Nitrate Toxicity

Causes

• Nitrate levels in plants may increase after rapid growth of crops following dry weather or any conditions that cause ‘stop-start’ crop growth. Even when plants are growing well, nitrates can build up in plants when weather is dull and overcast and there is not much photosynthesis occurring in the plant.
• Nitrate concentrations increase in the plant when it flowers, and is often higher in regrowth.
• Nitrogen fertiliser application elevates the risk of nitrate concentrations increasing, especially if the weather conditions described above follow the applications.

Preventative measures

• Be aware of the causes of nitrate toxicity, described above, and plan accordingly.
• Test the crop for nitrate levels. Consult your local agronomist for advice on how and when to test.
• Introduce animals gradually to the crop over 10 days.
• Monitor animals closely when they first go onto the crop.
• Don’t turn hungry animals onto the crop, feed pasture or supplements first.
• Feed hay to animals before and during access to the crop.
• Offer the crop as only part of the daily feed requirements of the animals.

Rape Scald

Causes

• Animals grazing brassicas can sometimes show signs of ‘scald’ or photosensitisation. While this condition is most often seen when stock graze rape, it may also occur on other types of brassicas.
• Scald is usually seen on unpigmented areas of the skin (for example, white patches of skin and especially the udder of black and white dairy cows). Sheep are particularly vulnerable on the eyelids, lips and ears but lesions may extend down the back and onto the rump.

Preventative measures

While it is not entirely understood what causes rape scald, some common sense steps can not only help reduce the risk of rape scald but also optimise animal performance from brassicas.

• Traditional recommendations indicate that risk of rape scald may be reduced by making sure rape has matured adequately before introducing animals to the crop.
• For root crops (bulb) strip graze to encourage stock to consume bulb and leaf, not just leaf alone.
• As a rule of thumb, avoid the application of sulphur if the soil test sulphur level is higher than 10ppm. Nitrogen applications may increase risk of scald, so applications to boost growth should be made with care.
• Good crop husbandry can help reduce risk of scald (and other health problems) on crops. Introduce animals gradually to the crop to allow adaptation to the high-quality feed, build up to desired ration over at least 7–10 days. Offer animals an adjacent runoff paddock, or hay or silage during adaptation.

Red Water

Causes

• Red water on brassicas is caused by a compound called S-methyl cysteine sulphoxide (SMCO) a compound found in all parts of the brassica plant.
• SMCO is broken down in the rumen of sheep and cattle to form a toxin (dimethyl disulphide) which passes into the blood stream of affected animals. This toxin damages the cell membrane of red blood cells, releasing haemoglobin (the red pigment inside red blood cells that carries oxygen). Free haemoglobin released from damaged red blood cells is lost in the urine, hence the name of the condition – red water.
• Although rare, most cases of red water are seen on kale, risk is much lower on non-kale species.

Preventative measures

• High levels of soil sulphur have been associated with increased concentrations of SMCO in brassicas. Use non-sulphate fertilisers at sowing to reduce the risk of high SMCO concentrations
• If soil sulphur levels are high, choose a non-kale crop e.g.: rape or a non-brassica species
• Application of nitrogen fertilisers appears to be associated with increased concentrations of SMCO. Care is needed following application of nitrogen not only with SMCO but also nitrate toxicity
• SMCO concentrations increase with maturity of the plant. Older standing kale crops late in the season are more likely to cause red water problems, particularly if flowers and/or pods are present

Choke

Causes

• Turnip crops grazed at an immature stage or where crops have been sown at a very high rate can have very small bulbs. These may become impacted at the back of the throat or even part way down the oesophagus (‘food pipe’ in the neck) of cattle.

Preventative measures

• Unfortunately, there is nothing that can be done to totally reduce the risk of choke
• If a crop is immature, delay grazing
• A veterinarian will be able to remove turnips impacted at the back of the throat, or pass a stomach tube and paraffin to move turnips impacted in the oesophagus

Clostridial Diseases

Clostridia are a group of bacteria that cause several diseases in livestock. Pulpy kidney (enterotoxaemia) is the most important clostridial disease which can cause problems when young animals first go onto a brassica crop. It is quite rare in cattle but more common in sheep, particularly lambs under the age of 6 months.

Causes

• Affected animals are usually found dead, typically after a sudden change of diet from poor quality to good quality feed, including brassica crops. This is the main reason animals should be vaccinated well before going onto a crop.

Preventative measures

• A veterinarian will provide the appropriate advice and recommendations as to the use of clostridial vaccines.
• An appropriate vaccine is one that offers protection against five major clostridial diseases: pulpy kidney, black leg, black disease, malignant oedema and tetanus.
• Importantly, animals should be vaccinated at least 10–14 days before going onto the crop, as it takes several days for animals to develop an immune response to the vaccine and to gain protection against the bacteria.