Pastures were very short so the day before he had put a big bale of high-quality pearl millet hay for the cows to eat. Later, the veterinarian diagnosed nitrate toxicity. A terrible loss. Why? Maybe it was ignorance, more likely it was failure to take the necessary precautions at the right time to avoid such a serious loss. Now during the autumn season is a good time to take stock of hay supplies and protect oneself from potential livestock losses as a result of nitrate toxicity.

Pastures and hay plants take up nitrogen from the soil, primarily in the form of nitrate. Under good growing conditions, the nitrate is rapidly converted into nitrite, then into ammonia, then into plant proteins. However, when plant growth is slowed or stopped because of drought or frost, nitrate continues to be taken up by the plant but it cannot be changed into other forms of nitrogen fast enough and nitrate accumulates. When the animals eat this forage as hay or pasture, nitrate poisoning can occur. Although drought and frost are the primary factors causing plants to accumulate nitrate, small grains may accumulate toxic levels during periods of very cloudy weather. Also, many weeds can accumulate toxic levels of nitrate after they have been sprayed with 2,4-D or 2,4-DB herbicides.

Certain plants are more likely to accumulate toxic levels of nitrate than others. Pearl millet, sorghum-sudangrass, johnsongrass and oats are known to be accumulators. Grasses that have been highly fertilized with nitrogen and harvested under severe moisture stress such as bermudagrass, corn, forage sorghum, or tall fescue may also have toxic levels. Weedy hay may accentuate the problem as certain weeds such as pigweed, smartweed, thistle, lambsquarter, goldenrod, nightshade, and stinging nettle are known accumulators of nitrate.

Nitrate levels can change rapidly from day to day, depending on soil moisture levels or frost. Once cut for hay, nitrate levels do not change appreciably. However, ensiling high-nitrate forage will cause a portion to be converted to a brown gas and lost. Thus, silage is safer to feed than hay but there may be some danger.

Nitrates are relatively non-toxic to cattle but after their conversion to nitrite in the digestive tract there is trouble. The nitrite ion oxidizes iron in blood hemoglobin to the ferrous state, forming methemoglobin, which is unable to function as an oxygen carrier. If enough of the hemoglobin is changed to methemoglobin, the animal will suffocate and die from lack of oxygen.

Sublethal symptoms of nitrate poisoning are seldom apparent to a cattleman. There is abdominal pain, diarrhea, muscular weakness, incoordination, accelerated heart rate, sometimes convulsions. Nitrate in the diet at moderate levels will cause reduced milk production, lowered rate of gain, and reproductive problems. Death may occur in a short time after consuming forage with high levels of nitrate. Dead animals will have discolored, dark, chocolate-colored blood. Animals in poor condition on low energy diets will gorge themselves and increase the severity of the problem on a given level of nitrate. Animals that have gradually increased levels of nitrate in the diet tend to be able to tolerate higher levels than animals first exposed to high levels.

The most important thing a cattleman can do to protect against cattle losses from nitrate toxicity is to test hay that was harvested during severe drought or just after frost. This is critical in making decisions on how to feed the hay or if it can be fed at all with safety. Take several representative samples, mix well, and send to a soil testing laboratory. When you receive your report from the laboratory, it will give the level of nitrate nitrogen (NO3-N) in the forage in parts per million (ppm) on a dry matter basis. In practical terms, the report will be able to assist you in determining what to do with the forage.

Finally, to summarize what to do with feeding nitrate-toxic hay:

  • Consider the class and condition of animals consuming the hay. Animals in poor condition will be affected by lower levels of nitrate than animals in excellent condition. Lactating cows and stockers may show reduced performance.
  • Do not feed nitrate-toxic hay to hungry cattle or they will eat excessive amounts in a short time.
  • Dilute the diet by feeding a non-toxic hay or by limiting the time animals have access to the toxic hay. If a big bale of high-nitrate hay is put alongside a low-nitrate bale, some animals will eat only one type and toxicity may result. Nitrate converts to nitrite in hay that has become wet, and this makes the hay more toxic.
  • Feed an energy supplement. Feeding corn grain will increase the rate of nitrate metabolism in the rumen, thereby detoxifying it.
  • If you observe symptoms of nitrate toxicity, give an intravenous injection of methylene blue solution immediately. This reducing agent converts methemoglobin to oxyhemoglobin and reverses the effect of nitrite. Treatment should be repeated in severe cases.

The tragedy reported at the beginning of this article doesn't have to happen. Keep records on when and under what conditions your hay was grown and harvested. If you have any suspicion that the hay might be toxic, sample and have it tested for nitrates. The loss of one cow makes the cost of a few nitrate tests seem cheap.

After reading the article you should be able to answer the following ten question to determine your comprehension of college-level text.

  1. What does nitrate convert into if good growing conditions exist?
  2. What are the two primary factors causing plants to accumulate nitrate?
  3. Name three weeds that are known accumulators.
  4. What does oxidizing mean?
  5. Why does a low-energy diet cause animals to be in a poor condition?
  6. Why should a farmer or cattleman mix his/her hay samples before sending them to the laboratory?
  7. How are lactating cows and stockers different?
  8. Explain how supplemental corn can detoxify the toxic hay?
  9. How might a cattleman diagnose nitrate toxicity?
  10. How might the location of the cattle ranch affect the number of nitrate toxicity cases?

After answering the above questions, you may go back and check your answers with the text. You should be able to answer 8 of the ten questions to easily comprehend college-level text.

The reading sample is an article from The Georgia Cattleman, October 1995. It contains almost 800 words and should take about 2-3 minutes to read. College readers should read at least 300 words per minute. High school students should read about 250 words per minute to handle the amount of reading involved in high school courses.