TURNIP PHOTOSENSITISATION IN DAIRY COWS.

Several causes of hepatogenous or secondary photosensitisation have been reported, such as from hepatotoxic plants, water-borne cyanobacteria and saprophytic fungi. In cattle, hepatogenous photosensitivity is associated with the feeding of crops comprising certain cultivars and/or hybrids of forage Brassica, namely turnip, rape and swedes. The bovine clinical case is referred to as Brassica-associated liver disease or BALD, which has been well-described in Australia and New Zealand, but not previously in South Africa. Recently, however, a case of infected Holstein cows in the Humansdorp district was reported.

The cows grazed a mixed forage crop dominated by bulb turnip (Brassica rapa, cultivar Barkant). The crop was grazed 45 days after planting and 10% of the herd developed symptoms. Initially, they developed reddened, painful teat skins and more characteristic non-pigmented skin lesions manifested one to two days later. The affected cows had elevated serum levels of gamma-glutamyl transferase (GGT), glutamate dehydrogenase and aspartate aminotransferase. These enzyme elevations confirmed secondary (hepatogenous) photosensitivity. Liver lesions from three culled cows showed oedematous concentric fibrosis around the medium-sized bile ducts and inflammatory infiltrates in portal tracts. Characteristic lesions associated with other known hepatobiliary toxicities were not found and no new cases were reported five days after the cattle were taken out of the turnip mixed pasture, confirming that the condition was due to BALD.

Farmers should be aware that Brassica species, particularly the Barkant and English Giant varieties, may cause BALD. It is therefore important to allow sufficient adaptation to mixed pastures with substantial proportions of turnip, by initially providing some other forages or hays to cows before grazing the turnip-containing pasture. If the rumen has adapted, the prevalence should be much lower or absent.