Effect of simulated treading and compaction on soil physical properties and ryegrass pasture yield

Abstract

Soil compaction by animal treading can have detrimental impacts on soil physical properties and plant growth. Studies investigating the effects of animal treading on pasture production and soil physical properties have often studied single intensive treading or pugging events, often in wet winter conditions. It is therefore difficult to determine the extent to which the pasture response was due to direct pasture damage from hoof action and plant burial from pugging, versus indirect effects of changes in soil physical conditions. The use of simulated treading however, is helpful to overcome these problems.

This project was therefore established to investigate the effects of simulated dairy cow treading on ryegrass pasture yield and soil physical properties. The study also aimed to establish response relationships between ryegrass yield and selected soil physical properties, particularly macroporosity, and determine critical and optimum values of soil physical properties for ryegrass yield. Macroporosity was defined in this study as the volumetric percentage of pores greater than 30 µm diameter.

The effects of simulated dairy cow treading on soil physical properties and Yatsyn perennial ryegrass pasture yield were studied over 16 months, on a Wakanui silt loam. This field study minimised soil pugging and plant damage, common with animal treading, in order to determine the effects of soil compaction alone. Treatments were rates of treading, namely, a control, 50%, 100% and 200% treaded. Pasture yield was decreased by up to 14% for individual harvests for the heavily treaded treatments. Yield in summer was decreased by 1.3%, 5% and 7.6% for the 50%, 100%, and 200% treaded treatments, respectively, compared with the control. The treading treatments decreased herbage yield for the total of all harvests by 1.5%, 7.1 % and 8.8% for the 50%, 100% and 200% treaded treatments, respectively, compared with the control.

Treading reduced macroporosity at 0-5 cm, in summer, from 20.5% (v/v) in the control, to 10.7% in the 200% treaded treatment. In winter, macroporosity at 10-15 cm was reduced from 8.5% in the control to 6.1% in the heavily treaded treatments. Similar trends for bulk density, penetration resistance, air permeability and saturated hydraulic conductivity were found to occur and are presented in this thesis.

A quadratic relationship between macroporosity at 0-5 cm and ryegrass yield indicated that at 5% and 10% macroporosity, yield was 75% and 88% of maximum, respectively. Optimum macroporosity was found to be 16-17%, while a critical macroporosity was found to be 9-11%. Penetration resistance associated with 9% macroporosity was 1.4 MPa, while an optimum was 0.89 MPa. Macroporosity and penetration resistance were found to be the most useful indicators describing relationships with pasture yield.

A small glasshouse study also aimed to establish response relationships between ryegrass yield and soil physical properties, under a range of soil bulk densities. Relationships between total relative yield and bulk density, macroporosity and saturated hydraulic conductivity were weak to moderate. Under the glasshouse conditions the optimum bulk density was found to be approximately 1.2 Mg m⁻³, and the optimum macroporosity was 12-14%.