Quantitative effects of feed protein reduction and methionine on nitrogen use by cows and nitrogen emission from slurry

The effects on N use and N volatilization from slurry were investigated in 24 early-lactation Brown Swiss cows (32 kg/d milk) fed four diets with 128, 124, 147 and 175 g/kg DM of crude protein (CP). All diets were supplemented with 0.75 g/kg of rumen-protected Met except for one of the low-protein rations (128 g/kg of CP). The unsupplemented low-protein ration was calculated to be deficient in Met by approximately 20%. No significant treatment effects on performance, water intake and excretion, and slurry quantities were observed. Differences in N intake were closely reflected in the daily excretions of total and urea N via urine, and in urine N as a proportion of total excretory N. These values were higher for the unsupplemented low-protein ration than for the Met-supplemented low-protein ration. The treatment effects on fecal N excretion were generally smaller, and milk N excretion and N balance were not affected. Feed N utilization for milk N excretion increased with decreasing CP content from 27% for the high-protein group to about 35% for the two low-protein groups. Comparing the Met supplemented rations only, ammonia N emission from fresh slurry (excreta:water = 1:0.5) decreased from 231 to 160 and 55 mug/s per square meter of surface with 175, 147 and 124 g/kg of CP, respectively, and the corresponding total N losses during 7 wk of slurry storage declined from 89 to 57 and 25 g/d per cow. Regression analysis demonstrated the basic suitability of milk urea N excretion to estimate urine N excretion and, consequently, potential N emissions.