Soil Hydrologic Response to Number of Pastures and Stock- ing Density under Intensive Rotation Grazing

InfiltraGon rde and sediment production were measured for 2 years on 3 pastures from an intensive rotational grazing system. The pastures were 32,24, and 16 ha in size. Stocking rate was held constant but stocking density at any given point in time varied due to pasture size. Stocking densities were 0.68,0.51, and 0.32 ha/AU, respectively. Within the respective treatments, midgrass interspa ces exhibited significantly higher infiltration rates and lower sediment production than shortgrass interspaces. Overall, the pasture grazed at the highest stocking density produced the lowest infiltmtion rates and the greatest sediment loss. However, there was no consistent trend in hydrologic responses over time and the differences appeared to be the result of random selection of a poorer condition site on 1 or 2 occasions rather than the result of stocking density. Regardless of whether the pasture grazed at the highest stocking density was in similar or poorer hydrologic condition in terms of treatment response, the data do not support the hypothesized beneficial hydrologic advantages of increased stocking density via manipulation of pasture size and numbers. Rest, rather than intensive livestock activity, appears to be the key to soil hydrologic stability. The potential for altering the length of the rest period is greatest where the number of pastures is small. Therefore, very little benefit in terms of soil hydrologic condition should be expected from large increases in the number of pastures within rotational grazing systems. Rotational grazing of some form or another has been practiced for centuries. Transhumant or nomadic grazing is the most ancient and widely used form of rotational grazing. However, the most frequently addressed form of rotational grazing in current grazing management literature is intensive short-duration grazing. The principles of intensive rotation grazing were taught as early as the 1700’s (Voisin 1959). Much of the revised interest in this system of management centers around proposed potential for significant increases in carrying capacity (Savory 1978, 1983). The principal objective of most rotational grazing systems is to improve or maintain the vigor and production of the forage resource and/or improve animal production. One method of enhancing forage production is to increase available soil water through improved rainfall infiltration rate and reduced runoff loss. Some modern proponents of intensive rotational grazing have hypothesized that intense trampling activity associated with high stocking density will enhance rainfall infiltration and reduce eroAuthors are presently rangeland watershed manager, USA-CERL, Environmental Division, P.O. Box 4005, Champaign, 111.61820; professor of watershed management, Department of Range Science, Texas A&M University, College Station 7784!; and superintendent, Texas Agricultural Experiment Station: Sonora 76950. At the time of research, the senior author was graduate research asnstant, Department of Range Science,, Texas A&M University. Pubhshed with approval of the Director, Texas Agricultural Experiment Station, as TA-20823. Manuscript accepted 27 January 1986. 500 sion (Goodloe 1969, Office of Technology and Assessment 1982, Savory and Parsons 1980, Walter 1984). Stocking density, which is a measure of grazing pressure at a given point in time, may be increased by adjusting stocking rates upward or by increasing the number of pastures within an existing rotation system. Both alternatives have been suggested (Goodloe 1969, Savory 1978, Savory and Parsons 1980). Heavy stocking rates under continuous grazing are almost universally deleterious to watershed condition, generally reducing rainfall infiltration rate and accelerating erosion (Alderfer and Robinson 1947, Blackburn 1984, Branson et al. 1981, Rauzi and Hanson 1966, Rhoades et al. 1964). Recent research has shown that the use of heavy stocking rates within rotational grazing systems is also detrimental to infiltration rate and sediment production, regardless of the system used (Gamougoun et al. 1984; M&alla et al. 1984a, 1984b; Pearson et al. 1975; Pluhar 1984; Smith 1980, Thurow 1985; Warren 1985; Weltz 1983). Some research has indicated that the incidence of heavily impacted trails may increase as the number of pastures is increased within an intensive rotational grazing system (Walker and Heitschmidt 1986). The effect that an increase in the number of pastures has on hydrologic parameters has not been addressed. The objective of this study was to evaluate infiltration rate and sediment production from pastures subjected to variable stocking density as affected by number of pastures rather than stocking rate.