Ask the expert: When should I chop frozen sudangrass?

Posted on October 28, 2019 in Forage Foundations
By Dr. Michelle Chang-Der Bedrosian, Vita Plus forage products and dairy technical specialist

Question:  My sudangrass (standing in the field) froze last week.  Should I chop it now, wait or forget about it?

Answer:  Sudangrass is typically planted in June and harvested in the fall as an emergency forage.  It is a C4 crop, which means it is common in tropical climates, so it grows great in hot years.  It is usually planted as an emergency crop and harvested young (less than 24 inches tall) for a higher quality, or you can harvest it tall for heifer feed.

Sudangrass, sorghum and sorghum-sudangrass hybrids produce cyanogenic glucosides (commonly known as dhurrin) during growth.  This compound is made of glucose bonded to a cyanide molecule, a rapidly acting and potentially deadly chemical.

Under normal growing conditions, the glycoside remains intact and doesn’t cause any problems.  However, when the plant is injured by frost, the molecules break into glucose and prussic acid, a highly toxic compound containing cyanide.  Prussic acid can be deadly in small quantities – 1 gram can kill a 1,400-pound cow very quickly.

Signs of prussic acid poisoning
The deadly effects of prussic acid poisoning are almost immediate, or within minutes of eating the feed.  Cyanide combines with hemoglobin in the blood and prevents the transportation of oxygen.  Some signs include increased respiration rate and pulse, muscular twitches and trembling, and foaming at the mouth.  The animal may be blue in the mouth, and it will eventually die from respiratory paralysis.  Blood often passes from the nostrils/mouth at the time of death.  Many times, death happens so quickly no symptoms are identified except for dead animals at the bunk.

What you can do
It’s important to understand these grasses pose no risk of prussic acid poisoning if they are handled correctly.  If you harvest these grasses when they are young (under 24 to 30 inches tall) and properly ensile or dry them, they pose no risk of prussic acid poisoning.  Additionally, once the plant reaches full maturity and no new growths are present, prussic acid stops accumulating and it is generally safe to feed or graze.

However, the problem is when the plant experiences trauma, such as from a frost.  After the plant experiences a frost, new growths can start, which have a lot of prussic acid.  In this scenario, we advise playing it safe.  Try one of the following methods to help minimize the risk of prussic acid poisoning.

  • Dry it.  Free prussic acid doesn’t decline until thawing and wilting begins.  Dry it for five to six days and it should be safe to feed.
  • Ensile it.  Let it ferment at least three weeks before feeding it to animals.  Please note, if the forage is frozen, it will not ferment.  Then, once the temperature starts to warm up, monitor it for spoilage.
  • Dilute it.  Animals weighing 1,000 pounds or more can detoxify about 0.5 grams of prussic acid per hour, so dilute, dilute, dilute.

Please note, these grasses can be grazed by animals, so long as they are taller than 24 to 30 inches and have not been recently frozen.

You can also check with your local county extension office to see if it offers prussic acid testing.  Some may offer to come to the field and test the sorghum for prussic acid.  If not, you can send a sample to Midwest Laboratories to be tested for $50.  If you choose to submit a sample, freeze it, package it with ice and send it overnight to Midwest Labs.  If the sample is exposed to light, it will lower the prussic acid content.

Contact your Vita Plus consultant if you have any questions regarding the harvest of these crops.

Category: Forage Foundations
Forage harvesting
Forage storage and management