Submitted to: Journal of Sustainable Agriculture
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: October 18, 2005
Publication Date: November 16, 2006
Citation: Sigua, G.C., Williams, M.J., Coleman, S.W. 2006. Long-term effects of grazing and haying on soil nutrient dynamics in forage-based beef cattle operations. Journal of Sustainable Agriculture. 29(3):115-134. 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 12%, 37%, 50% and 30%, but this decline varied with pasture management. Phosphorous levels in the grazed only bahiagrass pastures were essentially unchanged after 15 years without P fertilization. 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 added that can be used for productive forage growth, nutrients will build up in the soil, creating high risk for runoff and water contamination.
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 phosphorus (P) and other crop nutrients (OCN) in subtropical pastures planted with either bahiagrass (BG, Paspalum notatum Flügge) or rhizoma peanut (Arachis glabrata Benth) - mixed grass association (RP-G). The levels of soil P and OCN were significantly affected by pasture management (P' 0.001) and soil depth (P ' 0.001). A declining trend was observed for the levels of P and OCN from 1988 to 2002. The average decline of soil P from 1988 to 2002 was 0.7 mg/kg/yr (-12%), while K and soil pH declined at the rate of 1.1 mg/kg/yr (-37%) and 0.1 pH unit/yr (-21%), respectively. Calcium and Mg levels also had decreasing trends. Calcium was lost more rapidly at a rate of 11.5 mg/kg/yr whereas Mg declined about 0.8 mg/kg/ yr, leading to a significant increase in the Ca:Mg ratio (2.4 % per yr). Overall, there was no spatial and temporal build up of soil P and OCN in RP-G despite of the annual application of P-containing fertilizers and daily in-field loading of animal waste. Results of this study have renewed the focus on improving the fertilizer efficiency in subtropical beef cattle ecosystem, and to maintain a balance of nutrients removed with nutrients added to ensure healthy forage growth and control nutrient runoff.