Diagnose foliar para nitrogênio em capim-marandu em função de doses de ureia

Abstract

Leaf nitrogen (N) diagnosis is a technique used to evaluate the amount of N in plant leaves. It is important to identify the need for nitrogen fertilization in crops, such as Brachiaria brizantha cultivar Marandu pastures, which is one of the most used species in Brazilian livestock farming. The objective was to evaluate the concentration of N in the diagnostic leaves of Marandu grass as a result of the application of doses of urea on the surface, in the second year of evaluation. The experiment was conducted in randomized blocks with N doses: 0, 60, 120, 240, 180 and 300 kg ha-1 using urea as N source, with four replications. Each plot had a total area of 30 m2 , and a usable area of 0.5 m2 . The N doses were divided into five applications of 60 kg ha-1 of N each. After each application, Marandu grass growth was evaluated weekly and when it reached 35 cm in average height, it was collected 20 cm above the ground. The entire collected aerial part was taken to the laboratory, weighed to obtain the mass of green matter, later around 500 g were separated into the following parts: FE-expanding leaves; LR-newly expanded leaf blades; LM-mature leaf blades and CB-culm plus sheath. After each cut, the experimental area was lowered to 20 cm in height and all material was removed by raking. The LR, as they are the diagnostic leaves of pastures, were placed in a forced air circulation oven at 65ºC until constant weight, subsequently ground and analyzed for N concentrations. The data obtained were subjected to analysis of variance and regression using the SISVAR statistical program. Leaf diagnosis proved to be essential to define the best dose response for Marandu grass in relation to the foliar N content in the diagnostic leaves. N applications from control to maximum dose followed the first degree regression model, that is, with higher doses, higher N concentration in the diagnostic leaves.