Most animal health problems in brassicas are seen in the first few days of feeding them, particularly if hungry animals are put onto a crop. Stock intake of brassicas can be very high, and in some cases this has led to rapid death of animals.
Problems are much less likely to occur when animals have adjusted to a crop, and have access to another source of feed or pasture to eat in conjunction with the brassica crop. A small percentage of animals do not perform well on brassicas, so stock should be monitored, and poor performing animals removed from the crop.
What animal health problems can occur on brassicas?
Crops are relatively high in rapidly fermented energy (NSC or sugars) but low in fibre (NDF). Microbes convert the sugars into volatile fatty acids, which lowers the pH (increases the acidity) of the rumen. The lack of fibre lowers the amount of rumination that occurs; this results in less saliva, which helps buffer the rumen, and consequently lowers the pH even further. This can result in clinical acidosis, which can have the symptoms of dehydration, diarrhoea, abortion, lameness, and death.
To prevent clinical acidosis, ensure the cows gradually adapt to the crop and provide 25-40% of the diet as roughage. Often the poor performers in the first 2-3 weeks of crops suffer from sub-clinical acidosis – these animals should be identified and removed from the crop.
Bloat on crops is typically caused by acidosis that reduces the contractions of the rumen and prevents belching. The risk of bloat is increased when cows graze frosted brassica crops. It is good practice to wait until the frost lifts before offering the cows their break, and feed other fodder such as hay, straw of silage first.
Bloat on crops is different from the frothy bloat that occurs when cows graze clover. Frothy bloat results from the formation of foam that prevents the cow from belching out gases that form in the rumen.
Choking on bulbs
Choking can occur when cows are grazing small bulbs, usually only 4-8 cm in diameter. A range of factors can cause small bulbs including grazing the crop too early, sowing too late, or poor yield due to pests, diseases, drought, or insufficient fertiliser.
Animals literally choke or die of bloat because gas fails to escape from the rumen. If bulbs are small, the risk of choking can be reduced by ensuring cows are fed hay, straw or silage before they are shifted on to a new break; this discourages hungry cattle from gorging on the crop.
Brassicas can contain high levels of calcium and only marginal levels of phosphorus. They also contain low levels of magnesium but high levels of potassium. This imbalance in minerals is conducive to milk fever. It is best to avoid feeding brassicas to springing cows - springers should be transitioned to a grass based diet at least two weeks before calving.
Magnesium can be supplemented by supplying 60 grams/cow/day of magnesium chloride or sulphate in the water at least one month before calving. Silage or grass for springers can be dusted with an additional 60 grams/cow/day of magnesium oxide. Refer to Farmfact 3-1 for more information on magnesium supplementation.
Nitrate occurs naturally in plants but not usually in great amounts. Nitrate concentrations increase when plants take up more N than they need for leaf growth. Nitrates in the plants are converted to nitrites and ammonia by the microbes in the rumen. At high concentrations, the ability of rumen microflora to reduce nitrate to ammonia via the intermediate nitrite is overloaded. The nitrites are instead absorbed into the bloodstream which lowers the blood’s ability to carry oxygen to the tissues. Clinical signs of nitrate poisoning are sudden deaths, cows appearing uncoordinated and anoxia (lack of oxygen to tissues). Removal of animals suspected to be suffering from nitrate toxicity from the crop usually allows them to recover.
Periods of rapid growth after dry spells or frosting, as well as the recent or high application of nitrogenous fertilisers, can lead to an increase in nitrate concentration in the plant. Kits are available for testing nitrate levels of crops. The risk of nitrate poisoning is reduced by allowing cows to gradually adapt to the crop, and ensuring enough roughage is available to cows before feeding the breaks. Hungry cows that gorge themselves on crop are at greater risk of nitrate poisoning.
Red water or kale anaemia is caused by a high concentration of SMCO (high S-methyl cysteine sulphoxide). SMCO is found in all brassicas but is especially high in kale. In the rumen, SMCO is converted to an oxidative compound, called dimethyl sulphide, which reacts with red blood cells, lowering haemoglobin concentration. As this is excreted it results in red urine or the clinical condition known as “red water”. Other clinical signs can include weakness, diarrhoea, jaundice, decreased appetite, and poor performance. If red water is observed affected animals should be removed to alternative feed sources, to reduce SMCO intake. Ensure animals have adequate levels of selenium before going onto crops.
High concentrations of SMCOs most often occur where high concentrations of both nitrogen and sulphate are either present in the soil or applied as fertiliser, care should be taken to avoid excessive use of these nutrients. Avoid feeding cows flowering brassica crops, as flower heads have the greatest concentration of SMCOs in the plant.
High intake of fast growing immature brassica crops, usually rape or turnips, can induce a photosensitive reaction of the skin. White and non-pigmented skin areas of dairy cattle can be affected. Cows will recover rapidly when they are taken off the crop to a shaded area.
Scald can be avoided by waiting for the crop to mature. Slow growing crops such as kale appear less prone to this problem.
Trace mineral imbalance
Brassicas contain low levels of copper and are likely to induce copper deficiency in cows fed brassicas for prolonged periods. High levels of iron and sulphur in crops could further reduce copper uptake. Cows copper status should be checked before drying off and if necessary, topped up with copper injections before grazing crops.
Selenium is an essential element in the enzyme that helps prevent red water. Selenium levels in cows should also be checked prior to drying off and topped up with injections if required.
Brassicas also contain low levels of iodine, and may contain high levels of goitrogens, which can block the uptake of iodine resulting in an iodine deficiency in cows. Iodine deficiency may increase the risk of stillbirths or reduce the viability of new born calves. Iodine supplementation in drinking water is the only viable solution.