Many areas of the country that experience drought this summer are now being relieved with some rain, which can bring its own special issues with regrowth of lush grass.

Tom Troxel, PhD, University of Arkansas, says regrowth of sorghums or johnsongrass after these rains can poison cattle because they are high in prussic acid (hydrogen cyanide [HCN]).

Troxel offers these precautions for using sorghums or johnsongrass:

  • Do not allow animals to graze fields with succulent, young, short growth. Graze only after plants reach a height of 18 to 24 inches.
  • Do not harvest or feed 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 HCN in the young tissue and of nitrates in the stems is most likely to occur.
  • Do not graze 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. Uneven growth of hybrids as a result of drought can best be utilized as silage.
  • Do not use frost-damaged sorghum as pasture or green chop during the first seven days after the first killing frost. Delay pasturing for at least seven days or until the frosted material is completely dried out and paper brown colored. Do not rely on frosted material as the only source of feed. The toxin is usually dissipated within 48 hours. Do not graze at night when frost is likely.
  • Do not turn hungry cattle onto a pasture of sorghum, sorghum-sudan hybrid or johnsongrass. 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 that may be grazed down to a six-inch stubble within a ten-day period. This will mean cross fencing to provide short-term rotational or strip grazing.
  • Silage may contain toxic quantities of HCN, but it usually escapes in gaseous form 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 HCN potential of hay decreases during the curing stage and is only dangerous if hay is improperly cured.

Animal Treatment
Animals affected by prussic acid poisoning may be treated with a sodium nitrite-sodium thiosulfate combination. It must be injected intravenously and very slowly. The dosage and method of administration are critical. Consult a veterinarian to correctly diagnose prussic acid poisoning and to determine the proper treatment.

Blue-green algae
Another summer problem is blue-green algae (read more here). “We’ve had some cases were we’ve suspected blue-green algae toxic problems,” Troxel says. “It’s the time of year when we can see problems and with the drought it makes more reason to be on the alert.” 

Troxel says stagnant waters may contain excessive levels of blue-green algae, which may be toxic and result in death of cattle. Because of their stagnant, nutrient rich nature, small ponds and streams in late summer can have toxic algae blooms. Toxicity is most common following a rapid bloom in late summer when cattle are consuming a substantial amount of the algal surface scum. The problem is difficult to predict, and the first sign may be sudden animal death. Because of this, it is advisable to restrict cattle access to stagnant waters, especially when a substantial amount of algae scum is visible. Algae blooms can be controlled in ponds through the use of copper sulfate (blue stone), but the rapid dieoff of algae may result in a fish kill.

The best method to control algae is to eliminate the source of nutrients entering the pond. If copper sulfate is used, the recommended application rate to water depends on the alkalinity (total carbonates and bicarbonates) of the water. Copper ions can kill fish if the water’s total alkalinity is below 40 ppm. Copper sulfate treatment may be ineffective if alkalinity of the water is greater than 300 ppm. The maximum tolerable level of copper sulfate in water is 2.7 (sheep) and 6.8 (cattle) pounds of copper sulfate per acrefoot.

he formula to calculate the pounds of copper sulfate needed is: Total alkalinity (ppm) x 2.04 x acrefoot volume = 100 Do not exceed the application limits for livestock, especially sheep. Livestock (especially sheep) should not be watered for at least five days after the last visible evidence of the algae bloom. Care should be taken to avoid water that has algae cells, either from treatment with algicide or natural aging of the bloom, because most toxin is freed in the water only after breakdown of the intact algae cells.