Although odors have always been associated with animal production, only within the last decade has the interest to control them resulted in extensive research. Because odors are difficult to measure, this research has not achieved much more than suggestions as to what may work. What is known is that odors are predominantly the result of anaerobic fermentation of feed waste, indigested feed, and secretions by the animal. Although sulfur compounds are extremely malodorous and of concern for health, their role in odor sensation is controversial. Phenolics such as skatole and indole and volatile fatty acids are typically the compounds most highly correlated with odor sensation. Ammonia is derived from cleavage of urinary urea by fecal/bacterial urease. Ammonia is of health and environmental concern and should be reduced for those reasons. However, its correlation with odor sensation is mediocre and strategies effective at reducing ammonia may not positively affect odor.

Feed waste is a major contributor to waste carbohydrates and may be a substantial source of volatile fatty acids and methane. Thus, care should be taken to minimize feed waste, especially because this is also attractive economically. Non-starch polysaccharides in the feed are poorly utilized by non-ruminants and they will contribute to odor and methane production. For emission control, reducing non-starch polysaccharide intake may be important.

About half of the waste protein is from indigested feed, the remainder being from animal secretions. Selection of easily digestible feedstuffs can lower both the secretions by the animal and indigestible feed, the factor with the highest negative correlation with digestibility being the dietary fiber content. Thus, selecting feedstuffs with a low fiber content and thus a high digestibility, is preferred. Alternatively, fibers can be degraded using fiber-degrading enzymes such as xylanases. Low-protein feeds also have been shown to reduce both odor and ammonia emission without compromising animal performance.

An indirect method for reducing odorants is through manipulation of the microflora in both the intestines and in the waste. pH is a very important modifier, and lowering pH is suggested to be beneficial for reducing odor and ammonia emission. Microbial populations in the gut can also be manipulated using dietary fiber and pre, pro, and antibiotics. Although several experiments have demonstrated effective strategies for reducing odor through this mechanism, including the use of high fiber diets, our understanding of how to manipulate the microflora is still rudimentary.

Because odor production starts with the feed it is important that feed formulators start taking into consideration the impact of feed composition on waste production and odor. Options include formulating for lower crude protein contents, reducing dietary fiber, reducing excreta pH, and using compounds such as enzymes that improve digestibility.