Forage-related disorders in cattle

grazing pasture

Did you know that cattle and other livestock can get ill or even die simply from consuming forages?

For the last month, pastures and hay fields have been growing steadily.

Now that livestock are back on pastures, producers should be aware of common forage-related disorders to prevent them from happening.

Three common forage-related disorders producers often experience are bloat, grass tetany and nitrate poisoning.


Formation of a stable foam in the rumen prevents belching of gases that cause bloating (bloat) in the animal.

The gases are produced from microbial fermentation of the forage, which are then retained in the animal.

Then pressure in the rumen (left side) increases that cuts off the animal’s belching mechanism.

When this occurs, oxygen is cut off or reduced, resulting in suffocation. In severe cases, death can occur within an hour.

Cattle are especially susceptible grazing pastures dominated by certain legumes like white clover and alfalfa.

To prevent bloat, hungry animals should never be allowed to graze legume pastures or winter annual grass fields.

Producers should feed dry hay before turning animals to graze these types of fields and pastures.

Also, animals should not be placed on fields that have wet forage with dew or frost. Bloat hazards are greatly reduced when grazing on fields with 50% grass or more.

Also, a useful preventative measure for grazing pastures with heavy legumes is providing animals with salt-molasses blocks containing ingredients that reduce the formation of this stable foam in the rumen.

Grass tetany

Grass tetany occurs when low levels of magnesium exist in the blood of cattle and sheep that have been grazing cool-season pastures in the early spring (or late winter).

However, ewes and cows in early lactation are often affected the most.

Grass tetany results in animals grazing forages that have soil low in magnesium.

Signs of grass tetany are nervousness, muscle twitching, staggering and going down, with spasms or convulsions. If not treated, death can occur.

Soil testing the pasture will determine the field’s magnesium level. Pastures deficient in magnesium should be limed with dolomitic limestone.

Producers should also provide animals with a supplemental magnesium mineral mix, especially during early spring for dependable tetany control.

Nitrate poisoning

Animals grazing on forages with high levels of nitrate-nitrogen can experience nitrate poisoning.

If the weather is dry (low soil moisture and low humidity), nitrates can build up in forages that received a heavy fertilizer of nitrogen.

However, fertilizer applications with nitrogen during cool, wet, cloudy weather may also result in nitrate poisoning.

Some plants are more likely to accumulate nitrates like sudangrass, sorghum-sudan hybrids, corn, wheat and oats.

Also, certain weeds are more likely to accumulate nitrates in hay crops like pigweed, lambquarters, Canadian thistle and goldenrod.

Plants with high levels of nitrates are highly toxic to many classes of livestock. Animals can tolerate low levels of nitrates, but it can quickly increase in the blood, surpassing the tolerant threshold level.

Toxicity occurs because the nitrates are reduced to nitrites during digestion. Then nitrites oxidize the iron in blood hemoglobin, which prevents necessary oxygen movement.

Animal symptoms include labored breathing, muscle tremors, staggering, gasping for breath and, sometimes, death.

To prevent nitrate poisoning, animals should be watched closely, grazing on pastures that received a high level of nitrogen fertilizer.

If possible, delay grazing in fields that received heavy nitrogen fertilizer.

In hay fields that received nitrogen fertilizer during drought periods and then was baled, the hay should be analyzed in a laboratory to determine the nitrate level.

If hay is found to be toxic, then it can be grounded and mixed appropriately with other materials.

If you suspect your animal of having one of these conditions, contact a veterinarian right away. Consult your local Extension agent with questions about forage-related disorders.


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