OSU Sheep Team

Bob Hendershot, USDA NRCS, Grassland Conservationist

(Originally Published in Sheep Team Newsletter October 2003)

Toxic levels of prussic acid or otherwise known hydrocyanic acid (HCN) form naturally in the leaves of many annual warm season grasses as well as a few other plants.  Johnson grass, sorghum, sudangrasses, sorghum-sudan hybrids and even the leaves of wild cherry trees have this process occurring immediately after a killing frost.  Also the tender new growth that occurs after a drought can have toxic levels of prussic acid.  The young fast growing plants are more likely to be toxic than the older mature plants. Fields with these plants that were mowed or grazed in September will have a higher level of prussic acid after a frost than fields mowed earlier in the summer and allowed to regrow to a more mature stage.  Even herbicides like 2, 4-D can cause prussic acid to form in these plants.

Prussic acid causes death in the animals by interfering with the oxygen transferring ability of the red blood cells.  Symptoms include excessive salivation, rapid breathing and muscle spasms.  The animal will stagger, collapse and eventually die.  These symptoms can occur within ten to fifteen minutes after eating the plants with high levels of prussic acid.

Prussic acid levels will drop in the plants with time.  Standing plants killed by frost are normally save to graze after about a week.  The ensiling and hay curing process will also reduce the prussic acid to safe levels.  Care needs to be taken with scattered frosts.  Portions of the field may be initially killed and later frosts will cause the remaining plants to have high levels of prussic acid.  This will create problems in managing the grazing of the field.  Producers need to be able to identify the plants that have the potential for prussic acid poisoning.

Grazing management decisions need to consider waiting a week after a frost or the rain that breaks a drought to graze or green chop these plants.  Grazing animals should be introduced into the field slowly with their rumens full.