Major differences occurred in the physical and chemical parameters of three dung types, NGD, MGD, and HGD, from cattle fed diets containing no grain, and medium (35–40%), and high (>60%) amounts of grain, respectively. Both NGD and MGD differed from HGD in a majority of the parameters measured over a 3-d period, including face fly,Musca autumnalisDe Geer, larva survival to the pupal stage, production of large quantities of organic acids in HGD, substantial decreases in pH of>1 pH unit, and large increases in osmolality (ca. 200 mmol/kg) in HGD. Alteration experiments indicated that a complex of factors causes the increase in larva mortality associated with HGD, including low pH, elevated osmolality, and the production of organic acids such as volatile fatty acids and lactic acid.