Utilization of nitrogen from injected and surface-spread cattle slurry applied to grassland

Abstract

The utilization of nitrogen (N) from cattle slurry following injection into grassland, was assessed in 4 field experiments at different sites for 3–5 years. In two experiments, surface application of slurry was included; all slurry applications were made in the spring. In each experiment three rates of slurry application were used, the maximum rates were 40tha^-1 yr^-1 and 80 or 90t ha^-1yr^-1 for the surface-spread and injected treatments, respectively. In all experiments 0, 200, 400 or 600 kg fertilizer N ha^-1 yr^-1 was applied, both in combination with the slurry applications or alone.

Averaged over all experiments the mean apparent recoveries of N (ANRs) from surface-spread slurry, injected slurry and the lowest rate of fertilizer N were 23%, 49% and 90%, respectively. The corresponding mean apparent efficiencies of N (ANEs) were 6.0, 11.5 and 27.4 kg dry matter (DM) per kg N applied. In the experiments enabling direct comparison with surface spreading, injection consistently resulted in higher ANR and ANE values; the average increases were 2.3 fold and 1.9 fold, respectively. Injection had a relatively greater effect on N uptake than on DM yield; causes and consequences of this are discussed.

The ANRs and ANEs for each method of slurry application differed between sites and years, but were unaffected by slurry application rate. A lag phase of several weeks was observed in N uptake and DM yield responses following the spring injection of slurry. The largest responses occurred in the second of 5–7 cuts; the lag phase was more apparent in DM yield than in N uptake.

Analysis of the responses to combined applications of slurry and mineral fertilizer N, revealed that effects of slurry and fertilizer N on both N uptake and DM yield were additive up to optimal levels of N supply. At higher rates of N supply, fertilizer N could not fully replace the effect of slurry on N uptake and DM yield, indicating a specific effect of slurry.

Herbage from two experiments was examined for nitrate content. Herbage from the plots receiving injected slurry at rates of 80–90 t ha^-1 without fertilizer N contained excessive nitrate in the second cut. For the other treatments, accumulation of nitrate in herbage occurred when total N uptake exceeded 400–475 kg N ha^-1yr^-1. These results suggest that herbage nitrate contents should be considered when determining (i) maximum application rates for injected slurry when used without fertilizer N and (ii) optimal fertilizer N rates to be used with injected slurry.

The high recovery of available slurry N indicated that injection of slurry in spring considerably reduced the losses of N associated with both surface spreading and applications made well before the start of the growing season.