Agriculture Reference
In-Depth Information
0.12 ± 0.12 Mg m
−3
. Bartley et al. (2010) identified a clear indication of the ten-
dency for grazing practices to reduce saturated hydraulic conductivity by up to 90%.
Most notable was a relationship between bulk density and infiltration. The low bulk
densities of light grazing (10 years) and no grazing (16 years) were associated with
infiltration rates up to 1000 mm h
−1
, about 10 times greater than in denser soils asso-
ciated with more intense grazing. Adams (2009) found that removal of vegetation
in the Ecuadorian Andes, whether by tillage, grazing, or burning, reduced SOC and
increased the tendency to crust, which would result in increased runoff and erosion
risk. The authors called for leadership by nongovernmental organizations, research
institutions, and the government in supporting intensification projects such as rota-
tional grazing or silviculture to reduce pressure on the forested landscape.
Livestock can exert a significant mechanical load on the soil surface, especially
considering the small footprint of large animals, such as mature dairy cows. An adult
Friesian cow was determined to exert a pressure of 220 kPa on the soil (Scholefield
and Hall 1986). However, the pressure can vary significantly depending on type
and age of animal, land slope, and extent of movement. The range of hoof pressures
reported in the literature has been 130-350 kPa for cattle (Willatt and Pullar 1983;
Scholefield and Hall 1986; Nie et al. 1997), 331 kPa for horses (Cohron 1971), 83-124
kPa for sheep (Cohron 1971; Willatt and Pullar 1983), and 60 kPa for goats (Willatt
and Pullar 1983). This compares to a contemporary tractor tire exerting a pressure of
100-200 kPa (Schjønning et al. 2006).
Trampled soil from cattle traffic around drinking stations in Finland had greater
bulk density than in grazed pasture with no visible trampling; however, the effect
was depth and soil texture dependent (Table 4.4). In Texas, bulk density at a depth of
0-5 cm was greater with cattle grazing (240 kg head
−1
) than without, and the effect
increased with increasing stocking rate (Warren et al. 1986). Soil bulk density aver-
aged 0.91 Mg m
−3
without grazing, 1.00 Mg m
−3
with 0.12 animal units ha
−1
, and 1.04
Mg m
−3
with 0.24 animal units ha
−1
. Increasing sheep stocking rate from 0 to 22 head
ha
−1
in Victoria Australia resulted in an increase in soil bulk density from 0.89 to
1.05 Mg m
−3
(Willatt and Pullar 1983). These studies demonstrated increases in bulk
density with animal treading (but remaining <1.3 Mg m
−3
), which may have caused
disruption of aggregation and surface sealing, but should not have been debilitating
to root growth and/or air and water storage and transport.
From a survey of farms managed with continuous cropping and pasture-crop
rotations (8 years cropping + 4 years grazed pasture) in Argentina, SOC did not dif-
fer. However, relative compaction was lower in pasture-crop rotation than continuous
cropping in loamy/sandy loam soils (Fernandez et al. 2011). Penetration resistance of
the topsoil (0-7.5-cm depth) was greater under pasture-crop rotation, although not
necessarily due to surface compaction since bulk density was unaffected. This study
showed that the physical condition of soil was minimally affected by conversion of
continuous no-till crop management to pasture-crop rotation under no till.
Drewry et al. (2008) reviewed impacts of animal treading on soil physical proper-
ties and pasture productivity, and found a shortage of data to develop yield response
curves to support decision making on pastures. In plot studies, Miguel et al. (2009)
reported a 73% decrease in infiltration rate after 15 passes of cattle trampling. du
Toit et al. (2009) found infiltration rate to be inversely related to stocking rate, with
