Structure, morphogenesis and patterns of defoliation of Brachiaria decumbens Stapf. pastures

This study aimed to contribute to the understanding o f the morphogenesis and structure of signal grass (Brachiaria decumbens Stapf.) pastures, and how they are affected by the environment and cattle defoliation. A diagrammatic model was adopted to identify and relate the major morphogenetic and structural characteristics at different levels of organisation o f the sward: phytomer, tiller, plant and population. The morphogenetic characteristics studied were leaf elongation rate (LER), intemode elongation rate (IER), leaf senescence rate (LSR), leaf appearance rate (LAR), tiller birth rate (TBR) and tiller death rate (TDR). The structural characteristics analysed were length of phytomer components (blade, sheath and internode), number o f phytomers per tiller, total length and number o f live blades per tiller, number and size-age distributions o f tillers per plant, and number and size distribution o f plants and tillers per area. Experiments to analyse the effects o f temperature, nitrogen application, season and defoliation intensity on sward structural and morphogenetic variables were performed under controlled and field conditions, and using destructive and non-destructive measurements.

An initial experiment under controlled environments showed highest LAR, LER and IER values at 30°C, and strong mutual regulations between morphogenetic and structural characteristics at the phytomer and tiller levels. The effects of cattle grazing and nitrogen fertilisation on the dynamics o f signal grass morphology, and the patterns o f defoliation were monitored during one year in long established pastures in Santa Cruz, Bolivia. Plant and population demography were mainly determined by the season: high number of seedlings and new vegetative tillers at the beginning o f summer; new vegetative tillers, plant fragmentation, and flowering during summer (wet season); and low tiller and plant densities with low proportions o f live leaf towards the following winter. Defoliation affected the phytomer and tiller levels of sward organisation, mainly through increases in the population o f young tillers. All morphogenetic characteristics were mainly affected by season, with higher values during summer. LSR, IER and LAR were partly regulated by tiller size or age characteristics, while TBR and TDR were directly affected by the intensity of defoliation. During winter, soil water availability affected tiller growth more than temperature. The low doses o f nitrogen applied in the field were effective in increasing LER and TBR after short spells at the end o f winter, but not during the main growing season. Tillers o f signal grass showed very high turnover rates, mainly because o f their pattern o f growth with intemode elongation, the production o f weak aerial tillers, and the speed in developing flowers during the reproductive season. The pattern o f defoliation by cattle was best explained at the tiller level, due to the sward heterogeneity in tiller structure and the physical barrier imposed by the tiller stem.

A conceptual model integrating the information obtained from the experiments was developed as a synthesis for discussion. The applicability o f information on sward morphogenesis and structure as a pasture management decision support tool, especially in the diagnosis o f pasture degradation is finally considered.