Home > Scientific studies and reports > Disease

Cause and control of common diseases of pastured cattle


Cause and control of common diseases of pastured cattle



There are a number of diseases and conditions that dairymen should be aware of when pasturing cattle. While the pasture environment provides many benefits to animals, there are some diseases and conditions that adversely affect cattle on pasture. This article will give a brief overview of the causes and offer some control measures.


Methane and carbon dioxide are normal byproducts of rumen fermentation. Under normal circumstances, these gasses are expelled through belching (eructation). Bloat occurs when the animal cannot expel the gasses and they accumulate in the rumen.

There are several causes of bloat. Excessive foam production is the primary cause in cattle on pasture. Legumes pastures tend to cause bloat more often than other types of plants. Both the type and quantity of proteins in legume plants contribute to their foam-producing capacity. Excessive foam in the rumen blocks the gasses from escaping to the esophagus. While legumes are the primary culprits, other species, such as immature grasses can have a similar effect.

Symptoms of animals with bloat include: distension of the left side, discomfort, kicking of the belly, labored breathing, frequent urination and defecation and collapse.

Acute bloat can be treated by passing a stomach tube to help relieve the gas, or, in severe cases, using a bloat needle or trochar directly into the rumen through the animal’s side.

There are several management practices that can help reduce the incidence of bloat. Managing pastures so that they contain 50% or more of grass species will reduce the likelihood of bloat. If only legume pastures are available, supplementing with grass hay can help. It is better to gradually adapt animals to legume pastures and then allow them to continuously graze them, than to graze them for short periods of time and remove them, at night for example.

In addition, cattle can be fed poloxalene, an anti-foaming agent, if they are grazed on bloat promoting pastures. This is normally provided in a molasses-salt block, but it can be top dressed on feed or given as a liquid supplement. Studies have shown that poloxalene is effective in reducing the incidence and severity of bloat in pastured cattle. In order to be effective, cattle must consume poloxalene on a daily basis. As a general rule, one block should be available for every four or five animals. Blocks should be placed where the animals tend to congregate, but not near water sources.

Grass Tetany (or Grass Staggers)

Technically, grass tetany is a condition in which low levels of blood magnesium (hypomagnesaemia) cause a metabolic disorder. Often, however, simple magnesium deficiency is not the true cause. Many times low blood magnesium is precipitated by complex mineral interactions. Common culprits are high potassium and/or nitrogen content of grasses, which interfere with the body’s ability to absorb and metabolize magnesium.

Grass tetany occurs most often in the spring, when grass pastures are immature and rapidly growing, but also is prevalent during the fall re-growth period. Grass species, especially young cereals, have relatively low magnesium levels and are most often associated with this disorder. Any condition that favors the growth of grasses over legumes in the pasture sward will increase the risk of tetany occurring. Examples of this include, pesticide spraying of broadleaf weeds and nitrogen or potassium fertilizer application.

Symptoms of grass tetany include: wandering, staring, tremors, staggering, collapse, thrashing and death. Often animals are found dead in the pasture without any symptoms being observed. Usually when an animal is found dead, there is evidence of thrashing.

If an animal is found in the beginning stages of grass tetany, immediate attention is required. The most common treatment is intravenous injection of a calcium and magnesium solution. Consult with your veterinarian about dosages and procedures.

Several management practices can be employed to try to prevent this disorder from occurring. It is always important to introduce cattle to pasture slowly, allowing their digestive system and metabolism to adapt to the new feed. Allow grass to grow to at least 4 to 6 inches before allowing cattle access. Maintaining a sward with a healthy balance of grass and legume will also help. Avoid over fertilization of pasture swards with manure or nitrogen fertilizers. Supplementation of magnesium is also recommended, especially if grass tetany occurs regularly. Keep in mind that supplementation will not correct all causes of grass tetany. Magnesium oxide, the most common source of supplemental magnesium, is not particularly palatable. It is often added to a salt-mineral mix or a protein mix, but care must be taken to ensure that it is actually consumed.



Cattle that are grazed on pasture are often exposed to high parasite loads. Typically, it is young cattle that are most affected by internal parasites. Mature cattle seem to develop immunity and do not show the same effects of a heavy parasite burden. In addition, cattle that are under stress will be more affected. For example, cows in early lactation are more susceptible to internal parasites than in later stages. The signs of parasite infection can manifest as either clinical or subclinical. Clinical cases show symptoms such as: poor hair coat, diarrhea, potbellied appearance, anemia and edema. Subclinical cases do not show overt signs, but instead cause a decrease in productivity and performance, such as decreased milk production, conception rate, feed efficiency, etc. Often these effects are felt economically, but not attributed to parasite infection.

There are three major types of internal parasites that cause problems in pastured cattle: roundworms (nematodes), lungworms and liver flukes. There are other parasites that infect cattle, but they tend to have minimal effects on animal performance.

Roundworms mature to adult stage inside the intestines of the host animal. Eggs are produced, which are passed through the feces to contaminate the pasture. The eggs hatch and develop into larvae that cattle ingest as they graze. The most common and damaging types are stomach worms. In the gastrointestinal tract, some of these worms can attach to the stomach wall and feed on blood. In heavy infestations, this can lead to serious anemia. Other stomach worms destroy the stomach lining and interfere with digestion. Other types simply consume digested nutrients, causing a decrease in feed efficiency.

Lungworms are mostly a problem in the Southeast U.S. and areas with warm/humid climates. Lungworms live in the air passages to the lungs. Under a heavy parasite load, air passages can become blocked, making breathing difficult. Young stock tend to be particularly susceptible to this type of infection. In addition, outbreaks are heavily influenced by warm, wet climate conditions.

Liver flukes are a common problem in areas with wet marshy environments, especially in bottomlands along river systems and in coastal areas. Unlike the previous two parasites, the fluke requires an intermediate host, the snail, as part of its life cycle. Infection mostly occurs during the late fall and spring.

Control of internal parasites is essential for grazing cattle. Pasture management and strategic deworming programs go hand-in-hand for accomplishing control.

Pasture management suggestions

  1. Allocate pastures according to susceptibility. If you have pastures that you know are likely to be contaminated, reserve those for mature cows that are dry or in late lactation. Reserve pastures that are less likely to be contaminated (i.e. have not been grazed for at least a year previous) for young stock and early lactation cows.
  2. Keep stocking rates under control. High stocking rates, as are common with rotational grazing schemes, can force cattle to consume contaminated pasture. This will also occur with pastures that are overgrazed.
  3. Allow for pasture rest periods. Keeping fields rotated and allowing adequate rest time will let sunlight help kill exposed parasite larvae. Dragging the field to spread the manure can also aid in the process of exposing the larvae to sunlight.
  4. Provide water sources. Cattle with direct access to ponds, streams or springs will contaminate those water sources.
  5. Improve pasture drainage.
  6. For problems with liver flukes, snail populations must be controlled.

Strategic deworming

Treatment with chemical dewormers (anthelmintics) is an integral part of the entire parasite control scheme. For maximum effectiveness, treatment should be timed so as to interrupt the life cycle of the parasite, as well as killing the adults already infecting the animal. This approach significantly reduces pasture contamination. If parasite loads are high, it is important to treat all animals in the herd, as any untreated animals will be able to reinfect the pastures. Traditional products do not have a sustained effect and require repeat treatments. More modern products are stored in the body and released over a longer period. It is important to consult with you veterinarian about the appropriate treatment to use, taking into consideration withdrawal times, parasite load, type and cost effectiveness.


The two major pests that attach pastured cattle are horn flies and face flies. Both type of flies breed in fresh manure.

The horn fly feeds off of blood and spends almost all of it time on the animal. Horn flies tend to congregate on the shoulders, back and sides of the animal.

The face fly feeds off of animal secretions, primarily of the eyes and nose. They cluster on the face when feeding, but do not spend any other time on the animal. Because of the time spent around the eyes, face flies can transmit eye diseases, such as pinkeye.

Horse and deer flies can also be a problem. They tend to be found in pastures close to woodlands or wet marshy areas. When these flies bite, they leave a bleeding wound, which can attract additional face flies.

In general, typical control measures for house or stable flies do not work for horn or face flies. The best methods for control of these pests are ear tag and/or self-pplicating devices, such as back rubbers, that treat the animal with an oily insecticide preparation. Feed-through products tend to be less effective, especially on pasture flies. Read product labels carefully, as certain treatments may not be used on lactating dairy cattle. In addition, not all products are effective against both horn and face flies. Typically ear tags are best used in the summer, when fly populations reach their peak. It is recommended that ear tags be removed in the fall to avoid development of resistance to the insecticide.

Related Links:

Bloat Prevention & Treatment – NebGuide #G74-149-A
Rick Stock, Rick Rasby & Duane Rice

Grass Tetany – NebGuide #G73-32-A
Paul Q. Guyer, Alex Hogg & Gene White

Magnesium Deficiency – University of California Davis, Vet Med Extension
John H. Kirk

Grass Tetany in Cattle – treatment and prevention – NSW Agriculture
Mac Elliott

Internal Parasites of Cattle: Seasonal Patterns of Infection and Control – LSU AgCenter
James C. Williams, Alvin F. Loyacano, Andy A. DeRosa & Jeffrey A. Gurie

Parasite Problems of Dairy Replacements – University of Florida Cooperative Extension
B. Harris & J.K. Shearer

Dairy Cattle Insect Management – NebGuide #G93-1141-A
John B. Campbell