Compaction trial at McMaster Chris Guppy- UNE Grain and Graze activities at McMaster Research Station, Warialda fall into several of the Themes identified as priorities for research and extension through the Border Rivers-Gwydir catchment. These activities are found all over the research station on soil and land use types that are typical of the region. One of these activities is designed to investigate the question of whether or not cattle grazing on black cracking clays have an adverse effect on subsequent crops. Two trials are proposed to investigate this question – the first of which is outlined below. The site of the compaction trial at McMaster Research Station is indicated below, as are the other Grain and Graze activities (Figure 1). Land use types at McMaster Research Station, 2006. Legume – grass mixed pasture trial Compaction by livestock on cropping country Farmlet paddocks GPS tracking of livestock foraging Temperate behaviour legumes and lucerne variety trials Figure 1. Grain and Graze trials and activities at McMaster Research Station, Warialda. First experiment A split plot trial will be undertaken, essentially as a demonstration experiment on a uniform section of land equating to approximately 1.5 ha. The Gray Vertosol is currently under grazing triticale, and the uniform site was selected following an EM survey (Figure 2). Compaction site July 2006 (mS/m) 37 - 58 59 - 70 71 - 81 82 - 91 92 - 101 102 - 112 113 - 124 125 - 137 138 - 149 150 - 170 N W E 0 100 200 300 Meters S Figure 2. EM survey of selected paddock with potential trial site indicated in white The aim is to measure a number of related parameters designed to assess the impact of cattle on soil physical properties including saturated hydraulic conductivity, penetrometer resistance and aggregate size distribution using dry sieving. Following the trial a field day is planned to demonstrate the effects of cattle on soil physical properties where preliminary results can be presented, as well as a white paint pit study of pore size distribution on the variably compacted sites. Methodology. On Monday 4th September a group of academics and students will peg out the trial sites using GPS and line up a sampling transect across each of the paddocks. The identified trial site will be electrically fenced during the morning. At least 6 sites in each paddock will have hydraulic conductivity assessed using a disc permeameter. Following the disc permeameter measurements (and depending on how far the moisture has managed to penetrate) a field penetrometer will be used to assess soil strength. Visual assessment at each of the GPS identified sites on the transect will determine soil horizonation and subsequently identifiable layers (through aggregation) will be sampled uniformly and dry sieved to determine the size distribution. A separate, (fenced if Milton is going to graze the rest of the triticale around it?), series of control sites (at least 6) will be sampled external to the two paddocks in the same manner. Soil moisture content from 0-20 cm will also be determined both in the sampled sites and in dry adjacent soil. The fenced areas will then be grazed with different numbers of cattle (12 in one 0.75 ha paddock and 3 in the other) for 21 days. If the GPS livestock tags are ready in time they will be used to assess grazing pattern and allow a ‘density’ measure of cattle trampling, particularly around the watering point. Following grazing, on approximately the 27th September, the same GPS identified sites will be resampled using the same parameters to determine the impact of cattle grazing on soil physical properties in a Gray Vertosol. The white paint method will also be used to examine pore size distribution following grazing, digital photography being used to assess changes in porosity. Should any soil physical effects be observed, the same sites will again be sampled approximately 2 months after harvest to determine if there is significant recovery following wetting/drying cycles. In June 2007, a winter cereal (barley) will be sown over the top of the trial site. Prior to sowing an EM survey will be conducted to determine if profile moisture storage was affected by grazing, and subsequently whether this moisture storage difference resulted in reduced yields. A control on ungrazed soil, to remove the effects of differential stubble input (and hence moisture storage efficiency) should involve slashing and carting the stubble from an ungrazed section of the paddock as a control comparison on yield. Alternately, an average of the remaining stubble following harvest of the triticale can be used as the ‘baseline’ to which stubble levels should be lowered. The growth RATE of the subsequently sown barley will be monitored 4-5 times during the season, prior to complete canopy closure, using CropCircles (with destructive harvests of small areas for calibration). Any reduction in crop vigour due to compaction/profile moisture storage can then be recorded. The null hypothesis in the first trial is that self-mulching Gray Vertosols will remain unaffected by grazing, and have no impact on the following cereal crop. Second trial Presuning there is not heavy rain during the 21 days that the cattle are grazing in their separated paddocks, it is proposed that a second trial be undertaken to examine the effects of different pre-existing soil moisture content on the effect of cattle grazing on soil physical properties. This study would commence in January/February 2007 and involve the use of a rainfall simulator to apply different amounts of rainfall to the Gray Vertosol prior to the introduction of cattle. Systematic, repeated EM survey of the trial site, coupled with gravimetric measurement of soil moisture content in the upper 20cm of the soil profile will be used to monitor the impact of cattle on infiltration, aggregate stability and soil strength. Cattle will be allowed to wander (hay bales etc) overall the sites for 3 days to compact the soil, and will be GPS tagged to ensure reasonably even trampling distribution. Before and after compaction, soil physical parameters as indicated in the First trial, will be measured. An EM survey will be conducted to look at profile moisture levels, and after 7 days and 28 days the rainfall simulator will apply a uniform amount of water across all sites. Runoff, erosion and profile moisture retention can then be assessed, through both physical measurement and EM survey. The details of this second trial can be determined closer to the middle of summer! It may be possibly, given timely storm events to assess following storms?
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