Soil quality and plant yield under dryland and irrigated winter forage crops grazed by sheep or cattle

In New Zealand, the winter grazing of standing forage crops combines high animal stocking densities with soil water and climatic conditions conducive to soil compaction and pugging deformation. The extent of soil damage under winter forage cropping practices and impact of management factors such as stock type and irrigation on soil quality is relatively unknown. A research trial was established, on a Pallic soil type (Aeric Fragiaquept) in the North Otago Rolling Downlands of New Zealand, to compare cattle v. sheep and dryland v. irrigation management. Kale, Swedes, and triticale were direct-drilled in 3 consecutive years and soil physical (macroporosity, bulk density, structural condition score), chemical (total C, total N, C:N ratio), and biological (mineralisable N, mineralisable C, and earth worm mass and numbers) properties were assessed annually post grazing in midwinter. Increased soil compaction was evident following grazing of winter forage crops, with lower macroporosity (P < 0.01) measured at 0-50 mm under cattle grazing compared with sheep grazing for 2 of 3 years and greater bulk density (P < 0.05) measured under cattle grazing for all years. However, there was no affect of stock type on crop yield for all 3 forage crops as a result of the measured differences in soil compaction. There were few differences between treatments or through time in soil chemical or biological properties following 3 years of continuous winter forage cropping as pools of C and N are slow to change under a no-tillage cropping regime and not necessarily measurable over a relatively short time frame.