STRAIGHT FROM THE HORSES MOUTH

                          

Duane A. Lienemann
UNL Extension Educator

       

     Along with the dust from harvesting beans and dry-land corn exhibiting head smut, there is a feel in the air that I would describe as crisp. That’s right we have either already or well soon experience the end of the growing season with a frost. In driving across the country I have noticed that there have been some cover crops and in many cases some sorghum grasses planted to help out with the hay and winter feed situation with the lack of such brought on by the drought of this past two years.  When I scraped the soft ice off my pick-up a couple of days ago it made me think of several years ago when I got a call from a producer who had lost a couple of cows and we determined that it was likely Prussic Acid poisoning. The conditions were very similar so I opted this week to take another look at this potential problem.
     Prussic acid was discovered in the early 1900s when animal scientists noticed that under certain conditions sorghums are capable of releasing hydrocyanic acid (HCN) or commonly called prussic acid.  Prussic acid when ingested by cattle, is quickly absorbed into the blood stream, and blocks the animal's cells from utilizing oxygen.  Thus the animal dies from asphyxiation at the cellular level.  Animals affected by prussic acid poisoning exhibit a characteristic bright red blood just prior to and during death. 
     Lush young re-growth of sorghum plants is prone to accumulate prussic acid especially when the plants are stressed such as drought or freeze damage. Of course I don’t have to tell you that both of those conditions exist. What we have been experiencing over this past 3-4 is what I would call light frosts. These light frosts that stress the plant but do not kill it, are often associated with prussic acid poisonings.  Producers should avoid grazing fields with sorghum type plants following a light frost. That would include freshly cut milo fields and those fields with sorghum grass-type forage. The risk of prussic acid poisoning will be reduced, if grazing is delayed until at least one week after a "killing freeze".  
     Just how do we depict a killing freeze? Many plants are sensitive to freezing conditions, and will stop their production once temperatures reach or fall below 32 degrees. A freeze will occur when temperatures reach or fall below 32 degrees. While some plants are able to with stand a brief period at or just below 32 degrees, most plants will not survive once temperatures drop to or below 28 degrees for two consecutive hours, or more (often refer to as a killing freeze). 
     As the plants die and the cell walls rupture, the hydrocyanic acid (HCN) is released as a gas, and the amount is greatly reduced in the plants.  One can never be absolutely certain that a field of sorghum is 100% safe to graze.  Sun-curing of hay will reduce HCN, especially if the hay is crimped.  The complex that binds the HCN is hydrolyzed and the HCN evaporates in gaseous form. An easy way to understand the volatility of this gas – just think of cyanide, which it really is. Hydrogen cyanide has been discussed by scientists as a precursor to amino acids and nucleic acids. HCN is proposed to have played a part in the origin of life, so it should not surprise us that it would be present in plants.
     How can we utilize these feeds and reduce our fear of prussic acid? Let’s consider a few things. Cattle that must be grazed on sorghum pastures during this time of year should be fed another type of hay before turning in on the field, and should be watched closely for the first few hours after turn in.  If signs of labored breathing, such as would be found in asphyxiation, are noted, cattle should be removed immediately.  Call your local veterinarian for immediate help for those animals that are affected. 
     Grazing or feeding green sorghums, sudangrasses and sorghum-sudangrass crosses to cattle can be a bit worrisome because of the threat of prussic acid poisoning. This risk is most evident when the plant is at a young, dark green growth or regrowth (less than 18 to 24 inches) and the danger increases just after a frost. However, they can be safely grazed a few weeks after freezing if there is no substantial regrowth. Usually regular sudangrass contains the least amount of prussic acid, while sorghum-sundangrass is more acidic and sorghum has the most risk of poisoning, so it follows reason that if you can, it may be best to mix sorghum or sorghum-sudangrass with other feeds to reduce the danger of prussic acid poisoning. There are some other management practices that you can do should you have feed that needs to be put up. 
     I would consider putting it up as silage. Sorghum and sorghum-sudangrass silage is generally safe for feeding. Although it may have contained toxic levels of prussic acid when harvested, while in storage much of the poison escapes as a gas during fermentation and when being fed. However, as a precaution, do not feed new silage for at least three weeks after harvesting and storing. Another method would be to simply put it up as hay. The prussic acid content of sorghum and sorghum-sudangrass hay decreases as much as 75 percent while curing and is rarely hazardous when fed to livestock. Hay stored for two or more months gradually losses all its cyanide potential.
     I know that most people are considering grazing these fields and that is possible. However, do not graze sorghum or sorghum-sudangrass hybrids following a series of light frosts, as the potential for poisoning increases for a period of time after frosts.  The old rule of thumb is to allow seven to 10 days after a light frost before grazing. I definitely suggest that you don’t graze sorghum or sorghum-sudangrass hybrids following a killing frost until the plant has dried, approximately seven days. Probably the most important thing to do if grazing is to not graze hungry livestock on sorghum or sorghum-sudangrass hybrids. Poisoning potential increases with the amount of high-risk forage consumed. Just play it safe!

The preceding information comes from the research and personal observations of the writer which may or may not reflect the views of UNL or UNL Extension. For more further information on these or other topics contact D. A. Lienemann, UNL Extension Educator for Webster County in Red Cloud, (402) 746-3417 or email to: dlienemann2@unl.edu or go to the website at: http://www.webster.unl.edu/home