NAUDÉ, T.W., BOTHA, C.J., VORSTER, J.H., ROUX, C., VAN DER LINDE, E.J., VAN DER WALT, S.I., ROTTINGHAUS, G.E., VAN JAARSVELD, L. & LAWRENCE, A.N. 2005. Claviceps cyperi, a new cause of severe ergotism in dairy cattle consuming maize silage and teff hay contaminated with ergotised Cyperus esculentus (nut sedge) on the Highveld of South Africa. Onderstepoort Journal of Veterinary Research, During December/January 1996/97 typical summer syndrome (hyperthermia and a 30 % drop in milk yield) occurred in succession in two Holstein dairy herds (n = 240 and n = 150 milking cows, respectively) on the South African Highveld. These farms are situated in the midst of the prime maize and dairy farming areas of South Africa where this condition had never been diagnosed before.The individual components of the concentrate on both farms were negative for ergot alkaloids. Endophytic fungi and/or ergot infestation of teff and other grasses fed to the cows were then suspected of being involved, but neither endophytes nor ergot alkaloids could be implicated from these sources.By measuring the serum prolactin levels of groups of sheep (n = 5) fed the first farm's total mixed ration (TMR) or its three individual fibre components for a period of 11 days, the source of the ergot alkaloids was identified. A statistically significant decrease in the level of this hormone occurred only in the group on maize silage (which constituted 28 % on dry matter base of the TMR). The involvement of the maize silage was further chemically confirmed by the high levels of total ergot alkaloids, predominantly ergocryptine, found by LC-MS in the silage as well as in the TMR (115-975 ppb and 65-300 ppb, respectively). The ergot alkaloid content (mainly ergocryptine) of the maize silage on the second affected farm was 875 ppb. Withdrawal of contaminated silage resulted in gradual recovery of stock on both farms.Nut sedge (Cyperus esculentus and Cyperus rotundus of the family Cyperaceae) has a world-wide distribution and is a common weed in annual crops, and can be parasitized by Claviceps cyperi. Careful examination of the maize silage from both farms revealed that it was heavily contaminated 1 Division of Toxicology, ARC-Onderstepoort Veterinary Institute, Private Bag X05,