The soluble and dispersed nitrogen and carbon components in the filtrate fraction of cattle feedlot waste are a potential nutrient source from which single-cell protein could be produced for animal feeds. The ability of more than 200 fungi and streptomycetes to grow in this liquid was determined; these included isolates from the waste and associated sources, as well as organisms maintained in the Culture Collection of the Agricultural Research Service in Peoria, Ill. Utilization of waste nutrients was measured by changes in nitrogen content and chemical oxygen demand. Only 20% of the organisms were able to grow appreciably in the filtrate. Of these, dry-weight yields varied from 0.6 to 2.7 g of mycelium per liter; from 21 to 50% of the nitrogen in the filtrates was used during growth, whereas chemical oxygen demand levels diminished from 4 to 60%. In general, streptomycetes isolated from the feedlot used nutrients from the filtrates better than fungi did. Addition of readily available carbon sources such as glucose or whey significantly increased (as much as sixfold) cell yields of selected organisms and promoted better utilization of nitrogen (from two- to threefold); the effect on chemical oxygen demand varied (0 to 33% increase).
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