Consider Toxicity Issues When Grazing In The Fall

Consider Toxicity Issues When Grazing In The Fall
Oct 07, 2022

Many livestock producers are utilizing stockpiled pasture, hay regrowth and warm- or cool-season annuals to extend the grazing season this fall.

North Dakota State University Extension livestock specialists urge producers to consider grazing management and potential plant and animal health implications when grazing certain forages this time of year.

Bloat associated with alfalfa and other highly-digestible forages

Bloat can occur any time that cattle are grazing large quantities of highly-digestible forage. Gases produced through the fermentation of forage are trapped in the rumen, putting pressure on the lungs and nerves, which affects the animals’ breathing. Pastures that contain 50% or more legumes such as alfalfa or clover present a bloat risk, as do brassicas (canola, turnips, radish) and small grains.

The incidence of bloat tends to be greater early in the day, following a rain or after a frost. The risk is greatest in the first three to five days after a killing frost, but it is best to avoid grazing for at least a week. This gives the top half of the plant time to dry down and decreases bloat risk.

Janna Block, NDSU Extension livestock systems specialist based out of the Hettinger Research Extension Center, suggests using the following management practices:

  • Ensure that cattle are full of dry hay before allowing them to graze forage with bloat potential
  • Delay turnout until pastures are dry after dew or rain
  • Monitor animals every couple of hours after turnout for the first couple days

If possible, provide cattle with access to other grazing, such as a permanent pasture. Providing a poloxalene block prior to and during grazing and placing grass hay bales in the field for grazing are additional management considerations.

Sweetclover, a biennial legume common in pastures, also may present a bloat risk. However, the biggest health risk associated with sweetclover is when it is harvested as hay, notes Karl Hoppe, NDSU Extension livestock specialist based at the Carrington Research Extension Center.

“It contains a substance called coumarin that is converted to dicoumarol due to the presence of mold in hay that was baled too wet,” Hoppe says. “Dicoumarol is a potent blood anticoagulant that can cause hematomas, abortions, and excessive bleeding. It is important to note this toxicity is not an issue when grazing.”

Grazing crop aftermath or standing grain crops can present several risks. High grain intake by cattle that have not been properly adapted can lead to bloat, founder or death. If volunteer grains matured to seed formation or mature grain is present in a field, grain overload from selective grazing could be a problem.

Producers should scout fields before turning cattle out to determine how much grain is present.

“Using strip grazing to limit access and adapting cattle to grain a week or so prior to turnout can also help manage this issue,” Block says.

Grass tetany

Grass tetany is a potentially fatal condition in beef cattle caused by a magnesium (Mg) and calcium (Ca) deficiency combined with high levels of potassium (K). Although most producers associate grass tetany with grazing immature cool-season grasses in the early spring, it is possible for cattle to be affected by tetany when consuming lush fall regrowth in grass pastures or annual cereal forages. This situation is less common in North Dakota because the mineral profile of fall regrowth is not exactly like new spring growth; however, it is important to be vigilant. 

To prevent tetany, producers should consider supplementing their cattle’s feed with a mineral containing 8% to 12% magnesium. Most mineral supplements contain magnesium oxide, which is unpalatable and may need to be mixed with grain or molasses to encourage consumption. If available, magnesium sulfate also is a good source and may be more palatable.

Potential toxicity issues

“Many species of cover crops and small grains can be toxic,” notes Miranda Meehan, NDSU Extension livestock environmental stewardship specialist. “Potential toxicities include prussic acid poisoning, nitrate toxicity and sulfur toxicity (polioencephalomalacia or PEM). Brassicas in particular carry a high risk of multiple toxicities and should not constitute more than 70% of the diet.”

“It is important to understand that nitrates and prussic acid are two separate issues and are not directly related to each other,” Meehan says.

Prussic acid (also known as hydrogen cyanide or HCN) mainly is a concern with sorghums and sudangrass, while pearl and foxtail millet typically do not cause issues. Toxic levels most commonly are associated with frost, but can also be caused by damage from hail, insects and harvest. Damage to the plant ruptures cells and releases cyanide gas.

The first few frosts in the fall increase the potential for prussic acid poisoning. New growth from frosted or drought-stressed plants is palatable but also will be dangerously high in cyanide.

James Rogers, Extension forage specialist based at the North Central Research Extension Center in Minot, recommends that producers leave a stubble height of at least 6 inches and do not graze regrowth until it is 18 inches tall.

“Grazing should not occur for at least five to seven days following a killing frost,” Rogers says. “If harvesting for hay, cyanide risks are minimal assuming that hay is properly cured and baled.”

For more information on prussic acid toxicity, please refer to the NDSU Extension “Cyanide Poisoning” publication at

Nitrate-accumulating crops include small grains, millet, brassicas, corn, sorghum and sudangrass. Rangeland or pasture weeds such as pigweed, Russian thistle, lambsquarter and kochia also are nitrate accumulators. Reduced growth in annuals during the fall may slow conversion of soil nitrogen to protein and amino acids in the plant, causing high levels of nitrate to accumulate. Dangerous levels of nitrate occur several days after a light frost but typically decrease within 10 to 14 days if conditions improve and the plant starts actively growing again.

Unlike prussic acid, nitrate levels do not decrease after a killing freeze. When plants die off, nitrogen uptake by roots will cease, but nitrate that is in the plant at that time will remain because no further photosynthesis will take place.

Most county NDSU Extension offices offer a “Nitrate QuikTest” that can be used to determine presence or absence of nitrate prior to grazing or haying. For baled hay, the best option is to use a hay probe to collect samples and analyze them for nitrate content.

This year, many Extension offices are offering free nitrate analysis on forage samples as part of a statewide programming effort to gather information about the effect of environment and management on nitrate content. If producers are interested in participating in this program, they should contact their county Extension agent.

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