This study aimedto evaluate the growth index of massai grass during two regrowth cycles in agreenhouse under five levels of nitrogen (0, 150, 300, 450 and 600 mg N/dm3of soil) and eight growth ages (5, 8, 11, 14, 17, 20, 23 and 26 days) in a split-plotdesign with five replications. The following variables were evaluated: net assimilationrate, herbage growth rate, relative growth rate, leaf area ratio, leaf weightratio and specific leaf area. The effect of nitrogen was detected on the variablesnet assimilation rate (regrowths 1 and 2), leaf area ratio (regrowth 1), leafweight ratio (regrowths 1 and 2), specific leaf area (regrowths 1 and 2) andherbage growth rate (regrowths 1 and 2). The growth ages influenced the relativegrowth rate (regrowth 2) and the net assimilation rate (regrowth 2). The netassimilation rate presented linear effect, estimated in 0.70 and 2.86 g/m2leaf.day-1 for the 0 and 600 mg N/dm3 soil, respectively(regrowth 1). For each milligram of N/dm3 added, the leaf area ratiowas reduced in 0.0000258 m2 leaf/g plant (regrowth 1). The herbagegrowth rate was estimated in 1.38 and 37.02 g/m2.day-1(regrowth 1), and 4.37 and 32.20 g/m2.day-1 (regrowth2) for the 0 and 600 mg N/dm3 soil, respectively. The massai grass'growth is favored by nitrogen fertilization, and little do the growth ages affectthe growth index of the plants.
