Nitrogen variable rate fertilization in corn crop prescribed by optical sensor

Nitrogen (N) plays a key role in the optimization between corn grain yield and its economic return. Based on that, this study aimed to evaluate the technical feasibility of a portable chlorophyll meter for recommending an economic dose of N under variable rate using different threshold values of the nitrogen sufficiency index (NSI). The study was conducted in Januaria-MG (15 degrees 28 ' 55 '' S, 44 degrees 22 ' 41 '' W; 474 m) from February to June 2018. The experimental design was a randomized block with 5 treatments and 5 replicates, which were: T0: without N application; T1: 112 kg ha(-1) of N (pre-fixed doses), with 50% (56.00 kg) applied in the four expanded leaves stage (V4) and 50% in the V8 stage; T2: 44.8 kg ha(-1) of N in V4 + 22.4 kg ha(-1) of N through variable rate based on NSI (< 95%) in the stages V8, V10 and V14; T3: 44.8 kg ha(-1) of N in V4 + 22.4 kg ha(-1) of N through variable rate based on NSI (< 98, 95 and 93%) in the same stages; and T4: 33.6 kg ha(-1) of N in V4 + 33.6, 28.0 and 16.8 kg ha(-1) of N according to NSI (< 98, 95 e 93%) in V8, V10 and V14, respectively. Variable rate treatments were more efficient than the pre-fixed dose, as they consumed less N and resulted in the same grain yield. Furthermore, the NSI threshold of 95% resulted in higher N use efficiency in the evaluated stages.