Outcomes and underlying mechanisms of grassland responses to legume introduction and manipulation of defoliation intensity

Abstract

Bahiagrass (Paspalum notatum Flügge) is the most important forage species in Florida grazing systems, adapted to high grazing intensity, low soil fertility, and continuous stocking. However, it has production, economic and environmental limitations associated with inadequate soil nitrogen (N) and poor nutritive value. Alternatives to overcome these limitations include introducing rhizoma peanut (Arachis glabrata Benth.), a C3-perennial legume, into bahiagrass, and improving grazing management. In this context, the objectives were to (1) identify legume proportion maximizing plant and animal responses, and (2) study potential effects of short-term increases in defoliation intensity on bahiagrass leaf characteristics and digestibility. For two years, a grazing trial assessed N dynamics, radiation use efficiency (RUE), biomass accumulation and animal responses along a rhizoma peanut proportion gradient in rhizoma peanut-bahiagrass mixtures (0-60% legume; Objective 1); and a clipping trial quantified seasonal changes in bahiagrass leaf tissue proportions and fiber concentration in response to different defoliation intensities (Objective 2). Overall, moderate legume proportions (~30-40%) optimized N dynamics and plant and animal responses. Within this optimal range, atmospheric-derived N yield was 27-50% greater than in legume-dominated sites (~60% legume), while soil-derived N yield was 225-285% and 73-134% greater than a grass monoculture and legume-dominated sites, respectively. Consequently, RUE and biomass accumulation increased quadratically with increasing legume proportion up to ~40% legume, and RUE was 86-110% greater at this proportion than for legume-dominated sites. Responses suggest RUE is N-limited below 30% legume, and physiologically-limited above 30% legume, where C4-grasses are replaced by C3-legumes. At 40% legume, cattle actively selected legume (60% diet vs. 40% pasture), maximizing gain per animal (604 vs. 290 g day-1) and gain per hectare (350 vs. 154 kg LW ha-1) compared with the bahiagrass monoculture. In the clipping trial, longer bahiagrass leaves, in response to lower defoliation intensity, increased the proportion of non-degraded vascular bundle and sclerenchyma tissues, which increased cell wall concentration and decreased digestibility. Legume proportions of ~30-40% represent a reasonable target for optimizing plant and animal responses, while short-term increases in defoliation intensity during late summer may overcome increases in non-degraded tissues, the presence of which reduces nutritive value and animal production.