Morphophysiological characteristics and herbage accumulation in mombaça grass subjected to defoliation regimes / Características morfofisiológicas e acúmulo de forragem em capim-mombaça submetido a regimes de desfolhação

AUTOR(ES)
DATA DE PUBLICAÇÃO

2006

RESUMO

Grazing systems that take into account the rhythm of regrowth of forage plants after defoliations can optimise herbage accumulation and ensure the control of sward structure. In these context, the objective was to evaluate patterns of response of Panicum maximum cv. Mombaça to defoliation in two experiments using measurements of sward morphogenetic and structural characteristics, tiller dynamics, herbage accumulation and morphological composition of herbage produced. The fist experiment was carried out at Federal University of Viçosa, Viçosa, MG, from October 2003 to May 2004, and followed a complete randomised block design with three replications. Treatments corresponded to combinations of two cutting heights (25 and 50 cm) and three cutting frequencies (time necessary to allow for the appearance of two, three and four leaves per tiller), and were allocated to experimental units according to a 2 x 3 factorial arrangement. Because of the different intervals between successive cuts, a result the way treatments were planned, each experimental unit was subdivided into four, three and two sub-plots for the cutting frequencies of two, three and four leaves per tiller, respectively. Measurements comprised estimates of sward height just before cutting and herbage accumulation during every regrowth, morphogenetic and structural characteristics twice a week and tiller demography every 30 days. The second experiment was carried out at Embrapa Gado de Corte, Campo Grande, MS, from October 2004 to May 2005, and followed a complete randomised block design with three replications. Treatments corresponded to three grazing intensities, characterised by sward height post-grazing (30 and 50 cm throughout the experimental period and 50 cm during spring and summer being reduced to 30 cm over two successive grazings during autumn), and associated with a pre-grazing condition of 95% canopy light interception during regrowth. Measurements comprised estimates of sward height and herbage mass pre- and post-grazing, herbage accumulation, morphogenetic and structural characteristics and tiller demography. In Experiment 1, sward height increased with the decrease in frequency (79, 90 and 124 cm for two, three and four leaves per tiller, respectively) and intensity of defoliation (80 and 103 cm for 25 and 50 cm cutting height, respectively), and there was no difference in regrowth interval between defoliation intensities (41.8 and 39.9 for the 25 and 50 cm cutting heights, respectively). Rates of stem elongation (0.326 and 0.711 cm/tiller.day) and leaf senescence (0.701 and 1.309 cm/tiller.day), leaf lifespan (74.2 and 90.4 days), final leaf length (25.4 and 31.8 cm), number of live leaves per tiller (5.6 and 6.5) and tiller weight (2.27 and 4.89 g/tiller) were larger for the 50 cm than the 25 cm cutting height, also showed a lower turnover in tiller population. Cutting frequency affected only rates of stem elongation and leaf senescence and tiller weight, with lower values recorded for the two than three and four leaves per tiller. The lower defoliation frequencies did not control stem elongation and stem accumulation (0.05, 0.10 and 0.16 kg/m2 for two, three and four leaves per tiller, respectively), favouring the reproductive development of tillers with consequent reduction in leaf-to-stem ratio (8.50, 6.78 and 2.79 for two, three and four leaves per tiller, respectively). The accumulation of total dry matter, leaf lamina, stem and dead material increased in different proportions with the decrease in defoliation frequency. On the other hand, defoliation intensity did not affect the morphological composition of the herbage produced (69.4, 29,8 and 9.8% of leaf, stem and dead material, respectively) and the accumulation of leaf, stem and dead material, but affected total dry matter accumulation (0.47 and 0.56 kg/m2 for the 25 and 50 cm cutting height, respectively). Climatic conditions during the experimental period affected tissue flows in the swards (morphogenetic and structural characteristics and tiller dynamics) suggesting the importance of taking into account how fast plants recover from defoliation during different seasons of the year. More severe defoliations resulted in increased turnover of leaves and tillers, but defoliation frequency was more effective in controlling stem accumulation and development. In Experiment 2, grazing intensity did not affect any of the morphogenetic and structural sward characteristics, except final leaf length. However, lower post-grazing residues resulted in greater turnover in tiller population, favouring a younger profile of a population of lighter tillers, reduced the rates of stem elongation and senescence, and increased the interval between successive grazings (55, 41 and 41 days for the 30, 50 and 50-30 cm post-grazing residue, respectively). Tiller population density did not vary with post-grazing residue (580, 569 and 568 tillers/m2 for the 30, 50 and 50-30 cm, respectively), but increased throughout the experimental period. Average herbage accumulation was 2020, 1950 and 1800 kg/ha per grazing cycle for the 30, 50 and 50-30 cm, respectively, but grazing to 30 cm resulted in one less grazing cycle than grazing to 50 or 50-30 cm (3.7, 4.7 and 4.7 grazing cycles during the experimental period for the 30, 50 and 50-30 cm, respectively). Pre-grazing height was 70 cm for the 30 cm residue and 72 cm for the 50 and 50-30 cm residues, and varied with grazing cycles, being reduced from 90.5 during the first cycle to 60.4 cm during the last cycle. Throughout the experiment swards followed a common pattern of variation in structure that resulted in the reduction of the sward height at which the target of 95% of the incident light was reached during regrowth. These modifications allowed swards to recover and become more homogeneous, since their structure was highly variable because of mismanagement during a long period of time before the commencement of the experiment. In general, the proportion of bare ground was reduced from 30 to 10%, tiller population density (tillers/m2) and number of tillers/tussock increased 100%, and herbage bulk density pre-grazing increased (43.4 to 74%), indicating that plants needed time and were still adapting to grazing managements imposed during the experimental period. Plants tended to prioritize these structural modifications in relation to the morphogenetic changes commonly perceived in swards under control during longer periods of time and, or, recently established. Example of this was the absence of difference between treatments (grazing intensities) in sward morphogenetic and structural characteristics during Experiment 2, since in Experiment 1 defoliation intensity affected significantly most of the variables studied.

ASSUNTO(S)

panicum maximum interceptação luminosa morphogenetics morfogênese panicum maximum defoliation incident light pastagem e forragicultura

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