Dietary manipulation as a means of decreasing N losses and methane emissions and improving herbage N uptake following application of pig slurry to grassland

Abstract

Slurry was collected from two groups of finishing pigs fed either a standard commercial diet (containing 205 g/kg crude protein (CP)) or a specially formulated lower CP content diet (140 g/kg CP). The slurries were surface applied to grass/clover plots on a freely draining soil in SW England in mid-March 1995 at three application rates: 25, 50 and 70 m3/ha. Measurements were made from the 50 m3/ha plots of ammonia volatilization, denitrification, nitrous oxide and methane emissions and nitrate leaching. Measurements of herbage yield and apparent N recovery (ANR) were made from all plots. Decreasing the CP content of the pigs' diet reduced N excretion by the pigs and also changed other characteristics of the slurry. Slurry from pigs fed the lower CP diet (the slurry referred to hereafter as LS) had a higher dry matter (DM) content, lower pH, lower total ammoniacal N (TAN), total N and VFA content with a similar total C content compared with slurry from pigs fed the standard commercial diet (the slurry hereafter referred to as CS). From the 50 m3/ha treated plots, losses by ammonia volatilization represented 38 and 58% of the applied TAN and net losses through denitrification represented 5·3 and 12% of the applied TAN for LS and CS respectively. Nitrous oxide emission was similar from the two slurries, with net emissions of c. 0·5% of the applied TAN. Methane emission was significantly less from LS. No nitrate leaching was detected either in spring or in the following autumn. Yield and ANR increased with increasing slurry application rate up to 50 m3/ha. The best% N recovery was from the 50 m3/ha application rate with 58 and 47% of the applied TAN being recovered from LS- and CS-treated plots respectively. Changes in the slurry characteristics due to the lower CP diet resulted in lower losses to the environment and an improved utilization of the slurry N by the herbage.