Comparison of the Equations for Predicting nitrogen Excretion in Fattening Steers

Abstract

Because nitrogen excretion in fattening steers has been identified as a source of nitrogen pollution, ways to reduce it must be examined. The objectives of this study were to develop and evaluate mathematical equations for predicting nitrogen excretion into feces and urine in fattening steers and to investigate the possibility to reduce nitrogen excretion by simulation with the equations. Two new equations, N_f+u=N_s-CP_g/6.25 and N_f+u=(N_s-N_r)+b+N_f+u(b), and a published equation, N_f+u=16.74×DM+8.54×CP+0.108×W-154.3, were compared; where N_f+u is the amount of nitrogen excretion (kg/day), N_s is nitrogen intake (kg/day), N_f+u(b) is basal endogenous nitrogcn (g/day), N_r is nitrogen requirement (g/day), CP_g is nitrogen requirement for growth (g/day), DM, CP and W are dry matter intake (kg/day), crude protein content (% of DM) and body weight(kg), respectively. The CP_g, N_r and N_f+u(b) were estimated based on equations used in Japanese Feeding Standards and the parameters, a and b, were estimated from solution of the equations. The new equations were developed and evaluated using the results of nitrogen balance trials of 23 Japanese Black and 45 Holstein steers. Independent changes in daily gain (DG), crude protein (CP) and metabolizability (q) were used to simulate the effects an two environmental landing indexes expressed by total nitrogen excretion/total nitrogen intake (I ₁)and total nitrogen excretion/total nitrogen retention (I₂) during the fattening period. The simulation results showed that increased DG led to decreases in I₁ and I₂, and decreased CP and increased q led to decreases in I₂. The effects of CP and q on I₁ varied in equations used in this study.