To determine the minimum biological capacity for nitrogen (N) excretion for pigs, dietary regimens were created and fed that were designed to eliminate fecal excretion of undigested feed N, minimize endogenous N secretions and minimize intakes of nitrogenous compounds (amino acids) above those of the pigs biological needs. Excretion of undigested fecal N was eliminated by feeding ingredients containing highly digestible (≈100%) sources of nitrogen and by eliminating compounds that bind N. Endogenous secretions were minimized by eliminating antinutritional factors from the diet and by minimizing enteric bacterial populations. Intakes of amino acids above the animal's needs were minimized by providing a pattern of amino acids that closely matched that needed by the pig. Furthermore, dietary amino acids were provided to the pigs in amounts above, at, and below their biological needs based on the amounts of urea N excreted in the urine and the amounts of N accrued in the body. From two-slope breakpoint analysis, apparent digestible N intake resulting in maximum N retention (2.58g⋅kg^-1BW^.75⋅d^-1) was determined to be 3.66g⋅kg^-1BW^.75⋅d^-1. When daily apparent digestible nitrogen intakes were below the pigs' determined need, the amount of digestible N (mg⋅kg^-1BW^.75⋅d^-1) lost for body maintenance processes was estimated as .239 in urine and .080 in feces. The amount lost in urine as unusable for body N accretion was .206 g⋅kg^-1BW^.75⋅d^-1 of each gram of digestible N consumed above body maintenance needs. When daily apparent digestible N intakes were above the pigs determined need, .900 g of each gram of additional N consumed above that needed for body N accretion was excreted. The minimum biological capacity for N excretion in pigs, defined as total (urinary plus fecal) obligatory losses of N, is estimated to be .287g⋅kg-1BW^.75⋅d^-1 to support pigs at N maintenance and 1.11 g⋅kg^-1BW^.75⋅d^-1 for pigs at maximum body N accretion.

Using these estimates, the minimal biological capacity for N excretion can be calculated for pigs at various stages of growth. For example, in 60 kg pigs, minimum capacity for N excretion is estimated at .10 and .39 g/kg body weight/day in pigs in states of N maintenance and maximum body N accretion, respectively. These values are 12 and 44%, respectively, of N excretion relative to published standards (ASAE D384.1). Similarly, the N content of excreta from animals excreting N at their biological minimum was determined to be 2.57 g N/liter, or 18% of standard excreta N content (corrected for volatilization, ASAE D384.1). Based on these data, the minimum biological capacity for the excretion of N in pigs is substantially less than current estimates of N excretion and can be achieved by eliminating dietary factors that contribute to N excretion. Furthermore, dietary regimens that allow the minimum biological capacity for N excretion to be achieved are biologically capable of supporting maximum rates of body accretion (i.e., lean tissue growth) in pigs.