Discipline: forages/grazing; Keywords: Brassica rapa; forage turnip; dairy cattle; teat lesions; Barkant; Brassica-associated liver disease; hepatogenous photosensitivity.
There are several causes of hepatogenous or secondary photosensitisation in ruminants in South Africa, e.g. hepatotoxic plants, water-borne cyanobacteria, the mycotoxin, sporidesmin, found in spores produced by the saprophytic fungus Pithomyces chartarum etc. In cattle, hepatogenous photosensitivity associated with the feeding of crops comprising certain cultivars and/or hybrids of forage Brassica, namely turnip (Brassica rapa ssp. rapa), rape (B. napus ssp. biennis) and swedes (rutabaga, B. napus ssp. napobrassica) has been reported from Australia and New Zealand. This bovine clinical entity is now referred to as Brassica-associated liver disease, and it is also associated with various well-known clinical syndromes, including haemolytic anaemia, polioencephalomalacia, abortion, nitrate toxicity, ruminal acidosis, constipation or diarrhoea, goitre, fog fever and bloat as a result of choke, as well as the problem of Brassica milk taint.
In New Zealand, camps of these high-yielding Brassica forage crops are planted to supplement nutritional requirements when pasture growth is slow or of poor quality. In recent years, such crops have been introduced to the Eastern Cape Province to fill a forage gap, for example, in the transition between pastures or when feed supplies are limited. A case of photosensitivity has recently been picked up by the authors cited below in the Humansdorp area. Holstein cows on a farm developed reddened, painful teat skin three days after grazing a mixed forage crop dominated by bulb turnip (Brassica rapa, Barkant cultivar).
The crop was grazed 45 days after planting and 10% of the herd developed symptoms. More characteristic non-pigmented skin lesions started manifesting 1–2 days after the appearance of the teat lesions. Affected cows had elevated serum gamma-glutamyl transferase, glutamate dehydrogenase and aspartate aminotransferase. These blood chemistry findings confirmed a secondary (hepatogenous) photosensitivity. As a result of the severity of the teat and skin lesions, seven cows were slaughtered and tissue samples from five of them were collected for histopathological examination. Liver lesions in cows that were culled 3 or more weeks after the onset of the outbreak showed oedematous concentric fibrosis around medium-sized bile ducts and inflammatory infiltrates in portal tracts. Characteristic lesions associated with other known hepatobiliary toxicities were not found. No new cases were reported 5 days after the cattle were removed from the turnips. The sudden introduction of the cows, without any period of transitioning or adaptation to grazing turnips, as well as the short latent period, clinical signs of photosensitisation, blood chemistry and histopathology, confirmed a diagnosis of Brassica-associated liver disease, a condition seen in New Zealand but not previously described in South Africa.
Brassica forage crops are potentially toxic under certain conditions and farmers must be aware of these risks.
Davis, A.J., Collet, M.G., Steyl, J.C.A. & Myburgh, J.G., 2021, ‘Hepatogenous photosensitisation in cows grazing turnips (Brassica rapa) in South Africa’, Journal of the South African Veterinary Association 92(0), a2106. https://doi.org/ 10.4102/jsava.v92i0.2106