Nutritional Implications for Manure Nutrient Management Planning

Nutrient management planning is necessary for many livestock producers. In order for producers to accurately plan on-farm nutrient generation and utilization, reasonable estimates of manure production and composition must be available. Amounts of manure nutrients (e.g., N, P, and K) originally excreted are predicted more accurately with a nutritionally based input-output model than are the amounts recovered because the amounts that are recovered vary depending on climate, storage and handling practices, and other site-specific influences. Records of amounts of manure collected and composition determined from manure sampling are essential to determine the total of manure nutrients that must be managed in the plan. It is important to compare recovered amounts with manure production estimates to determine if losses are reasonable and acceptable. Using nutritional inputs in the prediction of manure nutrient outputs permits nutrient management planners to interact with producers to assess the environmental cost of overfeeding critical nutrients. Manure nutrients (e.g., N, P, and K) equal the amounts in feed consumed minus the amounts in products produced (e.g., milk, eggs, meat, or offspring) whereas, the amount of manure dry matter is an inverse function of the ration digestibility. The indigestible dry matter is the expected amount of fecal dry matter; additional dry matter in urine is small. The percentage compositions of nutrients in manure recovered (accounting for nutrient losses as well as uncollected portions) are much more difficult to predict than total amounts that should be collected because anaerobic digestion of carbon-containing compounds that was initiated in the large intestines of animals continues after excretion or the fermentation shifts to aerobic. Volume reduction occurs as carbon dioxide and methane are emitted and non-volatile nutrients such as P and K are concentrated in the remaining dry matter. From 40% to 75% of excreted N is in the urine as urea or uric acid (birds) and can be quickly volatilized as ammonia. Some losses of N to the atmosphere are unavoidable, at least 35% of excreted N in best case scenarios and 60%, or more, in most situations. Losses of non-volatiles such as P and K are small. Due to these changes, manure becomes increasingly P-rich relative to plant fertilization needs with N:P ratios usually below 3:1; whereas, ratios based on plant needs are much wider. Thus, acreages of crop production needed to recycle manure P are much greater than acreages needed for manure N. In the future, priority will be on reducing excretion of P and on retaining a higher percentage of excreted N. Dietary measures to impact P excretion will be increasingly important. To achieve environmentally acceptable nutrient balances, many animal production facilities will have to export manure or manure products or manipulate nutrient production to match nutrient needs. The role of diet will become increasingly important as producers establish whole-farm nutrient balance plans.