Summer 1999
Principles of diet manipulation
by Wendy Powers, assistant professor, Department of Animal Science
Manure malodor results from the anaerobic decomposition of organic material. During anaerobic degradation, odorous compounds (more than 200 identified) are formed that can ultimately be degraded to less odorous and nonodorous endproducts such as methane, carbon dioxide, water, ammonia, and hydrogen sulfide. Failure to provide a balanced system leads to the accumulation of these intermediate compounds and to malodor (Figure 1). Freshly excreted manure is less odorous than manure that has been stored for 1 or 2 days, indicating that malodorous compounds begin to accumulate immediately following excretion. Changing the composition of manure offers the potential to improve the odor of stored manure. Some dietary management steps a producer can take to minimize odor development are discussed in this article.
Feed a balanced diet. Most research has shown that protein metabolites appear to be the compounds most highly correlated with manure odor intensity. Several studies illustrate a trend of decreasing odor intensity with decreasing dietary crude protein. Use of crystalline amino acids allows total dietary crude protein to be reduced while still providing adequate amino acids to meet animal needs. Feeding according to the metabolizable protein system guidelines (ruminally degradable versus undegradable intake protein) for ruminants is another method to avoid overfeeding crude protein. By more nearly meeting the animal’s exact protein needs, less protein is excreted in the manure and lower concentrations of malodorous metabolites accumulate, thereby reducing manure odor intensity. Balancing dietary carbohydrate with protein is another strategy. By providing adequate sources of fermentable carbohydrate protein, use may be improved, resulting in reduced nitrogen excretion.
Group animals according to nutrient requirements. Grouping animals according to production needs reduces the gap between animal nutrient requirements for different groups and allows producers to formulate diets to more closely meet animal requirements. The same principles discussed in "Feed a balanced diet" regarding the impact of overfeeding nutrients then apply.
Feed ingredients. Based on their nutrient profile, digestibility, and inherent odor, specific feeds may predispose manure to malodorous conditions more so than other feeds or dietary regimes. For example, some work demonstrated less malodor when peppermint oil was added to the diet. Peppermint oil has a very strong, but not unpleasant odor. The peppermint oil may have been poorly digested due to its chemical structure, resulting in the pleasant odorants remaining intact and acting as a masking agent to malodorous compounds. Fishmeal is an example of a high-protein byproduct with a strong inherent odor. A study at Iowa State University considered the inclusion of bloodmeal in the diets of nursery pigs and observed a trend of increasing manure odor intensity with increasing level of bloodmeal. Others observed that manure from dairy cows fed diets containing bloodmeal smelled distinctly different from that excreted by cows fed diets without bloodmeal. These observations suggest that either the inherent odor derived from unabsorbed bloodmeal impacts manure odor or amino acids in bloodmeal metabolize to odorous intermediate compounds.
Figure 1. Anaerobic decomposition of manure.
Alteration
of rumen or hindgut fermentation may prove to be a plausible mechanism
of odor control. By shifting the fermentation, more desirable products—from
an odor intensity or offensiveness perspective—may result. Some work has
shown that manure from feedlot cattle fed barley-based diets was lower
in odor intensity than manure from cattle fed sorghum-based diets, probably
due to fermentation differences of each feed. Additives or feeding strategies
that improve feed efficiencies or alter rumen or hindgut fermentation
and function offer a great deal of potential as manipulators of manure
odor as well as a means of reducing nutrient excretions for nutrient management
purposes. Although recommendations on specific feeds or additives to avoid
or incorporate are not currently available, research is continuing and
recommendations are likely in the future.
Consider mineral and water sources. Often, mineral sources contain sulfur, a component of odorous mercaptans and thiols. It is important that producers consider mineral inputs and their impact on total dietary sulfur content. Research that considers the benefits of chelated mineral sources is underway. Water supplies also contain a fair amount of sulfur, depending on geographical location and water source. The contribution of dietary intake of sulfur from the water supply should be considered, if possible.
Keep feed fresh. Offering a supply of fresh feed and disposing of waste feed is another means of controlling odors. When wet, feed starts to ferment anaerobically and produces malodorous products in the same manner as stored manure. Proper cleaning of feeding areas and removal of the feed to a disposal area helps reduce odors from feeding areas as well as improving feed intake.
Future possibilities. Based on current research efforts, producers can expect to have recommendations available in the future for feed and mineral selection that will reduce odor potential. The possibility also exists for alteration of byproduct processing techniques to produce animal feeds with less potential for odor development. Although these strategies are still in their infancy the importance of byproduct feeds coupled with pressure to reduce odor from animal operations supports efforts to pursue this area of study.
