It has been known for years that johnsongrass has the potential to be a livestock killer because of two lethal issues: prussic acid poisoning and nitrate accumulation.
And even though the amount of johnsongrass has been increasing in pastures and along roadsides for a number of years there was minimal concern because veterinarians had not seen a case of prussic acid poisoning in years.
"Even though there has been an abundance of johnsongrass in past years too, cattle seemed to graze it with minimal problems," said Eldon Cole, a livestock specialist with University of Missouri Extension.
Apparently the summer of 2012 and the accompanying drought has changed that situation with johnsongrass According to Cole, there have been a number of deaths due to grazing johnsongrass reported in southwest Missouri in the last six to eight weeks. In most cases the diagnosis was by circumstantial evidence.
"Most of the reports were simply finding dead cows or yearlings near a patch of johnsongrass after they'd been turned into a field with short johnsongrass in it. As a rule, only a small percentage of the animals in the herd will die," said Cole.
In a couple of incidents the farmer was hauling a bale of hay into a pasture and a couple of hungry cows beat him through the gate and started eating lush, 12 inch tall johnsongrass along the road.
"The first farmer was fortunate, after the cows went down he contacted a veterinarian and was able to treat and save both cows," said Cole.
In the other situation the farmer went ahead and unloaded the bale and went back to get the cows back in the pasture. By that time they were shaking and struggling. One died, but the other survived without treatment according to Cole.
The weather stress this year likely contributed to an abnormally high accumulation of prussic acid in the tissue of johnsongrass, grain sorghum, sorghum-sudangrass crosses, forage sorghums and shattercane. Sudan and pearl millet generally have a very low prussic acid potential.
'The conditions that may enhance the poisoning risk is if the lush plant is damaged by freezing, chewing or traffic running over it. This frees a larger amount of the cyanide which is more lethal," said Cole.
Symptoms of prussic acid poisoning occur quickly, often within 15 minutes. Farmers who are turning cattle that have not been recently on johnsongrass into a field with lots of johnsongrass in it are advised to only turn a few head in at first.
"It's also a good idea to check with a veterinarian regarding actions in case cows do have a reaction. Make sure the cattle are full and wait until the johnsongrass is at least 24 inches tall before turning them in," said Cole.
Cows affected by cyanide will have bright, cherry red blood. It may clot slowly or not at all. Cows tend to bloat quickly.
Source: University of Missouri Extension