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Main Content

Prevent Cattle Bloat

Cattle bloat can range from mild to heavy to dangerous.

By Heather Smith Thomas

Illustration by Tom Milner

Cattle bloat can happen at any time on grazing pastures, but cattle are most susceptible to this deadly health hazard during times when grass growth is at its peak, particularly in the fall. Autumn's cool nights, heavy dew, and frost, along with moderate daytime temperatures, create pasture conditions that increase bloat risk.

Bloat is distension of the cow's rumen — part of its stomach — due to excessive gas production. In extreme cases, bloat can be fatal. Gas production is normal during fermentation/digestion of feeds, but if the cow can't get rid of the gas quickly enough by belching, bloat can occur. And if gas builds up faster than it can be belched out, the rumen may become so full that it puts pressure on the lungs — and the animal suffocates.

Certain types of forage plants are more likely to cause bloat. Legumes such as alfalfa, most types of clover, and winter wheat pasture can cause the rumen content to become frothy and sticky and block the opening into the esophagus, which prevents release of the gas.

Other factors that increase risk for bloat include fertilization of pastures — making them grow faster and lush — and moisture from dew, rain, or frost. The cow produces less saliva, which inhibits bloat because it contains sodium and bicarbonate, hindering excessive gas production. Short, lush plants with high protein content and very little fiber also cause a risk for bloat. If cattle are hungry when first put into a legume pasture, they are more likely to overeat and, therefore, bloat.

Avoid bloat on fall pastures by waiting until legumes are mature before grazing them. Bloat potential is highest when plants are in pre-bud stage, and decreases when they are at full flower or later. Make sure cattle are already full when they're moved into a new pasture. Put them into it mid-day instead of early morning when they are hungry. Choose a dry day and wait until any dew or frost is gone. Cattle bloat twice as often in October as during summer months, studies show, despite the fact that stockmen generally think alfalfa is safe to graze after a killing frost. But there is risk as long as plants are green. The first fall frosts actually increase risk for bloat, preserving the immature stage of plant growth. Frost also breaks cell walls, releasing bloat-causing agents and increasing rate of cell breakdown, hastening the fermentation process in the rumen, and the possibility for bloat. It usually takes several hard freezes before alfalfa is truly safe to graze.


  • Most risk — Legumes such as alfalfa and most types of clover, and winter wheat pasture.
  • Moderate risk — Spring wheat, oats, and perennial ryegrass.
  • Least risk — Most perennial grasses, birdsfoot trefoil, and lespedeza.

If legumes are 50 percent or less of the plant mix in a pasture, this can minimize pasture bloat unless cattle selectively graze the legume and avoid the grass. Legumes regrow faster than grass after being grazed, so when seeding a pasture, use a species of fast-recovering grass such as timothy or orchardgrass.

If bloat is a problem in a rotation system, mow one-fourth of the new paddock in the afternoon or evening — so it will be drying — and graze it the next day, using portable electric fence to make the animals eat the mowed part first.

Make sure cattle always have plenty of salt at pasture — preferably loose salt that they can eat quickly, rather than hard blocks. If cattle have adequate sodium, they are much less likely to bloat.

Heather Smith Thomas has authored several books on cattle care.