Biomass-Topography-Soil Strength Relationships in a Winter Grazed Hayfield.

Soil penetration resistance and herbage biomass were collected for three years in a five-year winter grazing experiment conducted at the West Virginia University Reedsville Experimental Animal Farm (Preston Co, WV).  The six ha experimental area was divided into six paddocks, hay was cut twice per year in the months of May and June, and forage stockpiled after the second cutting.  Nitrogen fertilizer was applied before and after each cutting at a rate of 84 kg N/ha.  Animals were admitted to paddocks at the end of November, at a stocking rate of 4 steers/ha.  On half of the total area, the animals were removed in January and in April from the remaining area.  Topography of the study area is relatively uniform with broad level ridges, and side-slopes ranging between 3 and 25%.  In our study area, two landscape positions were observed, but for our analysis the area was divided into three elevation zones, the lower elevation corresponded to the lower backslope, the medium elevation corresponded to the middle backslope, and the higher elevation to the summit.   Soil properties (bulk density, penetration resistance, moisture content, rock fragments, texture, and organic matter) were measured for each elevation zone.  A total of 300 soil penetration resistance locations, taken at 2 cm intervals to a depth of 50 cm, were measured at soil’s field capacity (two days after a soaking rain), as close as possible to the schedule biomass measurements.  Bulk density tended to be significantly higher at higher elevations, and rock fragments tended to be less; soil organic matter was not different among elevation zones.  Soil penetration resistance rarely exceeded 2.5 MPa. Because of this it was not considered limiting to herbage biomass production, explaining the weak negative relationship between biomass production and penetration resistance (r<0.10). Measured soil penetration resistance changes mostly at the soil surface (0-16 cm), and decreases as elevation increases.  Increased shallow (0-4 cm) penetration resistance was observed between November and March/April, no statistically significant differences were observed at deeper depths.  Interestingly, soil moisture during dry and wet periods was, on average, greater at higher elevations.  This might explain the presence of greater herbage biomass at higher elevation.  During the study period, herbage biomass production at lower landscape positions was more susceptible to extreme weather conditions (excess rain or drought) than higher and middle elevation areas.   High infield (paddock) penetration resistance, soil properties, and herbage biomass production variability resulted in site-specific soil-herbage biomass relationships.  A strong relationship between moisture content and soil penetration resistance was observed only for lower elevation areas; soil penetration resistance between 0 and 2 cm depth was slightly related to biomass production at high and medium, but not low, elevation.  Temporal stability in herbage biomass production was found to be related to elevation. Under our study conditions soil penetration resistance was not a limiting factor to herbage production, although changes in this property due to winter grazing were observed.