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Photosensitization in Lambs Grazing Switchgrass

Div. of Anim. and Vet. Sci., West Virginia Univ., Morgantown, WV 26506-6108

D. P. Belesky

USDA-ARS, Appalachian Soil and Water Conserv. Res. Lab., Beckley, WV 25802-0867.

^* Corresponding author.

Hepatogenous photosensitization has been recorded in herbivores grazing a number of Panicum species in different areas of the world. This condition was observed in weaned and suckling lambs (Ovis sp.) grazing switchgrass (Panicum virgatum L. cv. Cave-in-Rock) in Morgantown, WY, during the abnormally hot, dry summer of 1991. The first cases occurred in June, 35 d after initiation of grazing. Further cases occurred with a new group of lambs 12 d after their introduction to switchgrass pastures in late July. Of a total of 104 lambs, 17 showed photosensitization and eight died. Symptoms were similar to those reported for ‘geeldikkop’ in sheep and hepatotoxicity was confirmed by changes in blood metabolites. Mature ewes were not affected and no signs of photosensitization were noted in suckling lambs grazing switchgrass during the previous year under more normal temperature and rainfall conditions. Photosensitization does not appear to have been observed in cattle on switchgrass but monitoring of younger animals grazing this Panicum species seems desirable.

Received for publication December 2, 1991.