Photosensitization of cattle in Montana: is Descurainia pinnata the culprit?

Recurrent photosensitization of cattle in Montana has been blamed on Descurainia pinnata, tansy mustard. Two feeding trials were conducted to determine if tansy mustard was phototoxic. Pen-fed cattle consumed 2.4 and 4.1 kg/hd/day of tansy mustard in the 2 trials, and no photosensitization was detected. Liver clearance of BSP was within normal limits, as were blood chemistry values for AST, CK and GGT. Field cases have confirmed that tansy mustard was present and grazed in pastures where affected animals have grazed. We suspect that other factors may be necessary to predispose cattle to photosensitization by tansy mustard, and future work will attempt to determine the cause of the photosensitization.