The development of a sustainable, profitable, and competitive egg-production system is becoming a challenging issue facing the laying-hen industry.
The excretion and volatilization of nitrogen and the emission of volatile organic compounds of dietary origin are responsible for a large part of the environmental issues that have risen from the intensive poultry production. High levels of airborne ammonia in poultry houses negatively affect egg production, worker health, and the public perception of the egg-production industry.
Especially during winter, when ventilation rates are low, the ammonia concentrations in many high-rise houses will exceed the levels recommended by industry groups (of maximum 50 ppm at bird level). Moreover, emission of more than 100 lb of ammonia per day per site triggers federal reporting requirements through the Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA) and the Emergency Planning and Community Right-to-Know Act (EPCRA). Thus, strategies to lower ammonia emissions from laying-hen houses and manure-storage barns (containing manure from manure-belt houses) need to be identified and evaluated.
Laying-hen diets that are formulated using proteinaceous ingredients such as soybean meal and meat-and-bone meal to meet the recommended levels of essential amino acids contain relatively high levels of crude protein and excessive amounts of amino acids other than the first- and second-limiting amino acids (usually methionine and lysine, respectively). Because the birds have no storage mechanisms for amino acids consumed beyond the requirement for protein synthesis, the amino acids consumed in excess are deaminated and the amino acid–derived nitrogen is excreted in the urine mainly as uric acid (80%), ammonia (10%), and urea (5%). Once excreted, uric acid and urea are readily converted to ammonia by a series of microbial enzymes present in the manure.
Strategies to Lower Ammonia Emission
There are multiple methods to reduce ammonia concentrations in laying-hen houses; these include mechanical, manure management, and dietary approaches. Increasing the ventilation rate will remove the ammonia from the building and lower the hens’ exposure to ammonia. However, it may not always be practical to increase ventilation (e.g., during winter) and increasing the ventilation rate does nothing by itself to comply with the CERCLA and EPCRA regulations because the total pounds of ammonia emitted will not change. Other mechanical and management approaches include planting tree buffers around the perimeter of the laying-hen house or moving the manure out of the house (as occurs in manure-belt houses) where it can be treated either chemically (e.g., by acidification or addition of zeolite) or physically (by changing its surface-to-volume ratio). Pre-excretion ammonia-lowering strategies include dietary manipulation, which can be divided into two not mutually exclusive categories based on the method of action: Lowering the nitrogen (uric acid) excretion and sequestering the ammonia in the manure.
Phase feeding and dietary crude protein contents
Dietary strategies to lower nitrogen excretion include phase feeding and formulating diets without a crude protein minimum using crystalline amino acids. The amino acid requirements of young hens are relatively high and decrease over time due to changes in egg production rate and body weight. When phase feeding is implemented, the dietary nutrient profile is adjusted over time to better reflect that needed by the hens. As a result, consumption of excess amino acids is reduced and, with it, the excretion of uric acid and production of ammonia.
Theoretically, a new diet with adjusted nutrient contents should be formulated and fed every day; however, a more practical approach is to change the diet formulation at least four times during a production cycle. (Note that phase feeding is different from the often weekly re-formulation of the diets that takes into account price and availability of feed ingredients, but does not adjust the absolute amount of nutrients consumed by the hens.) The amount of nitrogen excreted by poultry can be dramatically lowered by not specifying a dietary crude-protein minimum and instead formulating the diet to meet the hens’ amino acid requirements.
Such ‘low crude protein’ diets can be obtained through a partial replacement of soybean meal with corn and crystalline amino acids or amino acid analogs. The inclusion of individual crystalline amino acids in the diet allows for a more precise formulation, more closely matching the dietary content of amino acids with that needed by the hens. As a result, the diet contains less excess amino acids (in relation the hens’ needs), meaning that fewer amino acids are deaminated and less nitrogen converted to uric acid and excreted. In research settings, a 3–5 percentage-unit reduction in the dietary crude protein content has resulted in up to a 60% reduction in the total nitrogen excretion from broilers and laying hens with concomitant reductions in ammonia emission. Although in practical laying-hen diets the reduction in the dietary crude protein content is typically only around one percentage unit (e.g., from 16 to 15% crude protein), the ammonia emission is nevertheless reduced by 8–10%.
As mentioned previously, ammonia emissions can be reduced from laying hens by minimizing the excretion of uric acid. However, some ammonia production is inevitable and additives can be added to the diet to sequester or trap the ammonia. Among these dietary additives is zeolite, a type of mineral with a porous or lattice-like structure, which binds ammonia in the feces and/or manure and prevents it from being emitted into the air. Another strategy is to acidify the diet through addition of gypsum or calcium benzoate, or by lowering the dietary electrolyte balance (calculated as sodium + potassium – chloride). The acidic diet will result in acidic manure (i.e., with a lower pH), causing ammonia (NH3) to be converted to ammonium (NH4+). Ammonium is more water soluble than ammonia and therefore not readily emitted into the air. Results from experiments conducted at Iowa State University indicate that feeding a low-protein diet that includes gypsum and zeolite reduces ammonia emission from laying hens by 40%.
Fibrous feed ingredients
Dietary inclusion of fermentable fiber shifts partitioning of nitrogen excretion from urea and uric acid in the urine to microbial protein in the feces. Because dietary fermentable fiber is not digested, it serves as an energy source for microbial growth in the large intestine. The nitrogen used in the (increased) microbial protein synthesis comes partly from nitrogen that diffuses from the blood across the intestinal wall—nitrogen that would otherwise have been excreted as uric acid. Although the combined nitrogen excretion in the urine and feces may not change with consumption of fermentable fiber, the fecal nitrogen is in the form of microbial protein, which is less likely than uric acid to be converted to ammonia and emitted into the air.
Additional benefits of the microbial fermentation of the dietary fermentable fiber include production of volatile fatty acids, which lowers the pH of the manure and affects the ammonia–ammonium equilibrium. In a recent study conducted at Iowa State University, ammonia emission from laying hens fed a diet containing corn distiller’s dried grains with solubles (DDGS), wheat middlings, or soybean hulls as sources of fiber was evaluated. Results of this study showed that addition of either 10% corn DDGS, 7% wheat middlings, or 5% soybean hulls to laying-hen diets lowered the ammonia emission and ammonia-emission rate from manure by up to 50%. The hens fed the fiber-containing diets consumed slightly more feed and, therefore, produced slightly more manure. However, when the ammonia emission was calculated on a per-hen basis (taking into account the extra manure produced), ammonia emission was reduced by 40% compared to that of hens fed a control diet. The ammonia-reducing effect of dietary fiber was mainly caused by a lower pH of the manure. Egg production, egg mass, and feed utilization were not adversely affected by the addition of dietary fiber. However, more information is needed about the level and type of fiber that most effectively lowers ammonia emission and for how long the fibrous ingredient must be fed.
Ammonia emission can be drastically reduced through the prudent use of dietary additives, choice of feed ingredients, and careful formulation of the diet to minimize nutrient excesses. Phase feeding and the use of crystalline amino acids can lower both ammonia emission and feed cost. Fibrous feed ingredients, such as corn DDGS, are attractive ways to reduce ammonia emission because they also contribute energy and nutrients to the diet. For instance, when corn DDGS is included on a least-cost basis in the diet, the ammonia emission can be lowered by 40–50% without increasing the cost of the diet. – Presented at the Pre-Show Nutrition Symposium of the 2006 Midwest Poultry Federation Convention.