More than 120 cattle on 12 properties in the Hunter died of nitrate poisoning during the autumn of 2007. The area had been in and out of drought for 4 years without any good soaking rain or a flush of feed. Irrigators were on water restrictions. Pastures were mostly dry and short and of poor quality and light showers provided a green pick in some parts. Deaths occurred when hungry animals were first introduced to hay and crops. New season millet and sorghum hay, sorghum and oat crops and sorghum stubble caused most of the mortalities. In some cases, the weather was cloudy. Only light rates of fertiliser were used in the paddocks that produced the toxic fodder.
Cattle were mostly found dead without signs of struggling and without abnormal bloating or gas changes. Blood was brown in fresh carcasses but changed to a red brown in older carcasses. Some were recumbent and calm with rapid breathing and some were staggering, agitated with severe respiratory stress. Mucous membranes were a light brown.
The toxic nitrate level is greater than 1200 to 1800 mg/kg (80% wet basis) or 6000 to 9000 mg/kg on an oven dry basis.
Mortalities occurred on the following crops and pastures;
• Millet and sorghum hay (new season) tested 40,000 and 64,000 mg/kg (approx 80% dry matter)
• Sorghum hay (5 yr old, dry and mouldy) tested 50,000 mg/kg (approx 90% dry matter)
• Oat crops tested 12,000-16,000 mg/kg (approx 25% dry matter)
• Sorghum crops which changed Merckoquant nitrate test strips in 5-10 seconds
• Sorghum stubble which changed Merckoquant nitrate test strips in 3-6 seconds
• Kikuyu pastures which changed Merckoquant nitrate test strips in 8 seconds.
While hungry cattle and high nitrate levels in the feed were important, the most consistent cause was a sudden change in diet from a low to a high nitrate content. The district provided this situation with the high nitrate levels in the crops on the river flats and the low levels in the native pastures away from the river flats. Nitrate levels mostly likely built up in the cropping soils because only a minimum amount of irrigation was used during the drought. Soil types in the Hunter away from the river flats are mostly poor and support grass varieties that quickly lose their nitrogen content when they mature.
High nitrate crops were utilised by feeding as a partial ration to cattle that were not hungry and by introducing stock to the crops gradually over 6 days. Lactating cattle that were being grain fed (without monensin), were also used to utilise high risk crops. The practice of making silage to reduce nitrate levels was considered but not applied.