Cattlemen should look out for johnsongrass, sudangrass, and other members of sorghum genus in October. It is usually in October that the first killing frost visits Arkansas. Crops such as johnsongrass, sudangrass, sorghum-sudangrass, grain sorghum, and sorgo types sorghums are very sensitive to temperatures below 32 degrees F. Plant cells of these crops are damaged by frost and hydrocyanic acid (HCN) or prussic acid is formed.
There is a chance that cattle may be killed by eating only a few pounds of forage from plants belonging to the sorghum genus if the plants have been killed by frost. The same crops are considered safe prior to frost if they are properly fed.
Hydrocyanic acid (HCN) is more abundant in sorghum leaves than in stems. Since young shoots and suckers consist mainly of leaves, they are more hazardous to feed than mature plants that contain large stems. Said another way, stems dilute the harmful effects of this potentially lethal compound found in sorghum leaves.
HCN released by frost is volatile and vaporizes quickly from frosted sorghum plants. Therefore, it is considered safe for grazing after having thawed from a killing frost for 7-10 days.
During October a light frost may occur that “burns back” only the uppermost leaves of sorghum plants. The lower leaves may remain green until a harder frost occurs several days later. Sometimes suckers will develop at the base of these plants. If cattle are removed from such fields immediately after frost and then returned five days later, they may selectively graze the young shoots only. When this occurs, there is a danger that cattle will consume high levels of the HCN from this leafy tissue and be killed.
Historically, only a few cattle have been killed in October as a result of eating sorghum that has been killed by frost. However, the producer should be aware of this hazard. By using some of the following precautions one may reduce the likelihood of poisoning cattle that consume sorghum forage and johnsongrass.
Each time a non-killing frost occurs on living sorghum crops, remove the cattle from these fields for fourteen days.
Do not graze frost-damaged sorghum or johnsongrass for at least seven days after the first killing frost. It is best to delay grazing until the frosted material is completely dried out and paper brown colored. Do not graze at night when frost is likely.
Do not harvest or feed drought-damaged plants in any form within four days following a good rain regardless of height. It is during this period of rapid growth that an accumulation of HCN in the young tissue and of nitrates in the stems is most likely to occur.
- Do not allow animals to graze fields with succulent, young, short growth. Graze only after plants reach a height of 18-24 inches
- Do not graze wilted plants (drought stressed) or plants with young regrowth
- Do not turn hungry cattle onto a pasture of sorghum or sorghum-sudan hybrid. Fill them up on hay first and begin grazing in the late afternoon
- Prevent selective grazing of the young regrowth, which may be highly toxic, by rotational grazing of small pastures which may be grazed down to a 6-inch stubble within a ten-day period. This will mean cross-fencing to provide short-term rotational or strip grazing
- Hay cured properly will not present any risk from prussic acid poisoning.