Periods of crop stress change forage chemistry.
Drought, temperature variation, slow growth and rapid regrowth after rainfall can cause prussic acid and nitrate to build up to levels that pose a real risk to livestock. This is particularly relevant for sorghum and related species at this time of the season, where toxin levels can change quickly with little or no visible warning.
1. Why is risk higher at this time of the season?
Late-season conditions often include moisture stress, temperature fluctuations, reduced growth and regrowth after rain. These disrupt normal plant function and increase the chance of both prussic acid formation and nitrate accumulation. Sorghum and related species are especially sensitive to these changes.
2. Which crops are most affected under stress?
Prussic acid risk is highest in sorghum, sorghum–sudan hybrids, sudangrass and johnsongrass, particularly when crops are drought-stressed, frost-affected or regrowing rapidly. Nitrate accumulation can occur in many forage species when growth is restricted or nitrogen uptake exceeds the plant’s ability to convert it into protein.
3. Can stressed crops become dangerous quickly?
Yes. Toxin levels can increase within hours of stress events such as frost, drought, chemical application or physical damage. Forage that previously tested low risk can become hazardous without any obvious visual change.
4. Can visual assessment identify risk?
No. Plant appearance, colour, height and maturity are not reliable indicators of prussic acid or nitrate levels. Chemical testing is required to determine actual risk.
5. Does harvesting reduce seasonal risk?
Prussic acid is volatile and usually dissipates as plant material dries, reducing risk in hay and silage. Nitrate does not dissipate with drying and remains in conserved fodder. Ensiling may reduce nitrate levels but does not remove the risk. Testing is still required.
6. How should high-risk forage be managed during this period?
When elevated levels are detected:
- Delay grazing or harvest where possible
- Dilute with low-risk forage to control intake
- Avoid feeding high-risk material to hungry animals
- Consider ensiling or controlled feeding strategies for nitrate-affected forage
- Retest after weather changes or regrowth
Seasonal conditions require active risk management based on measured values, not assumptions.
We are always on hand for advice when you need it. Check out our On Farm Testing Kits for Prussic Acid and Nitrates, and call us on 1300 669 429 if you have any questions at all.
This video shows you how easy the kits are to use, and if Prussic Acid and/or Nitrates are present, how Lab Testing will get you the information you need.
