Solid Substrate Fermentation of Feedlot Waste Combined with Feed Grains

A new process is being developed for recycling animal wastes based on a solid substrate fermentation with cracked grains. To date, the work has been done mainly with corn and the liquid fraction of cattle feedlot waste. This liquid, separated by an oscillating 30 mesh screen, contains 10% solids in the form of microbial cells and either fine or soluble waste components; it has a nitrogen content of 3-4 mg/ml. When added to cracked corn at a ratio of 1:2, the thick liquid adheres to grain surfaces. Grain waste mixtures at about 40% moisture, incubated aerobically by tumbling in a slowly revolving (0.5 rpm) vessel, such as a small cement mixer, rapidly undergo an acid fermentation caused by the selective growth of lactic acid bacteria originally present in the waste; other waste organisms die. The number of bacterial cells increases 100 fold with 24 hr; yeasts emerge after extended incubation. The fetid waste odor quickly disappears as 0.1 meq of acid per wet gram of fermented solids is generated while nitrogen is conserved. The acids formed are primarily lactic and acetic, with lesser amounts of propionic and butyric. Exploratory tests with milo and wheat, as well as with wastes from hogs and cattle on different rations, indicate that the new process may be widely applicable. Diluted whole waste may be used when inclusion of fiber in the fermented feed product is of no concern. The solid state system provides an easily established selective environment for microbial growth. Specific organisms and added inorganic nitrogen may be used for a continuous operation to generate higher protein content in grain based rations.