Absence of detectable levels of the cyanobacterial toxin (microcystin-LR) carry-over into milk

The potential for the carry-over of the cyanobacterial toxin, microcystin-LR, from feed to milk was assessed using four Holstein-Friesian cows in a 4 week feeding trial. Two cows were used as control and the other two dosed daily at increasing weekly concentrations of microcystins from zero to a maximum dosage of 13μg toxin kg^-1d^-1 (or 7.4mg toxin day^-1). The absence of any deviation from the control in terms of physiological response and plasma indicators (total bilirubin, γ-glutamyl transpeptidase and alkaline phosphatase) suggests that the microcystin-LR dosage did not have a detrimental effect on cattle liver function or milk yield during the course of the study. While the milk production did decrease over the period of the trial, no difference was observed between control and dosed cattle. Protein phosphatase inhibition assays were successfully used to determine the presence of microcystin-LR in prepared milk samples with an average recovery of 88% for samples spiked with 0.6μgl^-1 microcystin-LR. The level of microcystin-LR in all milk samples during the trial was less than 0.2μgl^-1. This suggests that after digestion, microcystin-LR is either not present in milk or sufficiently modified to render it non-toxic.