Whole-Farm Nutrient Budgeting: A
Nutritional Approach to Manure Management

Wendy Powers
Assistant Professor, Livestock Waste Management
Department of Animal Science
Iowa State University

H.H. (Jack) Van Horn
Professor, Dairy Nutrition and Management
Department of Dairy and Poultry Sciences
University of Florida

Environmental accountability requires that livestock producers formulate a nutrient management plan, or budget which includes number of animals to be produced, estimated nutrient excretion, nutrients recovered and applied for fertilizer (based on manure analyses), and a plan to export nutrients off-farm if there is excess manure nutrient production relative to on-farm crop production needs. Without any one of these components, a budget can not be achieved. Proposed budgets for example operations are available (Van Horn et al., 1996). Based on manure makeup and the overall management program, a producer can make the necessary alterations to achieve a properly balanced operation.

In a nutrient management scheme, a key point available to manipulate the budget is at the manure source. By minimizing manure quantity and nutrients excreted, especially in relation to animal food product produced, manure may be more easily managed through reduction of storage needs and acreage required for nutrient utilization. Manure production is influenced by the diet consumed, thus nutritional strategies to optimize manure production should be employed.

Digestibility is considered to be the percentage of the dry matter or particular nutrient in the diet that the animal could absorb from its digestive tract and have available to use for maintaining life and producing offspring, body weight gain, milk, eggs, wool, etc. By definition, apparent digestibility is considered to be the difference between amounts fed and amounts recovered in feces. Previous nutrition research has given us good estimates of apparent digestibilities of ingredients that can be combined to estimate total ration digestibilities. Farmers, often with help from a consulting nutritionist, formulate rations of known digestibility for animals based on performance expected from those animals.

Knowing digestibility and, hence, indigestibility of the ration dry matter and organic matter permits us to estimate the amounts of dry matter and organic matter excreted, components that determine manure volume. Digested carbon-containing compounds are the energy components of the diet that are either oxidized and the carbon exhaled as carbon dioxide or they are used for the synthesis of animal products. Carbon losses, other than losses in manure and respired carbon dioxide, do occur in ruminants primarily as methane. Very few carbon-containing compounds are excreted through the urine. Urine is the avenue of excretion for several metabolic end products, most importantly, with respect to nutrient management, urea or uric acid from protein degradation or from dietary nonprotein N sources. Urine is the major excretion pathway for readily available fertilizer-N (urea or uric acid), K, and Na. Excreted P, Ca, and slower-released N from undigested protein primarily are in feces (Morse et al., 1992; Tomlinson et al., 1996).

If animals are consuming dietary nutrients at maintenance levels, e.g., N, P, and K, they will excrete, on-average over time, the same amount of N, P, and K they consumed. Maintenance needs and equivalent excretions for those nutrients will be accounted for by turnover, without net gain or loss, of nutrients in current body contents. When animals are accumulating N, P, and K in body weight gain, offspring, or milk or eggs produced, the amount of those nutrients excreted in manure (feces plus urine) differ from what is fed by the amounts in products produced. Excreted amounts of nutrients are diluted in the indigestible residue of organic and mineral matter in manure and variable amounts of water excreted in urine and feces. Nutrition-based models also allow for production level consideration. Higher producing animals require greater feed intake to support greater production thereby excreting more manure as a percent of bodyweight. Thus, nutrition-based data coupled with good estimates of the nutrient content in food products leaving the farm permit accurate estimation of total nutrient excretions in feces plus urine by difference (Patterson and Lorenz, 1996; Tomlinson et al., 1996; Watts et al., 1994).

Nutrition-based models predict the amounts of nutrients in fresh manure excretions more accurately than collections from animal pens because of the dynamic state of manure after excretion. For example, 40 to 75% of the excreted N will be in urea or uric acid in the urine component and urease enzyme, that is of bacterial origin and is nearly ubiquitous in the environment, converts urea and uric acid N to ammonia which can be lost to the atmosphere. Also, anaerobic digestion that begins in the large intestine of animals before feces are voided, continues after excretion if environmental conditions permit. Or a shift to oxidative fermentation may take place, e.g. composting. Either way, volume reduction takes place as carbon compounds are emitted to the atmosphere, primarily carbon dioxide and methane and to lesser extent other odorous volatiles. Additionally, physical separations may take place in animal pens. For example, urine or urine plus added water may drain away from feces residues thus making solids collected from animal pens different from original excretion. While nutrient losses during collection and storage are significant, many operations do not have the opportunity to influence the magnitude of loss that occurs. However, measures of excreted and collected amounts are both important because differences give estimates of losses that occurred after excretion.

There is little difference expected on a dry matter basis between species when animals consume diets of similar nutrient composition and digestibility. Water content of collected manure usually is the biggest variable affecting wet-weight composition and total volume. Expressing manure nutrient composition on a DM basis reduces variation. The DM composition of manure in animal pens will change as diet and performance criteria are varied and, especially, as water evaporates. Additionally, manure changes in composition after excretion as a result of ammonia volatilization and volume reduction due to carbon dioxide and methane losses.

Predicted N concentrations (DM basis) in residual manure are lowest for dairy cattle compared with similar N concentrations expected in manure from other species consuming more digestible, higher-concentrate rations. Estimated P concentration is lowest for dairy cows and highest for laying hens with the ratio of N:P ranging from 1.28 to 2.47 (Table 1). These ratios illustrate that manures usually are P-rich relative to N because N:P ratios recommended in plant fertilizers usually are much wider, e.g., 9:1. Note that calculated ratios in freshly excreted manure range from 3.4 (hens) to 6.1 for broilers and hogs. These ratios, although still P-rich, are much closer to plant needs and point out that if N volatilization losses could be eliminated or greatly reduced and/or P excretion reduced, manures would be much closer to a complete fertilizer. If it becomes possible to reduce dietary P and still meet minimum animal requirements and to reduce N volatilization, production of manures with N:P ratios of 8:1 or greater like needed in plant fertilizers could result.

By optimally balancing the diet offered, manure production and composition can be altered. Offering a diet that most nearly meets the animals' needs without providing excess nutrients is the key to reducing manure nutrient concentrations. The major advantage of showing that manure nutrient production is a function of ration and performance is that it is easy to visualize the importance of ration management to minimize excretions. Van Horn et al. (1994, 1996) provide example budgets for dairy and beef operations. Book values for feedstuff, pasture, and product compositions were used in each example. Improvement of feedstuff digestibility and/or productivity in the example budgets would have resulted in reduced manure nutrient excretion relative to animal food products produced. Improvement in feed digestibility can occur as a result of feed processing such as grinding or ammoniation.

Supplementation of limiting amino acids, thereby permitting reduction of total dietary protein, would reduce excretion of N, especially urinary N. Development of protein systems which estimate ruminal and lower digestive tract needs individually are useful in more accurately predicting the protein needs of cattle thus reducing total protein intake. Selection of feedstuffs with highly digestible protein thereby improving protein utilization, such a high-oil corn, offer promise as well.

Because manures become more and more P-rich as more N volatilizes, ration management to minimize dietary P concentrations will become especially important. Utilization of phytase enzyme sources in poultry and swine rations makes organic P available to those animals and permits reduction of dietary P. Hopefully, it will become even more cost effective in the future. Phytase enzyme is inherent in ruminant rations because ruminal microorganisms provide it so dietary addition is not necessary. However, surveys indicate (e.g., Shaver and Howard, 1995; Watts et al., 1994) that dairy and beef producers usually feed more dietary P than animals require and, thus, excretions can be reduced by dietary reduction. By combining improved digestibility and optimal dietary nutrient content, nutrient and volume reduction, reductions in storage capacity and acreage for crop growth can be realized, resulting in reduced capital investment costs.

Consideration of nutritional management in nutrient management practices allows a producer to tailor the nutrient budget to more accurately reflect the operation's management practices. Wide variation in manure production may occur as a result of nutritional strategies.

Table 1. Manure and N production estimates based on ration and products produced.

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