Management effects on herbage responses, size of nutrient pools, and litter dynamics in grazed 'Tifton 85' bermudagrass (Cynodon spp.) pastures

Appropriate management is important to ensure efficient nutrient cycling and pasture sustainability. Two experiments were conducted to determine the effect of management strategies of Tifton 85 bermudagrass (Cynodon spp.) pastures on herbage characteristics, size of important nutrient pools, and litter dynamics. In Experiment 1, herbage accumulation, nutritive value, and persistence responses were measured from nine treatments that included all combinations of three post-graze stubble heights (SH; 8, 16, and 24 cm) and three lengths of grazing cycle (GC, <10h grazing time plus rest period; 14, 21, and 28 d). Pastures were fertilized with 250 kg N ha-1 yr-1. In Experiment 2, effects of N fertilization (50, 150, and 250 kg N ha-1) and SH (8, 16, and 24 cm) on size of nutrient pools and plant litter dynamics were measured on rotationally stocked Tifton 85 bermudagrass pastures. Experiment 1 showed that Tifton 85 bermudagrass is tolerant of a wide range of grazing management, making it a good candidate for grazed grasslands in warm climates. Greatest herbage accumulation was associated with close grazing (8-cm SH) followed by a relatively long rest period (28 d) or with lax grazing (24-cm SH) followed by a short GC (14 d). Persistence data showed that a 24-cm SH should not be used because of greater incidence of stand loss. If the 24-cm SH is ruled out, then grazing to 8-cm SH every 28 d is recommended to maximize herbage accumulation, or to 16-cm SH every 14 to 21 d for greater nutritive value without negative impact on persistence or a major reduction in herbage accumulation. Nutrient pools in live herbage, plant litter, root-rhizome, and soil pools increased with taller SH, while increasing N fertilization increased N accumulation in live herbage, plant litter, and root-rhizome pools, but did not affect N content in soil. Carbon content in live herbage and soil pools increased with increasing N fertilizer rate. These data suggest that lower grazing intensity (i.e., taller stubble heights) and greater N fertilization increase C sequestration. Thus, grassland management has potential to increase C sequestration in soils, but the changes are likely to be relatively small. Considering litter dynamics, increasing SH increased existing litter mass and litter mass deposition rate. Nitrogen fertilization had little effect on litter mass, but it had an important effect on litter quality and decomposition. In grasslands in which the proportion of herbage mass consumed is low, grass litter plays a major role in nutrient dynamics, particularly nutrient immobilization and slowing release for plant uptake. Immobilization can have negative effects on grassland productivity and persistence by reducing quantity of nutrients available for plant growth. High levels of N fertilization are thought to increase litter quality and nutrient release, but based on results with Tifton 85 bermudagrass, there was little difference in short-term nutrient contribution from litter of low- and high-input pastures. Although litter quality is greater under high N inputs, it remains sufficiently low that nutrient immobilization continues to occur. In conclusion, Tifton 85 bermudagrass thrives under a wide range of grazing management allowing producers to choose a strategy that fits their objectives. Pasture management has a significant but relatively small effect on C sequestration, but the effect of grazing and fertilization on litter mass and decomposition is significant and can impact long-term pasture sustainability. (Full text of this dissertation may be available via the University of Florida Libraries web site. Please check http://www.uflib.ufl.edu/etd.html)...