Grazing Sorghum Crops

25th February 2019

With the recent period of hot, dry weather many producers have seen promising sorghum crops (forage/grain/SSS/etc) start to struggle and may be looking at grazing or baling them to try and get what value from them they can. The major risk with grazing these types of crops are the potential for nitrate/nitrite toxicity and cyanide (prussic acid) toxicity.

What Causes Nitrate/Nitrite Toxicity?

Nitrate is a nitrogen based compound absorbed from the soil by plants. It forms the major source of nitrogen for growing plants and is usually rapidly converted into proteins, preventing its build up in plant matter. When ingested by ruminants, the rumen microflora convert nitrate into nitrite and then into ammonia. Issues arise under certain circumstances which result in the plant having reduced ‘energy’ levels required to convert nitrate into ammonia and thereby the nitrate can accumulate to dangerous levels. If the conversion of nitrite into ammonia by the rumen microflora is overwhelmed, nitrite levels in the rumen rise and are absorbed into the bloodstream. Once in the bloodstream, nitrite reacts with haemoglobin in red blood cells which prevents the binding and transport of oxygen throughout the body. The end result is death by oxygen deprivation.

Nitrate/nitrite toxicity is usually associated with events that cause a decrease in energy within plants that allows for the accumulation of nitrate. Some common risk periods include:

  • Cloudy, overcast conditions – favourable for plant growth and nitrate uptake but reduced photosynthesis means reduced energy to convert nitrates to ammonia
  • Frost damage
  • Exposure to herbicides
  • Insect damage
  • Stress from heat/dry causing plant wilting/stunting
  • Application of nitrogen fertilizers – most dangerous period is 10-14 days post application

Some plants commonly associated with nitrate/nitrite poisoning include crops commonly grown in this region such as cereals, sorghums (including SSS/etc), millet, brassica species and some weeds such mintweed and pigweed which accumulate nitrate naturally.

Some considerations to take into account about the potential toxicity of a crop are:

  • Young plants and regrowth contain highest concentrations of nitrate
  • Rapid growth following a stress/stunting event is a high risk period for nitrate toxicity
  • Plant parts closest to the ground generally have highest concentrations of nitrates (stem, roots). Leaves contain lower concentrations (may still be dangerous) whilst seeds and flowers generally contain low levels.
  • Cutting for hay DOES NOT reduce nitrate levels – always consider testing hay for nitrate levels if suspicious

What Are the Signs of Nitrate/Nitrite Toxicity?

The most common sign of nitrate/nitrite toxicity is sudden death in sheep and cattle. When observed in the acute phases of the toxicity, animals are often open mouth breathing, recumbent, have diarrhoea, salivating excessively and convulsing. Mucous membranes are often a brown colour instead of the usual pink. Death can occur from 1 hour post ingestion up to 24 hours depending on the amount of nitrates ingested. Affected animals may be treated with intravenous injections of methylene blue in the face of an outbreak however this drug is becoming near impossible to source and generally animals are dead prior to veterinary attention arriving.

Diagnosis of nitrate/nitrite poisoning involves testing of the eye fluid for excess levels of nitrates and can usually be done in house.

Minimising the Risk of Nitrate/Nitrite Poisoning

Whilst it will never to possible to completely eliminate the risk of nitrate/nitrite toxicity when grazing suspect crops there are a number of ways in which to minimize the risk including:

  • Testing plant material prior to grazing – samples can be taken from several locations and sent to a laboratory for testing for nitrate levels (and prussic acid levels) – we can arrange for the transport to the lab and test requests. Levels of >1.5% nitrates (on a dry matter basis) are considered toxic. IT IS IMPORTANT TO NOTE THAT WHILST TESTING PROVIDES SOME CONFIDENCE IT DOES NOT GUARANTEE THE WHOLE PADDOCK IS SAFE
  • Avoid grazing rapidly growing crops post a stress event – i.e. following a good rain event now after being affected by drought conditions
  • Provide some extra carbohydrates (grain) whilst introducing onto crops as this will result in a decrease in rumen pH which slows the conversion of nitrate into nitrite
  • A degree of tolerance can be built up over time with regular low level exposures to nitrates – therefore if possible restrict access to a new crop to 1-2 hours a day for a week before allowing unrestricted access
  • Never allow hungry stock onto a new pasture as they will likely gorge themselves – try and fill them with hay/grain/dry grass/etc
  • Introduce stock in the late morning/early afternoon

What Causes Cyanide Toxicity?

Cyanide (prussic acid) toxicity is caused by grazing some plants which accumulate cyanide. The most common of these in our area are the sorghum species (including forage sorghum, sudan grasses and Johnson grass). Like nitrite, cyanide prevents the transport of oxygen around the body and results in rapid death from hypoxia.

Some important points regarding the potential toxicity are:

  • Leaves are much higher in cyanide concentration than stems. Seeds do not contain cyanide
  • Rapidly growing plants are much more dangerous than mature crops, especially if growing after a stress period
  • Wilted or stressed plants can accumulate large concentrations of cyanide

Minimising the Risk of Cyanide Poisoning

Again, you will never be able to guarantee that a sorghum crop is completely safe to graze but there are a number of steps to take that may minimize the risk:

  • Test plant material for the presence of cyanide pre-grazing
  • Allow crops to mature before grazing – for Sudan Grass do not graze until 35-50cm tall. For forage sorghum do not graze until 75cm tall.
  • Cutting for silage greatly reduces the concentration of cyanide. Cutting for hay can also reduce cyanide levels but is not as safe
  • Providing Sulphur containing lick blocks as the Sulphur will provide some protection against the cyanide
  • Taking care to avoid grazing rapidly growing crops, especially after a stress event (drought)
  • As per the recommendations for nitrate prevention in regard to introducing stock slowly over a few days and on a full stomach.

Testing for Nitrate + Cyanide in Plants

Plant samples can be sent to the DPI laboratory in Menangle for testing of nitrate and nitrite levels. It is important to note that a normal test result is not a guarantee that the crops are safe for grazing as concentrations of nitrate and cyanide will vary in plants across a paddock however it does give a degree of assurance. A positive result is enough indication that the crop is unsafe at that point in time.

To test a crop for the presence of nitrates and cyanide, collect some plant material from several locations (ideally 5 or so) around the paddock. Tear the material collected into smaller pieces and mix together and then collect some if this into a large zip-lock back.

Bring into the clinic and we can send the sample to the lab via courier with the correct submission forms. If possible, submit samples to the clinic before 10am on Monday/Tuesday/Wednesday and results will generally be available in 48 hours. If sent on Thursdays, they may not be processed until the following week.

If you have any queries regarding nitrate/nitrite toxicity or cyanide toxicity or are interested in testing your crops please do not hesitate to call the clinic.