Critério de manejo da desfolhação do capim Marandu: número de folhas aparecidas e adubação nitrogenada

Abstract

The difference in soil fertility promotes a varied defoliation management for grasses, due to the effect of nitrogen on the structural and productive responses of forage plants, where the management of defoliation based on number of leaves serves as a tool to determine the best moment of the Marandu grass pasture. The objective of this study was to evaluate the growth and production dynamics of Marandu grass (Urochloa brizantha cv. Marandu) as a function of the management of defoliation, based on the time for the appearance of two, three or four leaves completely expanded by tiller, and nitrogen fertilization, average of 180 or high of 360 kg ha-1 of N, in the period from 11/24/2017 to 06/07/2018. The experiment was arranged in a completely randomized design under a 3 x 2 factorial design (2, 3 and 4 leaves x 180 and 360 kg ha-1 N), with six treatments and four replications, totaling 24 experimental units. The response variables were morphogenic, structural, forage accumulation, spad index and tiller demography. The grass responded with different growth rates between medium and high fertilization, with smaller sprout periods between treatments with high fertilization (360 kg ha-1 N), except for the four leaf strategy. The height showed differences in vertical growth, in the intervals of three and four leaves the height was higher, 37.81 and 46.56 cm, respectively, at high fertilization (360 kg ha-1 N). The morphogenic variables responded with higher rates of appearance (LAR) and leaf length (LER) and mean leaf length (MLF) in the two and three leaf appeared and sheath length (SL) strategies in the four - leaf strategy. Tiller population density (TPD) presented higher value in the treatment of two leaves and leaf area index (LAI) from three leaves at high fertilization (360 kg ha-1 N). The four - leaf strategy showed higher total dry matter (DMM) of forage and leaf blade (ALB), as well as higher dry matter of stalk (DMS) dead material (DMDM). On the other hand, the total dry mass and cumulative foliar leaf mass that was higher in the strategy of two leaves appeared, and under high fertilization (360 kg ha-1 N), as well as the forage accumulation rate (FAR), also presented greater results in this strategy. The four - leaf strategy seems to direct the highest contribution of N to root growth with higher root dry mass (DMR) value. The three-leaf strategy presented higher SPAD index (SPAD) under higher fertilization (360 kg ha-1 N). The same pattern of response was observed for tillering rate (TAR), tillering survival (TSR) and population density of tillers (TPD), which contributed to the higher forage volume density (VDF) in the two - leaf strategy associated with high fertilization (360 kg ha-1 N). Thus, nitrogen fertilization can promote changes in forage plant growth and production, which in the present study, high fertilization (360 kg ha-1 N) promotes better morphogenic responses and higher yields in two and three leaf strategies.