Soil Erosion Benefits of Pastures

The National Sedimentation Laboratory in Oxford, Mississippi is a prime USDA-ARS research station in the loess belt east of the Mississippi River. The station has a mandate to do research on soil erosion and conservation methods. The undulating loess soils are known for their high erodibility, while the precipitation in Mississippi is also highly erosive. Because many of our most intensively used agricultural soils also have silt loam surface horizons like those in loess soils, and because precipitation totals and intensity are expected to become very similar to those in Mississippi in the past, research from this laboratory is highly relevant to Pennsylvania. 

In the 1960s, a 5-yr erosion study was done on loess soils with slopes of 5% and 10%. Average annual precipitation was 47 inches during this period (in the past, PA precipitation varied from 34-50 inches, but it is inching upwards). The researchers investigated total annual runoff and erosion from 'USLE plots,' which measured 72.6 downslope x 30ft across (used to calibrate the United States Soil Loss Equation) and a three-acre watershed.

The research contrasted improved with poor pasture management (never grazed lower than 4 inches and fertilized versus overgrazed to 2 inches without fertilization) on 5% and 10% slopes. They also included a three-acre watershed under poor grazing management with 9% average slope. All plots started out with the same initial conditions in 1957 (Dallisgrass, a warm-season bunchgrass, and Kobe lespedeza, a legume). After 1957, the improved managed pasture stand became thicker, while the poorly managed pasture thinned out. Runoff and erosion from 1959 to 1963 under natural rainfall are shown in Figures 1 and 2.

Runoff varied from less than 5 to 30 inches per year. The highest runoff was collected in the wet year of 1961 (58 inches precipitation), while the lowest runoff was collected in the dry year of 1963 (37 inches precipitation). The greatest runoff was collected on the 10% slopes, while there was no difference in runoff from 5% or 10% slopes on the well-managed pastures. Runoff amounted to more than half of the annual precipitation on 10% slopes with poor pasture management (year 1961), but this was reduced to 32% with good pasture management, which meant 9 inches of additional precipitation was available to the pastures under good management.

Soil erosion was very low (less than 1 T/A/yr) irrespective of management on these pastures in the USLE plots (Figure 2). The researchers noted that on nearby cropland, erosion higher than 40 T/A/yr was measured. However, 40 T/A soil loss is totally unacceptable under today’s conservation standards. Tolerable soil loss on deep soils is 5 T/A/yr in Pennsylvania, but we believe that 1 T/A/yr should be achievable, even in annual crop production, if the soil is left undisturbed and covered with mulch. Interestingly, the 3-acre pasture under poor management showed increasing soil loss over the years (Figure 2). This was attributed to increasing rills and gullies in this larger field, which do not have a chance to form on short slopes such as in the USLE plots. 

This research is still relevant today. We learn that you can increase your effective rainfall with improved pasture management. Runoff can be substantial on pastures on steep slopes, but with good pasture management, more water will infiltrate and benefit pasture productivity. We also learned the benefits of pastures for soil conservation – even under poor management, pastures had relatively low soil erosion loss. Therefore, pastures should be a consideration in the design of our cropping systems. However, this research also shows the importance of avoiding rills and gully formation in pastures. The best way to accomplish this is by using Management Intensive Grazing practices, which include never overgrazing your pasture.

Reference:

Carter, C.E., Dendy, F.E., and Doty, C.W. 1966. Runoff and soil loss from pastured loess soils in north Mississippi. Reprint of a Paper presented at the Mississippi Water Resources Conference in Jackson, MS. USDA Sedimentation Laboratory, Southern Branch.