RUMINANT NUTRITION FROM AN ENVIRONMENTAL PERSPECTIVE - FACTORS AFFECTING WHOLE-FARM NUTRIENT BALANCE

Nutrient budgeting strategies focus primarily on recycling manure to l and as fertilizer for crop production. Critical elements for determini ng environmental balance and accountability require knowledge of nutri ents excreted, potential nutrient removal by plants, acceptable losses of nutrients within the manure management and crop production systems , and alternatives that permit export of nutrients off-farm, if necess ary. Nutrient excretions are closely related to nutrient intake and ca n be predicted by subtracting predicted nutrients in food animal produ cts exported from the farm from total nutrients consumed. Intensifying crop production with double- or triple-cropping often is necessary fo r high-density food animal production units to use manure without bein g forced to export manure or fertilizer coproducts to other farms. Mos t manures are P-rich relative to N largely because of 1) relatively la rge losses of volatilized NH3, most of it converted from urea in urine , 2) denitrification losses in soil under wet, anaerobic conditions, a nd 3) ability of many crops to luxury-consume much more N than P. Most soils bind P effectively and P usually is permitted to accumulate, al lowing for budgets to be based on N. However, P budgeting may be requi red in regions where surface runoff of P contributes to algae growth a nd eutrophication of surface waters or where soil P increases to level s of concern. Research is needed to determine whether dietary P allowa nces can be lowered without detriment to animal production or health i n order to lower P intake and improve N:P ratios in manure relative to fertilization needs.