Plasticidade fenotípica do capim-marandu em resposta à desfolhação e à adubação

Abstract

Brachiaria brizantha cv. Marandu (marandu grass) is one of the most used grasses in systems of pasture production in the tropics. With this, two works were realized to evaluate the phenotypic plasticity of this grass in relation to defoliation and nitrogen fertilization. In the first, for better understanding of the phenotypic plasticity of this plant, aimed characterize morphological responses of tillers of marandu grass before the nitrogen fertilization and frequency of defoliation. The study was conducted in two experimental years. The treatments in the first consisted of different cutting intervals (seven, 14, 28, 56 and 112 days), while the second, the same cutting intervals and phosphorus and nitrogen fertilization conditions (low and high). Were adopted the randomized block design and in experiment two, was used the factorial design with four repetitions. In both experiments the responses to stem length, leaf blade length, space between leaf blade and leaf area and weight of tiller increased with cutting intervals. In experiment two, high doses of fertilization provided larger number of vegetative tillers, space between leaf blades, weight of tiller, longer leaves and stems, and higher population densities of reproductive tillers with greater cutting intervals. Was concluded that the defoliation and the nitrogen fertilization induce to morphologic modifications in the tillers of marandu grass suggesting the phenotypic plasticity of this grass. In the second study aimed to characterize the changes in the way of growth subject marandu grass the cutting intervals and fertilization, as well to evaluate the effect of these variables in the type of growing on forage removed and remaining after of cut. In the first experimental year, the highest horizontal index found in plants subjected to more frequent cuts and, when cut with longer intervals (56 and 112 days), a reduction of the index. In the second year, the high fertilization increased the horizontal index only in the plant cutted every seven days. In general, in both experiments was increased residual forage biomass with increased of cutting intervals. In the second experimental year, the residual forage biomass increased with high fertilization, as well as the percentage of living stems. In the two experiments, the forage biomass and leaf area index were higher in the plants removed in higher cutting intervals. But when subjected to high fertilization, in the experiment two, the forage biomass and leaf area index were higher compared to the low fertilization condition. In the second experimental year when fertilization condition was high, was a low percentage of live leaf under intermediate cutting intervals (28 and 56 days). Was concluded that when the defoliation is very frequent, the increased availability of nutrients in the soil contributes to the Brachiaria brizantha cv. Marandu more sharply modify their way of growth, from upright to prostrate, compared to the condition of low availability of nutrients in the soil. Furthermore, the increased cutting interval and fertilization increment the removal of the fodder marandugrass.