Forage-related animal disorders are caused by anti-quality constituents. Anti-quality constituents are: “Chemical compounds that have negative effects on forage intake or produce negative responses in animals consuming the forage.” Some toxic compounds occur naturally. Others result from insect infestations, while some result from microbial activity. Three main groups of plants that cause animal health problems are:  1. Poisonous plants, 2. Seasonal or conditional disorders, and 3. Species related disorders.

Poisonous Plants

Poisonous plants contain some toxic compound that is harmful to livestock, either chronic or acute.  

Example poisonous plants: 

  • common groundsel 
  • poison hemlock 
  • tansy ragwort 
  • black cherry 
  • chokecherry nightshade

Herbivores tend to select nutritionally superior diets and have some ability to avoid toxins.  However, they are unable to consistently avoid harmful plants, so managers must be diligent to remove harmful plants.  In addition, more problems occur under conditions of overgrazing, since animals are less able to avoid harmful plants.

Tansy Ragwort

Seasonal or Conditional Disorders

  • Occur only under certain environmental conditions, at certain plant growth stages, or susceptible animal stages.

Grass Tetany (hypomagnesemia) 

" ... metabolic disorder characterized by low blood serum magnesium levels."

  • Mg concentration in forage is < 0.20% DM 

  • K concentration is high 
  • high rates of N fertilizer are applied ratio of K/Ca + Mg is above 2.2  (on an equivalent basis)

  Strategies for avoiding:

  •  direct supplementation of animals with Mg
  •  careful N and K fertilizer management
  •  supplemental dietary energy

Treatment: Ca-Mg-gluconate solution injection


“Legume frothy bloat occurs when a stable foam forms at the surface of the floating raft of actively digesting forage in the rumen and blocks access to the esophagus, causing gases to accumulate.”

  • Ruminal gas production (0.5 gal/min in cattle, 0.2 gal/min in sheep)
  • Normally gas escapes through eructation
  • Blocked distal esophageal sphincter
  • Death may occur within minutes
  • No single causative factor:
    • lush stands of legumes
    • formation of stable foam
    • high levels of soluble protein
  • Management to reduce:

    • grass-legume mixtures

    • bloat-safe legumes (trefoils, sainfoin)

    • provide hay or other feed

    • use “bloat-guard” blocks

Treatment: vegetable oil or other antifoaming into rumen with 0.75-in. diameter hose.

Nitrate toxicity 

“… toxicity occurs when ruminants ingest nitrate in excess of the ability of rumen microbes to convert the nitrite intermediate to ammonia.” 

  • nitrate is common form of N absorbed

  • nitrate intermediate transforms hemoglobin into methemoglobin, restricting oxygen transport

  • forages < 0.44% DM nitrate are safe

  • nitrate poisoning blood is “chocolate brown”

  • problems usually with grasses due to over-fertilization

  • drought stressed sorghums commonly high in nitrate

Treatment: remove animals to another forage source, feed energy supplement, intravenous methylene blue.

Prussic acid (HCN) poisoning

“… also referred to as hydrocyanic acid poisoning or cyanide poisoning, occurs when consuming cyanogenic glycosides.”

  • occurs in sorghums < 18” or plants recovering from frost or drought stress

  • epidermal cells contain cyanogenic glucoside

  • mastication mixes enzymes and dhurrin leaving aglycone from which HCN is released

  • cyanide is absorbed through rumen wall

  • symptoms include rapid, then labored slow breathing, muscle spasms, dilated pupils

  • death occurs in 15 minutes à 2 hours from asphyxiation (inhibits cytochrome oxidase); blood color is “cherry red”

Prevention: split N applications, make hay or silage, delay grazing for 2 weeks following frost


“… naturally occurring plant phenolic compounds that mimic estrogen hormones in animals.”

  • phytoestrogens cause infertility (formononetin, biochanin A, and genistein)

  • more common in sheep than cattle

  • subterranean clover may have up to 5% DM

  • low phytoestrogen cultivars developed

  • red clover, sub, white, and berseem clover contain phytoestrogens


"… primary photosensitization occurs when compounds in the plant move directly to the skin and cause the reaction (sunburned, reddened, or blistered)."

  • occurs in cattle, sheep, horses, and other livestock

  • secondary photosensitization occurs when liver damage prevents normal metabolism (facial eczema caused by Pithomyces chartarum)


Grass Tetany

Species-related Disorders

  • Forage species are known to contain antiquality compounds that limit animal productivity. 

 Tall fescue toxicosis

“… poor weight gains and reproductive performance problems sometimes exhibited by cattle grazing tall fescue is referred to as fescue toxicosis.”

  • tall fescue infected with a fungal endophyte (Neotyphodium coenophialum)
  • fungus-plant association is symbiotic (fungal symbiont derives nutrients, plant derives increased environmental stress tolerance)
  • plant appearance not affected
  • testing required to determine infection level and toxin concentration (ergovaline the prime toxin)
  • symptoms: reduced intake, gain, and production, increased respiration and elevated body temperature, rough haircoat, poor reproductive performance
  • avoid difficulties by planting an endophyte free, forage-type tall fescue
 Ryegrass staggers

Some perennial ryegrass cultivars contain a fungal endophyte (Neotyphodium lolii).

  • Not to be confused with grass staggers (grass tetany)

  • Ryegrass staggers is caused by lolitrem alkaloids

  • Causes staggering, swaying, trembling, and collapse

  • Treatment involves moving affected animals to other forages and/or feeding hay or silage of some other species

 Reed canarygrass alkaloids

Some reed canarygrass cultivars contain the alkaloid gramine which can reduce forage intake. 

  • alkaloids found primarily in leaf tissue
  • symptoms include muscle tremors and coordination loss
  • low alkaloid cultivars have been developed
Sorhum/sudangrass prussic acid (HCN) poisoning

Described above.

Clovers containing phytoestrogens

Described above. 

Brassica species with sulfur-containing anti-quality constituents

Glucosinolates and S-methyl cystein sulfoxide can negatively affect animals.

  • Found in Brassica species (turnips, swedes, forage rape, kale)
  • Enzymatic hydrolysis produces thiocyanate, isothiocyanate, or nitrile products
  • S-methylcystein sulfoxide can cause a form of anemia
Tannins and other phenolic compounds

Compounds that contain a free hydroxyl group on an aromatic ring structure.

   Examples found in plants:

  • amino acids tyrosine and phenylalanine
  • lignin
  • phytoestrogens
  • dicoumarol

  Negative effects of tannins:

  • Reduced intake
  • Reduced animal growth rates
  • Reduced fiber digestion

  Beneficial effects of tannins:

  • Enhanced forage protein utilization (by-pass protein)
  • Reduced bloat problems

Disorders with Stored Forages

  • In many cases, hay curing and ensiling reduces toxic compounds. However, sometimes antiquality factors arise that are exclusively with stored forages.


  •  Botulism: Clostridium botulinum produces toxins; do not feed hay containing dead animals, ensure low pH silage
  •  Listeriosis: Listeria monocytogenes proliferate in aerobically deteriorated or soil-containing silages; ensure anaerobic and low pH silage
  •  Moldy hay and silage: mold spores and/or mycotoxins; horses are highly susceptible; bale at proper moisture content, keep hay protected from rain and snow, keep silage anaerobic
  •  Blister beetle: beetles contain cantharidin, sometimes found in alfalfa, horses most susceptible; treat fields, check hay
  •  Red clover slobbers: Rhizoctonia leguminicola causes black patch, alkaloids (slaframine and swainsonine) found in infected plants


  • Anti-quality constituents are chemical compounds that have negative effects on forage intake or produce negative responses in animals consuming the forage.
  • Some occur naturally, some from insect infestations, some from microbes.
  • Three main groups: (1) poisonous plants, (2) seasonal or conditional disorders, and (3) species related disorders.
  • Poisonous plant problems are more severe with overgrazing
  • Seasonal and conditional disorders occur only under certain environmental conditions, at certain plant growth stages, or susceptible animal stages.
    • Examples: grass tetany (hypomagnesemia), bloat, nitrate toxicity, prussic acid (HCN) poisoning, phytoestrogens, photosensitization
  • Species-related disorders: some forage species are known to contain antiquality compounds that limit animal productivity.
    • Examples: tall fescue toxicosis, ryegrass staggers, reed canarygrass alkaloids, prussic acid (HCN) poisoning (sorhum, sudangrass), phytoestrogens (clovers), and sulfur-containing constituents (Brassicas)
  • Tannins and other phenolics can be positive or negative.
  • Although hay curing and ensiling often reduces toxic compounds, sometimes anti-quality factors may arise that are exclusively found in stored forages.
    • Examples: Botulism, Clostridium, Listeriosis, Moldy hay and silage, Blister beetle infestation, and Red clover slobbers.

Oregon Veterinarian Information System (OVIS)

Main Office
Animal Health
635 Capitol Street NE
Salem, OR 97301
Phone: 503-986-4680