Effects of tillage, compaction and nitrogen inputs on crop production and nitrogen losses following simulated forage crop grazing

Abstract

Soil compaction from livestock treading during winter grazing of forage crops on cultivated soils may be harmful to subsequent crop production and nitrogen (N) loss. This study focussed on determining: (1) how compaction and high inputs of N from livestock urine (urea) during winter grazing affect subsequent crop production and N loss and (2) whether establishing winter-grazed forage crops with no-tillage (NT) can mitigate potential adverse effects on subsequent crop production and N loss. A field trial was conducted on a Templeton silt loam soil in Canterbury, New Zealand from April 2016 to February 2017, comprising of forage crop (oat), grazing, and subsequent crop (barley) phases. The forage crop phase represented the production of forage oat crops established using NT or intensive tillage (IT) in autumn. The grazing phase was simulated by removal of the forage crop, followed by simulated grazing that comprised five compaction levels (0–261 kPa) and two urea-N treatments (0 versus 400 kg N ha−1). A subsequent barley crop was sown following shallow (<15 cm) cultivation of soil in spring. Results indicated relatively little net effect of compaction on crop yield and total N losses over the combined grazing and barley phases. However, moderate compaction (180–220 kPa) increased the water use efficiency and grain yield of the subsequent barley crop. Simulated urinary-N (urea) inputs during grazing promoted vegetative growth of barley, but restricted its reproductive growth (i.e. grain yield) and increased N loss during the barley phase. The use of NT amplified the adverse effect of simulated urinary-N (urea) inputs on yield and N loss. This suggests that variations in urinary-N inputs should be taken into account when applying fertiliser for subsequent crops. Adoption of NT during forage crop establishment did little to mitigate the effects of compaction on subsequent crop production, but it enhanced crop growth and water use efficiency in the early stages of the subsequent barley crop without sacrificing the final grain yield. In addition, NT crop had a significantly lower screenings than IT at final harvest. Compared with IT, NT increased N loss during the grazing phase but reduced N loss during the barley phase, resulting in no significant net effect of tillage on total N losses over the two phases. However, the lower cost of NT crop establishment and greater marketable grain yield as well as the reduced risk of soil compaction and N2O emissions reported in previous studies provide agronomic and environmental incentives to adopt NT in forage cropping systems. Our study also highlighted important temporal differences in the effects of agricultural management factors (i.e. tillage, compaction, and urinary-N inputs) on crop production and N losses to the environment.