Effects of severe cattle treading on soil physical properties and pasture productivity

Severe cattle treading can result in compaction and pugging, of which the latter was the focus of this thesis. Pugging is one of the severest forms of treading damage to soil, and occurs when soil is grazed while near saturation, resulting in plastic and liquid soil deformation. The research objectives were to quantify effects from one-off treading events, at different severities, on soil physical properties and sward characteristics, to monitor recovery from the one-off treading event, and to develop methods of estimating potential effects of treading on pasture productivity. Three field experiments were carried out on a Te Kowhai silt loam soil (NZ soil classification, Typic Orthic Gley; USDA soil taxonomic classification, Typic Endoaqualj) supporting a mixed sward of ryegrass (Lolium perenne L.) and white clover (Trifolium repens L.). A oneoff treading event of either O ( control), 3, 9, or 24 hours was carried out using lactating Holstein-Friesian cattle (300 cows ha- 1) at three soil moisture contents (65%, 71 %, and 81 % gravimetric soil moisture content). Soil physical properties, soil surface features, and sward characteristics were monitored for up to 34 weeks after the treading event. Longer treading durations at wetter soil conditions had increasingly detrimental effects on soil macroporosity, saturated hydraulic conductivity (Ksai), unsaturated hydraulic conductivity (K_40), surface roughness, depths of pug prints, bare ground area, ryegrass tiller density, and herbage accumulation. Soil dry bulk density and total porosity did not change, even under the severest of treading damage, because decreases in macroporosity were offset by increases in microporosity (r2 = 0.63; P <0.001). The area of bare ground increased to up to 87% of the soil surface and correlated with decreased in herbage accumulation (r2 = 0.73; P <0.001). Sward botanical composition did not change after treading damage, except when large patches of bare ground persisted into spring, resulting in the establishment of broad-leaved plantain (Plantago major L.). Soil and sward recovered (i.e. no significant difference compared to simultaneous controls) following treading damage and when the pasture continued to be rotationally grazed. The recovery of macroporosity, Ksat, and K_40 indicated that the soil had an increased susceptibility to further treading damage for up to 13 weeks after the initial treading event. However, soil susceptibility to direct hoof damage after a one-off severe treading damage event may persist for up to 25 weeks, as sward recovery (decreases in area of bare ground) took longer than the recovery of soil physical properties. When the gravimetric soil moisture content was >71 %, a three hour grazing caused a total decline of 1,100 kg OM ha- 1 in pasture productivity, however, at lower soil moisture contents grazing of ::,;3 hours was unlikely to cause significant pasture productivity decline. When ~50% of soil penetrations using the AgResearch Penetrometer are ~2 cm, a three hour grazing with 300 cows ha -I is likely to result in pasture productivity decline. Statistical models (multivariate regression models) were used to describe the relationship of soil physical and sward properties with pasture productivity after treading. Assuming that during the grazing rotation no further severe treading damage takes place before recovery is complete, the models can be used to estimate potential pasture productivity decline from a one-off severe treading event on a pugging susceptible Te Kowhai soil using variables measured immediately before or after cattle treading. The models with the best potential for practical applications were: DPP*= -1770.98 + l 96.35HR - 4.47HR2 + 32.1 lAP r2 = 0.72, P <0.001 DPP = -1057.55 + 43.17BG r2 = 0.61, P <0.001 DPP = -863.78 + 179.lOSR r2 = 0.78, P <0.001 DPP = -701.16- 155.89HR + 5.49HR2 + 26.61PD + 122.68SR r2 = 0.84, P <0.001 RT* = -6.94 + l .35HR - 0.032HR2 + 0.16AP r2 = 0.78, P <0.001 RT = -6.10 + 0.36HR + 0.1 OAP+ 0.58SR r2 = 0.80, P <0.001 Where: DPP= total decline in pasture productivity (kg OM ha- 1) RT= time (weeks) required until no significant difference in herbage accumulation compared to controls HR = proposed or actual hours of treading AP = AgResearch Penetrometer results (% of readings penetrating soil by ~2 cm) immediately before treading BG= proportion of bare ground ('Yc,) after treading SR= soil surface roughness index(%) after treading PD= mean depth of pug prints (mm) after treading * = models using only pre-treading variables The declines in spring pasture productivity correlated with declines in macroporosity (r2 = 0.70, P <0.001) and Ksat (r2 = 0.61, P