Aniaml Health Problems: Life-Threatening Illnesses


Excerpt from Publication 19, Pasture Production, Order this publication

Table of Contents

  1. Grass Tetany
  2. Nitrate Poisoning
  3. Bloat
  4. Prussic Acid Poisoning
  5. Other Animal Health Problem Recommendations
  6. Related Links

Grass Tetany

Grass tetany or hypomagnesemic tetany is a condition that occurs when an animal's daily diet does not provide enough magnesium to maintain a normal level of magnesium in the blood serum. It is seen in adult animals sooner than young animals, and most often in cows and ewes shortly after giving birth. Cows milking heavily or sheep with twin lambs or triplets are more susceptible than lower-producing animals.


Symptoms of grass tetany are usually observed 5-10 days after animals are turned out to pasture. Lush spring grass pastures can be low in magnesium, with their high moisture content reducing dry matter intake - thereby reducing the intake of available magnesium to a critical level. Typical symptoms are: nervousness, tremors and twitching, staggering, convulsions and death. Death occurs quickly, within 6-10 hours after the onset of the first symptoms. An animal dead from grass tetany is usually found with the ground about it churned up, foam on its mouth, and a pile of loose manure behind it. Blood serum magnesium levels can drop so quickly it is possible for tetany to occur on the first day out to pasture. Chronic hypomagnesemia can also occur, with the plasma magnesium level dropping slowly over a relatively long time. The clinical symptoms of the disorder occur when the animal is under additional stress.


The magnesium supply from pastures is affected by the botanical composition of the pasture, the stage of maturity of the plants, soil conditions and fertilizer treatments. Legumes and herbaceous weeds tend to have a higher magnesium content than grasses. White clover is one of the best accumulators. Most of the commonly used grasses tend to have similar abilities to accumulate magnesium, with the exception of tall fescue and orchardgrass. Tall fescue is a better accumulator than most grasses, while orchardgrass is much worse. With both legumes and grasses there is a decline in magnesium content as the plants mature. Wet, cold soils increase the chances of tetany occurring. Keep your animals off poorly drained pastures, particularly during periods of rainfall in spring and fall.

Fertilization affects the magnesium content in pastures in 2 ways:

  • it changes the botanical composition of the pasture. Use of nitrogen on a legume-grass stand encourages grasses and can lead to a decline in the legume content.
  • it directly affects the magnesium concentration in the plants.
Nitrogen can increase the magnesium concentration in grasses when the soil magnesium level is not limited. However, if the soil magnesium concentration is low and there is a high potassium concentration, high nitrogen fertility may result in an excessive uptake of nitrogen and potassium and a reduced uptake of magnesium. Heavy potassium fertilization, alone, may also decrease magnesium uptake by plants growing in soil low in magnesium. Soil testing to determine the potassium and magnesium soil levels before fertilizing is an important step in preventing grass tetany.

Other options to prevent grass tetany include:

  • feeding supplemental magnesium
  • offering hay before turning the animals out onto lush grass pastures
  • avoiding high rates of nitrogen by applying split applications
  • applying potassium in the fall, not spring.

If you suspect grass tetany, contact your veterinarian as soon possible.

Nitrate Poisoning

Nitrate is a normal plant metabolite. In a ruminant's digestive tract nitrates are broken down into nitrites and ammonia. Toxicity may occur when the rate of production of nitrites exceeds the rate of conversion to ammonia. The formation of large amounts of nitrites is dangerous to the animal. Nitrite is absorbed into the bloodstream where it reacts with haemoglobin to form methaemoglobin. This compound is incapable of releasing oxygen and, in lethal cases, the animal is killed by oxygen starvation. Cattle are more susceptible to nitrate poisoning on pasture than sheep, goats, or horses. Stressed animals are more vulnerable to nitrate poisoning.


The symptoms of acute poisoning are trembling, staggers, rapid breathing and dark-looking mucous membranes. Death can occur in as little as 3-4 hrs. Nonfatal poisonings can cause poor growth, poor milk production and abortions. Vitamin A storage is also affected in cattle. Contact a veterinarian as soon as nitrate poisoning is suspected.


Normally nitrate levels are low in the grasses and legumes commonly used in pastures. However, occasionally there can be sufficient nitrate accumulation to cause poisoning. Several factors influence nitrate accumulation by plants:

  • Environmental conditions such as drought, uneven rainfall distribution, long periods of cloudy weather, high temperatures and frost increase nitrate accumulation by slowing down the plant's normal growth rate. If conditions improve and plants start actively growing, some of the accumulated nitrates will dissipate and the danger of poisoning decreases.
  • Some plants are naturally good accumulators of nitrates. These include species commonly used for annual pastures (small cereals, sorghum, sudangrass, forage rape, and kale) and such weeds as lamb's quarters, thistles, pigweeds and witchgrass.
  • Nitrate levels are highest in vegetative plants, with nitrate accumulation peaking just before the plants start to bloom.
  • There is a direct response in plant nitrate levels to increasing levels of nitrogen fertilizer. High nitrogen rates on grass pastures can cause some grass species (orchardgrass, tall fescue, meadow foxtail and reed canarygrass) to accumulate unsafe levels of nitrates. Nitrate accumulation is also greater with nitrate forms of fertilizer than with urea or ammonium sulphate.



Due to the diverse factors influencing nitrate accumulation in plants, it is not possible to easily predict when nitrate poisoning may occur. To minimize the chance of poisoning, do not graze pastures containing a high percentage of plants that are good nitrate accumulators, or grass pastures treated with high amounts of nitrogen fertilizer, during or after periods of poor growth. Wait 10-14 days for the pasture to recover. A check on nitrate levels can be made by sending a sample (made up of pasture clippings) to a feed-testing laboratory. Water may also be a source of nitrates to grazing animals. If nitrate poisoning is suspected, test both the plants and water to determine the source of the problem.


Bloat results when an animal's rumen and reticulum become distended with gas. With pasture bloat the gas is trapped in millions of bubbles, giving it a foamy appearance.


The animal is unable to belch or burp up these small bubbles. The visible result is that the animal's left flank starts to swell. The animal becomes uncomfortable, may kick at its belly, or get up and down more frequently than normal. Breathing becomes difficult and the animal takes shallow, rapid breaths. The tongue may protrude and there is some slobbering. Death can take place within 2-3 hrs after the consumption of the bloating forage.


Bloating normally occurs on lush pastures with high alfalfa or clover content, but can also happen when animals graze cereal crops, forage rape, and young grass pastures with a high protein content. Alfalfa and ladino clover are considered more hazardous than white Dutch, red or alsike clovers. Cattle appear to be more susceptible to bloat than other ruminants. In addition, some individual animals are more likely to bloat than others.


The bloat hazard may be reduced in a number of ways.

  • Plant pasture mixtures that do not have high percentages of bloat-causing legumes. In cattle pastures limit the amount of bloat-causing legumes to 30%, while in pastures for other ruminants do not exceed 50%.
  • Use birdsfoot trefoil. It is the only commonly used legume that does not cause bloat.
  • Never turn hungry animals onto suspect pastures. Feed hay to animals before they enter the pasture to prevent them from gorging on bloat-causing plants.
  • Introduce animals to pastures that have bloat-causing legumes for only a short time. During the next few days, gradually lengthen the time the animals may remain in the field.
  • Introduce animals to suspect pastures when the plants are dry. Heavy dew or moisture from rain increases the chances of bloating taking place.
  • Wait until bloat-causing plants are flowering before grazing. Vegetative plants and legumes in pre-bud to bud stage are more likely to cause bloat.
  • Use rotational grazing to maintain a steady rate of feed intake and to reduce selective grazing.
  • Wait a few days after a killing frost before grazing suspect pastures. The risk of bloat occurring increases immediately after a killing frost.
  • Use an anti-foaming agent such as Poloxalene when the potential for bloat is great.

Bloat can happen at any time during the grazing season. A close watch is needed at all times, as the bloat-producing potential of a pasture can change quickly.



Bloated animals must be treated quickly. With the first signs of bloat, remove the animals from the pasture and consult a veterinarian. Keep the animals on their feet and dose the animals with an anti-foaming agent or surfactant such as mineral oil, raw linseed oil or liquid dishwashing detergent. In severe cases, when the animal has gone down and death is imminent, you must puncture the rumen and allow the gas to escape.

Prussic Acid Poisoning

Several plant species contain glycosides that can poison animals. Cyanogenic glycosides can be found in wild cherries, marsh-arrow grass, a few strains of New Zealand white clover and in plants in the sorghum family. The amount of cyanogenic glycosides found in these plants is influenced by plant genetics, stage of growth and environmental conditions.


When ingested, cyanogenic glycosides form prussic acid, which is highly toxic. The prussic acid interferes with the exchange of oxygen from the lungs to the body tissues.


The symptoms of acute poisoning are muscle tremours, difficult, rapid respiration, and convulsions. Death can occur so quickly that the other symptoms are not observed.


The highest levels of glycosides are usually found at early stages of plant growth. Do not graze sorghum-sudan hybrids until it reaches a minimum height of 75 cm and sudan grass a minimum of 45 cm in height. Environmental stress of any kind can increase the amount of cyanogenic glycoside to hazardous levels. After stresses such as drought, long periods of cloudy weather or frost, do not graze sorghum for at least a week. A check on cyanogenic glycoside levels can be done at a feed-testing laboratory.

Related Links

... on forages and pastures, visit Forages and Pastures (OMAFRA)
... on weed control, order Publication 75 Guide to Weed Control
... on agronomy for field crops, order Pub. 811 Agronomy Guide for Field Crops: Chapter 3 Forages
... on field crop protection, order Publication 812, Field Crop Protection Guide
... on livestock, visit Livestock (OMAFRA)


For more information:
Toll Free: 1-877-424-1300
Author: OMAFRA Staff
Creation Date: 01 February 2000
Last Reviewed: 15 July 2004