Forage induced animal disorders

Naturally occurring plant toxins are not unfamiliar or scarce and it may sometimes give a negative impact on animals as these compounds are frequently present in forages they consume. Herbivores are unable to consistently avoid harmful plants. So, the controller must be aware of forage related factors that cause serious injury to animal health. One of the major causes of animal health demolition is the imbalance of nutrients. Interaction between plant species, environmental conditions, and management practices may cause the occurrence of health problems. Different anti-quality compounds affect different biochemical pathways of animals. This may lead to different symptoms and severity of impact. One should understand the biochemistry involved to get more approximate and preventive measures.

Why do plants contain harmful compounds?

One suggestion is that such compounds developed over time as a mechanism for prevention of herbivore feeding and thus produce anti-herbivore agents. This characteristic could be beneficial if reducing defoliation is required for plant survival and reproduction. One of its best examples is the availability of high levels of HCN (hydrogen cyanide) in seeds of certain sorghum genotypes that greatly reduce feeding by birds. Forage related animal disorders can be categorized in three main groups:

  • Poisonous animal disorders
  • Seasonal or conditional disorders
  • Species related disorders

Poisonous plant disorders

These are unpleasant and may be present in pasture and rough land conditions. These kinds of disorders resulting from poisonous plants are not unfamiliar especially under conditions of overgrazing when forage availability is limited. These toxic effects in plants may cause short term damage to animal health or even sometimes may cause rapid death. 

Following are some forage plant families and species that contains toxic compounds:   

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It seems natural that animals should avoid consuming toxic plants. Herbivores tend to select nutritionally superior diets especially concerning energy and proteins. They naturally have some ability to avoid toxins in their diet. Young animals in herd learn from their dams which plant to consume and which to avoid. Animal disorders usually occur when animals are hungry and unable to avoid harmful plants. These disorders may be reduced if these poisonous plants are eliminated from the pasture and fields.

Seasonal and conditional disorders

These kinds of disorders occur only under certain environmental conditions at specific plant growth stages or certain vulnerable stages for animals. Mineral uptake pattern of forage plants is disturbed by environmental temperature and may cause grass tetany. Animals are more sensitive to this disorder at some age or reproductive stage as compared to other disorders. Tolerance towards forage toxins is sometimes lower during gestation, lactation, and weaning than during other life stages.

Grass Tetany

Grass tetany also called hypomagnesemic tetany is a metabolic disorder due to low blood magnesium level. It occurs usually during transition from winter to spring when temperature rises to the range of 40-60°F encouraging a rapid flush of grass growth. It is most common in grass pastures but may also occur in pasture containing legumes or non-grassy hays. 


Cattle with early symptoms may exhibit nervous behavior or graze away from the herd. In extreme conditions, animals collapse and go into convulsions. Affected animals may arch their head back and trash the legs may be called paddling. Older cows nursing calves under 2 months of age are more prone because recombination of magnesium from their tissue is less effective.

How to avoid Grass tetany?

Strategies to avoid grass tetany include direct supplementation of animals with Mg. Supplementation is usually accomplished by adding 75-150 pounds MgO per ton of salt per mineral mixture. For dairy cows, an intake of 30g Mg per day is recommended for lactating ewes an intake of 3g of Mg per day is recommended. Animals showing early symptoms of grass tetany (before coma) can be given intravenous injections of Ca Mg gluconate solution. Subcutaneous injections of a saturated solution of MgSO₄ may also be given. 


Legume bloat occurs when stable foam forms at the surface of the floating raft actively digesting forage in the rumen and blocks the access to the esophagus carrying gases to accumulate. Death losses calculated due to bloat are 0.5% of the cattle population annually. 

The large amount of gases released during fermentation in the rumen are CO₂ and CH₄. Bloat occurs in animals consuming hay.


Bloating animals exhibit distension of the rumen on the left side. Other symptoms of bloat include cessation of grazing, frequent urination, labored breathing and restless movements. In extreme conditions distension may also occur on the right side. Onset of acute bloat may cause death within minutes. Sub-acute bloat may cause distension of the left side with no negative effect on the animal. 

How bloat can be treated?

Bloat can be cured by adding vegetable oil or any other anti-foaming agent directly to the rumen. Bloat can normally be controlled by feeding anti-foaming agents like poloxalene. Supplementation of cattle diet with monensin, an ionophore reduce the bloat incidents. Another way to reduce bloat is providing animals with the dry forage before grazing. 

Nitrate toxicity

Negative effects including death can occur when ruminants ingest more than the ability of rumen microbes to convert into nitrate to ammonium form. When nitrate is produced faster, the rumen microbes can utilize it, some nitrate is absorbed through the walls of rumen and enters the bloodstream. Once it enters the bloodstream, nitrate transforms hemoglobin into the form methemoglobin, thereby restricting its ability to transport oxygen to body tissues.


Rapid breathing, muscle incoordination, diarrhea, and frequent urination are symptoms of nitrate poisoning. Blood from affected animals changes its color from normal bright red to chocolate brown. 

How nitrate toxicity can be treated?

Treatment involves intravenous injections of methylene blue solution. Feeding an energy supplement such as corn can hasten microbial utilization of nitrate thus reducing nitrate accumulation in the rumen. Nitrate test can be done on forages such as corn and sorghum by cutting a shoot near the soil surface, splitting the stem lengthwise and dropping a solution of diphenylamine in H₂SO₄. Onto the exposed pith. 

Development of dark blue color indicates the presence of nitrate. If this Qualitative field test is positive, a sample should be collected for lab analysis and sent to the lab for nitrate testing.

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