Submitted to: Journal of Animal Science
Publication Type: Peer reviewed journal
Publication Acceptance Date: 4/21/2006
Publication Date: 9/1/2006
Citation: Miller, D.N., Berry, E.D., Wells, J.E., Ferrell, C.L., Archibeque, S.L., Freetly, H.C. 2006. Influence of genotype and diet on steer performance, manure odor, and carriage of pathogenic and other fecal bacteria. III. Odorous compound production. Journal of Animal Science. 84:2533-2545. Interpretive Summary: Diet strongly affects feces composition, which in turn affects the production of odorous fermentation products. Excreted starch is a primary substrate for odor compound and lactic acid production, particularly in feces from animals fed finishing and growing diets high in corn content. The combination of high volatile fatty acid and lactic acid act to reduce pH and enhance volatile fatty acid emission. However, in feces where starch is less available, as was observed in feces from animals fed a bromegrass diet, protein in the feces is increasingly important as a substrate for odor compound production. As protein is fermented, highly malodorous branched-chain volatile fatty acid and aromatic compounds accumulate at proportionately greater rates than when starch is the primary substrate. Diet manipulation to control cattle feedlot odors is a valid approach, but future application will require human olfactometry to confirm laboratory results.
Technical Abstract: Three beef cattle diets were assessed for the potential to produce odorous compounds from feces excreted during the growing and finishing periods. Eight pens containing 51 steers of varying proportions of Brahman and MARC III were fed either a chopped bromegrass hay diet (BG) or corn silage diet (CS) for a 119 d growing period. After the growing period, all steers were switched to the same high corn finishing diet (HC) and fed to a target weight of 560 kg (finishing period). Fecal slurries were prepared from a composite of fresh fecal pats collected in each pen during both periods and incubated anaerobically. In additional flasks, starch, protein, or cellulose was added to composite fecal subsamples in order to determine preferred substrates for fermentation and odor compound production. The content and composition of fermentation products varied both initially and during the incubation depending upon the diet fed to the steers. The CS and HC feces had the greater initial content of VFA (381.0 and 524.4 µmoles gm-1 DM, respectively) compared to the BG feces (139.3 µmoles gm 1 DM) and accumulated more VFA than the BG feces during the incubation. L-lactic acid and VFA accumulation in the HC and CS feces was at the expense of starch, based upon starch loss and the production of straight-chain VFA. In the BG feces, the accumulation of branched-chain VFA and aromatic compounds and the low starch loss indicated that protein in the feces was the primary source for odor compound production. Substrate additions confirmed these conclusions. We conclude that starch availability was the primary factor determining the accumulation and composition of malodorous fermentation products.