Stock grazing crops high in nitrate can develop toxicity disorders. Plant nitrate levels are directly affected by nitrogen (N) fertiliser rates. Targeting high grain yields with additions of nitrogen fertiliser can elevate nitrate levels in plants to dangerous levels for livestock.
What can be done to reduce risk to stock
- Avoid grazing paddocks that have received high levels of nitrogen fertiliser.
- Apply supplementary N after grazing rather than before.
- Introduce animals gradually to the new feed source to allow rumen microbes to adjust – start with 1-2 hours/day and build to unrestricted access over 7-10 days.
- Introduce stock in the early afternoon on a sunny day, and ensure they are not hungry.
- Allow stock access to an alternate feed source with lower nitrate such as cereal hay.
- Be aware that sheep are generally more tolerant of high nitrate feeds than cattle and that pregnant stock are more susceptible.
- Only allow livestock in good health to graze crops.
What leads to high nitrate and nitrite in plants?
Plants take up nitrogen from the soil as nitrate and ammonia. Typically nitrate levels build up during the night and then rapidly decline during the day when photosynthesis is active.
Nitrate depletion is impacted by the rate and duration of photosynthesis. When photosynthesis slows nitrate is not depleted in the plant tissues, leaving high plant nitrate levels that increase the risk of nitrate and/or nitrite toxicity in livestock. The rate of photosynthesis is reduced by stresses to the plant such as:
- short days and long nights
- cloudy days
- frost or extended periods of unseasonal cold or dry conditions
- phenoxy herbicides, such as 2,4-D
Nitrate accumulation in plant tissues often increases with nitrogen-based fertiliser applications, as shown in the chart – nitrate levels in canola across different urea rates, or where soils have had unusually high levels of nitrogen mineralisation (e.g. following rainfall after drought or cropping sequences after pure pasture legume stands).
Nitrate accumulation varies in different plant species, plant growth stages and plant parts – lower stems and younger plants usually having higher nitrate levels than leaves and mature plants.
Caption: Nitrogen fertiliser rates affect plant nitrate concentrations. The measured concentration of nitrate (NO3-N) in leaf petiole sap of Canola for four different pre-sowing nitrogen treatments.
How livestock are affected
Plant nitrates are converted by rumen microbes to nitrite and then to ammonia which is, in turn, converted to protein.
- Nitrite toxicity occurs when the conversion of nitrate to nitrite happens faster than the conversion of nitrite to ammonia. Nitrite enters the animal’s blood and reacts with haemoglobin, forming methaemoglobin. This reduces the capacity of blood to carry oxygen. In an acute toxicity, animals can ‘suffocate’ (run out of oxygen) and die. Other symptoms include difficulty in breathing, rapid breaths, trembling and staggering.
- Nitrate toxicity occurs when large quantities of nitrate are eaten causing inflammation of the gut. Diarrhoea, salivation and abdominal pain may be observed.
Graeme Sandral, NSW DPI, Rob Norton, IPNI, Rohan Brill, NSW DPI, Shawn McGrath, Charles Sturt University, Bruce Allworth, Charles Sturt University
Luke Gaynor, NSW DPI, Alli Elliot, BCG
Photo courtesy of the GRDC