Rumen bacteria are highly effective at using plant nitrates, but in excessive amounts the bacteria are overwhelmed, resulting in accumulation and toxicity.
While failed canola crops can be useful for grazing, hay or silage, farmers are being warned to consider the potential risk from high levels of nitrates in their crop before feeding it to livestock.
District Veterinarian, Dr. Emily Stearman said many farmers were looking at canola crops that had suffered from severe frosts and prolonged water stress to salvage them for livestock feed.
“There are plant and animal factors to consider: the nitrogen and sulphur levels in the crop, and the adaptation of the animal to canola as a feed,” she said.
“Animals should be introduced to the canola for short periods of grazing with access to cereal hay during this period,” she added.
Across the district canola-growers will be managing many different scenarios with an inherent risk in making any assumptions about safety. If nitrogen fertilizer was applied just prior to the last rain event, plant uptake will have occurred, and these plants are a higher risk to livestock when cut or grazed.
Nutrient supply to the ruminant animal requires a healthy population of bacteria in the gut; sudden changes in diet can detrimentally alter these bacteria, resulting in decreased rate of digestion or clinical disease.
Poor gut adaptation can result in a neurological condition, polioencephalomalacia (PEM). The effect of this can also be induced or exacerbated by innately high plant sulphur. After 7 days animals can be left on the crop provided they have on-going access to hay – this will both buffer the nitrate content of the plant and provide an alternative source of sulphur in the diet.
Rumen bacteria are highly effective at using plant nitrates, but in excessive amounts the bacteria are overwhelmed, resulting in accumulation and toxicity. Respiratory distress is a symptom of toxicity, and can be accompanied by neurological signs when the animal is close to death. However, a gradual increase in available nitrate levels can be well tolerated by ruminants.
Tips for grazing
To help prevent animal health issues, follow the recommendations:
- Ensure animals do not graze the crop hungry – fill with hay prior to introduction to the crop
- Provide ad lib access to hay at all times while grazing the crop
- Graze between 10am-2pm each day for 7 days to ensure gradual adaption to the change in diet
- After 7 days animals can be left on the crop provided they have on-going access to hay – this will both buffer the nitrate content of the plant and provide an alternative source of sulphur in the diet
- Continue to monitor stock daily – while plant nitrate levels are not likely to change in the current environmental conditions, ongoing monitoring for signs of nitrate toxicity is required.
Nitrates in hay do not break down in storage so hay and silage made from plants containing high levels of nitrate are a risk for toxicity when fed to stock. Fodder testing is recommended.
When feeding canola fodder, other sources of roughage, such as cereal hay, should be provided. Similar to grazing recommendations, a gradual increase in the amount fed will allow rumen adaptation to increased nitrates.
If any animal develops signs of illness, remove from the paddock immediately, provide access to hay and water and contact your local district or private Veterinarian, who can also provide further information on grazing management or fodder nutrition.