Effects of Livestock Grazing on Infiltration Rates, Edwards Plateau of Texas

The influence of short duration grazing (SDG), moderate continuous grazing (MCG), heavy continuous grazing (HCG), and gmzingexcusionon iralonntesof -ig--and s asdominated communities was evaluated over a 20-month period on the Texas Agricultural Research Station, located near Sonora in the Edwards Plateau, Texas. A combination of cattle, sheep, and goats were used in each grazing treatment. Infiltration rates were consistently less in the midgrass (bunchgrass) than in the shortgrass (sodgrass) community. The HCG pasture was severely overgrazed and infiltration rates were reduced to about one-half those in the MCG pasture. The midgrasses in this pasture were destroyed after 26 months of overgrazing. Infiltration rates in the SDG pasture, stocked at double the recommended rate, decreased during the study period. Infiltration rates in the SDG pasture shortgrass community, near the end of the study, approached those in the HCG pasture. The greatest infiltration rates for both communities were maintained in the MCG pasture. Infiltration rates for the midgrass community remained relatively stable during the study when the general trend in the SDG and HCG pastures was toward reduced infiltration rates. The nongrazed pasture subsequent to the 1980 drought had a general increase in infiltration rates. Infiltration rates vary naturally in time and space because of normal variations in climate, vegetation, and intensity and duration of livestock use. Few studies have attempted to account for these natural variations. The literature is filled with examples of the adverse impact of heavy or abusive grazing on infiltration rates. Few research projects have studied seasonal or long-term impacts of grazing systems or proper grazing management on infiltration rates (Blackburn et al. 1982). Much interest has been generated by specialized grazing systems and their potentials. Short duration grazing and its potential economic benefits to the ranching industry have become extremely popular (Savory 1978). Little information is available, however, to support many of the claims concerning specialized grazing systems. Gifford and Hawkins (1978) found no published evidence to show that any single grazing system consistently or significantly increased plant and litter cover on watersheds. Other reviews (Van Pollen and Lacey 1979, Beck 1980) of the impacts of grazing support Gifford and Hawkins' conclusions. Most of the information on the impact of specialized grazing systems on infiltration rates come from studies conducted in the Rolling Plains and Edwards Plateau of Texas. The results of these studies indicate that infiltration rates of pastures grazed under a deferred-rotation system (4-3:12:4 mo)1 were similar to those of livestock exclosures and greater than high intensity, low frequency pastures (8-1:17:119 days) or moderate or heavy continuously grazed pastures (McGinty Authors are graduate research assistant and professor of watershed management, Range Science Department, Texas AM and professor of Range Science, Texas Agricultural Experiment Station, Sonora 76950. Published with approval of the Director, Texas Agricultural Experiment Station, as TA-I 8727. Manuscript received May 31, 1983. 'Nomenclature follows Kothmann 1974. et al. 1978, Wood and Blackburn 1981). Infiltration rates of pastures grazed under a high intensity, low frequency system were similar to moderate continuously grazed pastures (Wood and Blackburn 1981, Blackburn et al. 1982). Livestock grazing can alter infiltration rates of rangeland soils by removing protective plant cover and by trampling. Vegetation and mulch cover serve to protect the soil surface from raindrop impact and influence soil surface properties such as bulk density, organic matter content, and aggregation (Osborn 1954, Copeland 1963, Blackburn 1975, Meeuwig and Packer 1976, Blackburn et al. 1982). Reduced infiltration rates as a result of livestock grazing have been attributed to: ( loss of vegetation cover, (2) decreased mulch cover, (3) decreased amounts of vegetation standing crop and mulch, (4) increased bare ground, and (5) increased bulk density as a result of trampling (Alderfer and Robinson 1947, Knoll and Hopkins 1959, Reed and Peterson 1961, Branson et al. 1962, Copeland 1963, Dee et al. 1966, Rauzi and Hanson 1966, Smith 1967, Blackburn et al. 1982). Type of vegetation is important in determining infiltration rates (Blackburn 1975). Infiltration rates are consistently higher in bunchgrass-dominated areas than in sodgrass-dominated areas (Blackburn et al. 1980, Wood and Blackburn 1981). Livestock grazing has the greatest potential impact on bunchgrasses. Not only are bunchgrasses usually the better forage species but they are usually more sensitive to heavy grazing pressure than low-growing sodgrasses (Wood and Blackburn 1981). Objectives of this study were to determine: (1) infiltration rates of pasture soils under short duration grazing; moderate continuous grazing; and heavy continuous grazing; and grazing exclusion; and (2) the variables influencing infiltration.