Diet Modifications as a Means to Address Air Emissions

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By Wendy Powers, Department of Animal Science

Diet modification can be a useful tool in any operation's plan to reduce air emissions. While diet modification approaches may not be the only tool needed to obtain air emission objectives, changing diet formulation is a proven method. The observed reduction will, however, depend upon the initial and modified formulation and how that compares to animal nutrient needs.

While reducing air emissions is important, the diet strategy employed must maintain animal performance and be affordable. For this reason, most studies to date have focused on reducing excess nutrients in the diet. Ammonia and hydrogen sulfide emissions result from the excretion of unused dietary protein as urine and feces. Odor, too, is believed to be formed largely from the excretion of what was fed as dietary protein. Therefore, research has focused on meeting dietary protein needs without overfeeding protein.

In a study conducted at Iowa State University, growing pigs were fed diets containing no supplemental amino acids (17.4 percent crude protein), added lysine (17.0 percent crude protein) or added lysine, methionine, threonine and tryptophan (14.5 percent crude protein), recognizing that the large majority of the swine industry in Iowa is adding lysine to the diet to reduce total protein fed and still meet the lysine requirement. Ammonia emission rates from the three diet formulations were 2.5, 2.2, and 1.1 mg per minute, respectively. No animal performance differences were observed. Adding lysine to the diet decreased feed costs by $3 per ton when compared to the diet with no amino acids. The reduced cost reflects a replacement of soybean meal with lysine and corn. Adding the four amino acids increased diet cost by over $4 per ton due to the cost of the amino acids, particularly tryptophan. However, the added feed cost was associated with a decrease in ammonia emission rate of over 50 percent, which may be important to operations in the near future.

A portion of the industry is adding lysine, methionine and threonine and this portion increases as threonine becomes more affordable. In a follow-up study ammonia and hydrogen sulfide emissions from pigs were measured over the entire grow-finish period. Diets offered contained 1) lysine, 2) lysine, methionine and threonine or 3) lysine, methionine, threonine, tryptophan, and valine or isoleucine. Ammonia emissions (pounds per day) were reduced 22 percent with the three amino acid diet, compared to the lysine only diet and 48 percent for the five amino acid diet compared to the lysine only diet. No animal performance differences or hydrogen sulfide effects were found. While the five amino acid diet is unrealistically expensive at the present time, the three amino acid diet may offer an option to producers who are above reporting thresholds and feeding lysine only at the present time.

Similar work is currently underway with laying hens with plans to quantify diet potential in broiler chickens, turkeys and cattle in pending studies. The use of carbohydrates in feed formulation and the associated impacts on air emissions is being studied at various locations in the U.S. and will be the subject of a future article.