Rayner Reckons

Mar 19

Know the risks of Nitrate & Prussic Acid in your feed

Posted on Tuesday, March 19, 2019

Feeding stock is a task that requires some prior preparation.  While most feeds can be provided to ruminants, it doesn’t mean that you can feed them without following a few simple rules.  

 

The rumen is a living environment, which hosts the micro-flora, fungi and other organisms that actually work to break feed down so that it can be absorbed and used by the animal.  Sudden changes in feed type, lack or roughage and reduced water intake can all create a situation where the environment of the rumen becomes unhealthy to the micro-flora and results in digestive upsets and illness.

 

Mostly the rumen remains fairly stable as livestock select diets that allow the rumen micro-flora to thrive and do their job of breaking down material for absorption and digestion.  Problems start to arise when diets and rations are offered that create unhealthy rumen environments. 

 

As mentioned before common issues are changes in feed types, particularly to including grains that have high levels of starch.  It also occurs when fibre is lacking or if rations are less than the animal requires and as it becomes hungrier it eats plants that may contain toxins that can result in illness or death.

 

Poisoning is a risk that many producers have had to consider this year. Common issues have been weeds that have been eaten as hungry stock eat whatever they can chew.  It also has been an issue as new weeds arrive in drought feeds. Stock may consume plants that are poisonous simply because they have never seen them before.  

 

However the biggest issue has been with sorghum crops that have been grazed or cut for fodder.  The cause has been either from Nitrate poisoning or from Prussic Acid.  

 

What is Nitrate Poisoning?

 

Nitrogen is needs by plants for growth.  They absorb nitrogen through the soil and root system.  Young plants and leaves have high levels of nitrates as they are growing. However when plants are stressed or not growing at a rate that allows the nitrogen to be used, the plant stores this as nitrate.  Some plants are more prone than others to do this (they are known as ‘nitrate accumulators’), but most plants will accumulate nitrates to some degree if stressed.

 

The issue for livestock is that when the material is eaten that Nitrate is converted to nitrite.  This chemical change allows the nitrite to be quickly absorbed from digested feed into the blood system where it attaches to hemoglobin.  These nitrites replace oxygen cells in the blood and cause rapid impacts on the animal.  

 

Within 15-20 minutes symptoms like staggering, difficulty breathing, spasms and foaming at the mouth start to occur. Many affected animals will lie down while some may thrash about.  I’ve had it described to me that the cattle looked drunk.  

 

Its mainly sheep and cattle impacted in this way.  Horses and pigs are less affected by nitrate because they don’t convert it to nitrite. If levels are high though, the nitrate can damage the lining of their gut.

 

According to a number of sources, most of the species commonly grazed in Australia can cause nitrate poisoning if stressed.  These are species that include oats, sorghum, maize, sudan grass, Johnson grass, canola, lucerne, kikuyu, turnip and sugar beet tops, soybean, wheat, barley and a range of weeds.

 

It’s essential that you consider feed testing any fodder that you purchase to see what level of nitrate is in the feed.  Ask a few questions from the vendor?  Was it treated with a big application of fertilizer or manure?  Was it stressed before bailing?  These questions can help you decide if it is suitable to feed to livestock

 

Prussic Acid

 

Prussic acid is a major concern for producers who graze or rely on sorghum varieties for fodder.  It is present in most sorghum, although some varieties will have lower levels.   

 

At a chemical level within the plant, prussic acids exist as a non-poisonous chemical called Dhurrin.  This chemical can react with another plant-based material known as Emulsion.  Under the right conditions, these two materials will react and create Prussic Acid.  It’s also known as Hydrocyanic Acid.  In simplest terms this is Cyanide Poisoning!

 

Damage to the pant through mechanical impact, environmental stress, trampling and even insect damage results in the mixing of these materials and the release of Cyanide.  

While sometimes this can evaporate from the plant, it doesn’t all disappear.  It also means that further damage, such as harvesting, or grazing will result in more Cyanide being released. 

 

The concern with Prussic Acid is its high level of toxicity.  Feed Central suggests that amounts greater than 0.1 percent (1000 ppm or mg/kg) of plant dry matter is considered highly dangerous. Some levels from the Washington State University place that level even lower at 750 ppm.

 

The effect on animals is very similar to that of nitrate poisoning. The acid is readily absorbed into the bloodstream and it then attaches to the hemoglobin and displaces oxygen.  

 

Since many producers look to graze or use sorghum forage there are some basic considerations to be factored into the decision making process.  Remember that: 

  • Leaf blades normally contain higher levels than leaf sheaths or stems
  • Younger (upper) leaves have more prussic acid than older leaves
  • Tillers and branches (“suckers”) have the highest levels, because they are more leaf than stalk

 

Most sorghum should be grazed when they are more mature.  Often this is over 3ft in height.  As plants mature, there are more stalks than leaves in the overall plant causing prussic acid content in the plant as a whole to decrease.

 

With so much drought-affected crops its important to remember levels will be much higher as the pants are mostly leaves. Sorghum grown in drought may retain high levels of prussic acid, even if made into hay or silage.

 

My advice to all producers thinking about using or grazing sorghum is to get it tested first!  Know the levels before you feed it out.  There may be alternative uses to this feed. 


If you do have concerns, or you want some more advice, then get in touch with me. Asking questions can save you a lot of risk and the potential of stock losses.

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