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 util ization reasonable estimates of manure production and composition must be a vailable. Amounts of manure nutrients (e.g., N, P, and K) originally excret ed are predicted more accurately with a nutritionally based input-output mo del than are the amounts recovered because the amounts that are recovered v ary depending on climate, storage and handling practices, and other site-sp ecific influences. Records of amounts of manure collected and composition d etermined from manure sampling are essential to determine the total of manu re nutrients that must be managed in the plan. It is important to compare r ecovered amounts with manure production estimates to determine if losses ar e reasonable and acceptable. Using nutritional inputs in the prediction of manure nutrient outputs permits nutrient management planners to interact wi th producers to assess the environmental cost of overfeeding critical nutri ents. Manure nutrients (e.g., N, P, and K) equal the amounts in feed consum ed minus the amounts in products produced (e.g., milk, eggs, meat, or offsp ring) whereas, the amount of manure dry matter is an inverse function of th e ration digestibility. The indigestible dry matter is the expected amount of fecal dry matter; additional dry matter in urine is small. The percentag e compositions of nutrients in manure recovered (accounting for nutrient lo sses as well as uncollected portions) are much more difficult to predict th an total amounts that should be collected because anaerobic digestion of ca rbon-containing compounds that was initiated in the large intestines of ani mals continues after excretion or the fermentation shifts to aerobic. Volum e reduction occurs as carbon dioxide and methane are emitted and non-volati le nutrients such as P and K are concentrated in the remaining dry matter f rom 40% to 75% of excreted N is in the urine as urea or uric acid (birds) a nd can be quickly volatilized as ammonia. Some losses of N to the atmospher e 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 t o plant fertilization needs with N:P ratios usually below 3:1; whereas, rat ios 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 retai ning a higher percentage of excreted N. Dietary measures to impact P excret ion will be increasingly important. To achieve environmentally acceptable n utrient balances, many animal production facilities will have to export man ure or manure products or manipulate nutrient production to match nutrient needs. The role of diet will become increasingly important as producers est ablish whole-farm nutrient balance plans.