Source: University of Missouri Extension
Many landowners have had questions about the risk of grazing or haying johnsongrass or sorghum sudan forage.
The exceptionally dry weather has placed a lot of stress on the plants and increased the risk of prussic acid and/or nitrate poisoning according to Eldon Cole, livestock specialist with University of Missouri Extension.
"The principal causes of this risk are different but some plants may exhibit a risk for both. As other forages dry up, these more dry-weather tolerant plants can become the most palatable and available forage for cattle," said Cole.
Prussic acid, sometimes referred to as cyanide poisoning, typically occurs shortly after cattle are turned on a field that contains the high risk plants such as johnsongrass or sorghum sudan. It is more risky when the forage is less than 18 to 20 inches tall.
"The potential is increased when high levels of nitrate fertilizer have been applied, especially in phosphorus-deficient soils," said Cole.
There is not a simple field test available for prussic acid. The general recommendation is to only turn a few head into the pasture and watch them closely for 30 to 45 minutes. It is also wise to have your cell phone and your vet's number handy in case one or more animals go down.
"The good news is we've not seen many problems directly attributable to prussic acid recently. However, it is always best to error on the side of safety," said Cole.
Cattle seem to adapt to prussic acid if they are on it continuously from springtime through the grazing season. There is a risk in the fall if plants send up new, tender growth following the dry weather or a frost.
"The process of hay making does eliminate the prussic acid risk in most instances," said Cole.
Impact of Nitrates:
Nitrates probably create more of a risk in hay than in grazed material. They also accumulate in dry stressful weather conditions.
"When grazed, mainly the leafy parts of the plant are eaten, and problems are not noticed. However, when harvested for hay more of the total plant is eaten and the poisoning risk increases," said Cole.
Symptoms of prussic acid poisoning generally are rapid respiration rates, salivation, excitement, staggering, convulsions and death. This process doesn't take more than 45 minutes. Animals that live 2 hours after the onset of these signs recover in most cases.
Nitrates have more subtle symptoms unless the forage contains a large amount which would be above 1.76 percent nitrate. That level, and above, can be fatal in a short time. For lesser amounts symptoms can include unthrifty appearance, poor growth, reduced milk production in cows and abortion.
Dilution of the nitrate containing forage with other feeds can be challenging if the cattle are grazing it. Detection of nitrates is possible with a quick field test. If the color reaction is positive a quantitative test should be sought at a laboratory for $10.
"Cattle prices are too good now to risk performance or death loss with these high risk feeds, so take the necessary precautions. There is even a product available that reduces the risk on high nitrate feeds. It may either be fed or administered as a bolus," said Cole.