Critical concentration and management of nitrogen fertilization in the establishment of Brachiaria hybrid Mavuno

Resumo

The critical range of leaf nitrogen (%NLeaf) concentration can be used to monitor the plant’s nitrogen (N) status, being an essential
tool to defi ne strategic fertilization protocols, where the time and amounts of fertilizers are applied according to pasture’s demand. The productivity
parameters and two methods to determine N suffi ciency – (i) the N nutrition index (NNI) and (ii) the normalized N suffi ciency index (NSI) – were
evaluated in Brachiaria hybrid Mavuno (Brachiaria spp. Syn. Urochloa spp.) along the regrowth cycles of the fi rst growth season after seeding
(establishment phase). The aims of the study were to estimate the critical %NLeaf for Mavuno grass pastures through productivity parameters, considering
the sampling of the youngest expanded leaf as reference. Fertilization rates were defi ned as follow: no-nitrogen (N0), 15 (N15), 30 (N30) and 45 (N45)
kg ha-1 of N and were applied after each cutting. The Falker chlorophyll index (FCI) was obtained from a portable chlorophyll meter Falker Clorofi LOG®
CFL1030, and results were used to calculate the NSI. The regression analysis, on a regrowth cycle basis, provided the parameters to estimate the critical
range of %NLeaf and the fertilization rates to attain the maximum forage accumulation. Population density of tiller was the main response contributing to
increasing leaf and stem mass, leaf area index and forage accumulation as N fertilization increased. The FCI and NSI were unable to capture differences
in %NLeaf, but NNI was a suitable tool to detect N status in Mavuno grass. The critical %NLeaf range varied from 2.75 to 3.07% and may be indicated
to monitor pasture’s N status. The strategic N fertilization protocol suggested for the growth season after seeding is to apply 42.0 to 46.7 kg ha-1 of
N at the fi rst regrowth, 27.0 to 32.5 kg ha-1 of N during the two following regrowth cycles, whereas at the end of the season (April to May regrowth)
only 22.2 to 24.7 kg ha-1 of N would be enough to sustain a maximum growth.