Drought may cause livestock to graze toxic plants

Sep 24, 2010
by WLJ

Heat-shriveled pastures may prompt cattle to graze on toxic plants they’d normally avoid, said John Jennings, professor-forages for the University of Arkansas Division of Agriculture.

“We are seeing a lot of toxic plants in pastures this year, so producers should use caution when grazing,” he said.

Perilla mint, black cherry and poison hemlock are all more prominent this year and are all toxic to grazing livestock, Jennings said. All three can be controlled with an herbicide.

Johnsongrass, which is good forage under normal conditions, can become very toxic when stressed. Johnsongrass can develop and concentrate prussic acid, or hydrogen cyanide, during drought or following a killing frost. Sorhgum/sudan, greengraze, and forage sorghum can also develop prussic acid under the same conditions.

“The amount of prussic acid is higher in young plants than in older ones, and the toxicity of leaves is higher than that of stems,” Jennings said. “Upper leaves contain more than the lower ones.”

Plants taller than 18 to 24 inches have lower concentrations of the toxin.

Young plants and regrowth following haying or grazing have the highest levels. Plants that have received more than 75 pounds of nitrogen per acre in one application may contain more toxins.

“It is difficult to predict how much toxin may be present in the forage,” he said. “The grass may appear normal in the morning, but can wilt during afternoon heat, which increases toxic potential.”

Jennings added that while many producers believe the white powdery substance commonly seen on johnsongrass stems in late summer is prussic acid residue, it is only common powdery mildew fungus and is not considered toxic to livestock.

Precautions for grazing fields with significant amounts of johnsongrass include:

• Not allowing animals to graze fields with succulent, young, short growth. Graze only after plants reach a height of 18 to 24 inches.

• Drying mowed johnsongrass completely to allow the prussic acid to dissipate before grazing.

• Not harvesting or feeding drought-damaged plants in any form, regardless of height, within four days following a good rain. It is during this period of rapid growth that an accumulation of prussic acid in the young tissue and of nitrates in the stems is most likely to occur.

• Not grazing herds on wilted plants or plants with young regrowth. Do not rely on drought-damaged material as the only source of feed. Keep either dry forage or green chop from other crops available at all times. Even when this material is mildly toxic, it can be fed safely to animals receiving some other forage or grain source.

• Not turning hungry cattle in pastures of john songrass.

Fill them with hay first and begin grazing in the late afternoon.

• Preventing selective grazing of the young regrowth by rotational grazing of small pastures.

These pastures may be grazed down to a 6-inch stubble within a 10-day period. This will mean cross fencing to provide short-term rotational or strip grazing.

• Silage may contain toxic quantities of prussic acid, but it usually escapes as a gas while being moved and fed. If frosted forage is ensiled, allow fermentation to take place for at least six to eight weeks before feeding.

• The prussic acid potential of hay decreases during the curing stage and is only dangerous if hay is improperly cured.

For more information, ask for publication FSA 3069, “Prussic Acid,” at your county extension office, or find it online at www.uaex.edu/Other_Ar eas/publications/PDF/ FSA-3069.pdf. — WLJ