Photosensitivity is also possible with dairy cows grazing summer turnips and with other stock classes on summer turnips and leafy turnips. The cause of this condition is not well understood, for dairy cows the risk factors include: consuming large volumes of turnips (greater than one third of the diet) and feeding on crops under environmental stress.

• Crops should be accurately measured and allocation for dairy cows should be no more than one third of diet
• Animals with photosensitivity should be removed from crop and offered shade as soon as possible

In lambs grazing summer turnips (including Hunter) this condition is rare and unpredictable but may be associated with adverse and overcast weather conditions. This may be of particular concern to stud stock owners, where photosensitivity may cause cosmetic issues to sale animals.

When protein manufacture cannot keep up with nitrogen uptake in plants, the excess accumulates as nitrates, which when consumed are converted to nitrites in the rumen and can cause toxicity problems to grazing animals. When animals ingest high levels of nitrates, nitrites build up in the bloodstream. Here they bind with the oxygen carrying compound, haemoglobin, to form a compound that no longer is able to carry oxygen. Simply, the animal suffers oxygen deprivation.

The most common symptom of nitrate toxicity is sudden death, but prior to death, excessive salivation, rapid gasping breath, rapid pulse (>150 beats/min), pale blue or brown colouration of membranes, tremors and muscle weakness may occur. Pregnant animals surviving may abort.

Nitrates can build up in any situation where environmental conditions promote plant growth but limit photosynthetic activity. These include sudden temperature changes, dry periods followed by rain, frost, shading, overcast days, insect damage, some herbicides, some nutrient deficient soils, excessive nitrogen fertiliser use, soils with deficiencies in sulphur, phosphorus, molybdenum, or high acidity levels. Nitrate toxicity can occur on a range of grasses, brassicas, forage cereals and weeds. Young plants and plant material close to the ground are more likely to have high nitrate levels.

Treatment of Nitrate Toxicity

• Remove stock to low risk pasture
• Seek emergency veterinary assistance

Key Tips

• Recognise environmental conditions that cause nitrate build up and check animals more regularly during these periods
• Get suspect crops tested before grazing
• Introduce stock gradually to allow rumen adjustment
• Do not put hungry animals onto suspect crops
• Avoid overstocking of suspect crops as high grazing pressure will increase the amount of high-nitrate plant parts eaten
• When strip grazing, watch utilisation levels or graze for short periods
• Dilute high nitrate feed with a low nitrate feed source, e.g. hay, pasture, silage
• Make high nitrate forages into silage. Ensiling does not decrease nitrate levels but may be a way of managing high nitrate pastures by reducing the proportion in the diet
• Manage nitrogen applications carefully to match plant requirements and therefore avoid excess uptake and nitrate build up
• Do not allow animals access to nitrogen fertilisers, fertiliser storage areas, fertiliser spills or grazing on recently fertilised paddocks
• Take care when using nitrogen fertiliser around waterways, to avoid nitrate build up in drinking water
• Ensure that soil nutrient levels are in the optimum range for your farming system, as some nutrient deficiencies lead to nitrate build up
• Healthy animals are less likely to be affected than animals in poor health
• Remember that nitrate levels in animals are a combination of the nitrate consumed in their feed and their drinking water

In some situations, iodine deficiency can occur when livestock are fed on brassica crops. This is because brassicas are naturally low in iodine and contain plant chemicals which are goitrogenic and inhibit iodine uptake. Iodine is important for growth and cell differentiation of tissues through its inclusion in thyroid hormones. Consequently, iodine deficiency has its greatest effect on the developing foetus and therefore may play an important role where pregnant livestock graze brassicas for extended periods in the final stages of pregnancy. The most marked sign of iodine deficiency is enlarged thyroid glands (goitre) but weak newborn lambs, low birth weights and a high rate of perinatal mortality may be subclinical signs along with poor wool growth and lower fertility in older stock.

Be aware of the iodine status of pregnant livestock grazing a brassica crop and consider an iodine supplement. Look for productivity clues that may signal an iodine deficiency.

There is some evidence that animals grazing solely brassica crops do not receive sufficient trace elements and begin to deplete their liver stores. A trace element supplementation programme should be considered if animals are offered a sole diet of brassica for an extended period, or animals have a low trace element status prior to crop introduction. This may require soil, herbage and blood analysis and consultation with your veterinarian to establish current trace element status and the appropriate supplementation programme.

Mixing herbs and clovers with brassica crops is a strategy that may assist with increasing trace element availability to stock.