Submitted to: Soil Science Society of America Journal
Publication Type: Abstract only
Publication Acceptance Date: 10/1/2003
Publication Date: 11/4/2003
Citation: Sigua, G.C., Williams, M.J., Coleman, S.W. 2003. Impact of grazing and haying on soil nutrients in the subtropical beef cattle pastures. 2003 Soil Science Society of America Journal. Denver, CO. Interpretive Summary: Nutrient dynamics in various agro-animal-ecosystems are continually evolving in response to changing management practices. Efficient utilization of pastures through intensive grazing during the early season may cause a buildup of mineralized soil nutrients during the late summer and fall when plant growth and nutrient uptake is slow. Grazing animals have a dominant effect on the movement and utilization of nutrients through the soil and plant system, and thus on the fertility of pasture soils. Grazing has been documented to modify both the magnitude and distribution of soil organic carbon, nitrogen, and phosphorus. Environmentally, soil P levels in STARS are declining. During the past 15 years, average soil test values for P, K, Ca and Mg have declined by about 32%, 51%, 61% and 58%, but this decline varied with pasture management. Phosphorous levels in the grazed only bahiagrass pastures were essentially unchanged after 15 years without P fertilization. The combination of grazing in spring and haying in early fall can be considered a good pasture management (i.e., BMP) in maintaining nutrient balance in the pasture fields, thereby avoiding the potential negative impact to the environment. Maintaining a balance between the amount of nutrients added to the soil as manure and fertilizer and the amount of nutrients removed as forages, hay, or livestock is critical for productive crop growth and water/environmental quality protection. If more nutrients are removed through hay production, additional nutrients must be added for productive forage growth, without increasing stored nutrients.
Technical Abstract: Soil fertility levels over a 15-year period (1988-2002) were compared to evaluate the long-term effects of differing fertility (P or no P) and management treatments (grazed only or grazed and hayed) on soil P and other crop nutrients (OCN) in subtropical pastures planted with either bahiagrass (BG) or rhizoma peanut-mixed grass association (RP-G). The levels of soil P and OCN were significantly affected by pasture management and soil depth. Levels of P declined from 1988 to 2002 by 1.7 mg/kg/yr (-30.4%), while K and soil pH declined at the rate of 1.6 mg kg per yr (-54.1%) and 0.1 pH unit/yr (-24.4%), respectively. Calcium levels declined while mg declined about 1.6 mg per kg per yr, leading to a significant increase in the Ca:Mg ratio of 2.4 % per yr. Overall, there was no spatial and temporal build up of soil P and OCN despite the annual application of P-containing fertilizers in RP-G and daily in-field loading of animal waste.