This research committed to evaluate the gas exchange and growth index of Brachiaria decumbens under five water supply (3.84, 4.39, 6.19, 8.62 and 10.46 mm day-1). A complete randomised with split-plot design with five irrigation depth being the parcels and ages (10, 17, 24 and 31 days), the subparcels, with four replicates. It was observed interaction between irrigation depth and ages for leaf transmission, leaf temperature, rate of photosynthesis and leaf weight ratio. Table responses with minimum values have been observed to the internal CO2 concentration and specific leaf area (0.0055 m² leaf g-1 leaf) with advance in ages. The internal concentration of CO2 revised values of 220.09 and 176.20 ppm to 10 and 31 days, correspondingly, withminimum (173.68 ppm) to 27 days. It was observed square response with maximum value (25.04 g m-2 leaf day-1) to the net Assimilation rate with increasing ages. Decreasing linear responses were observed to the relative growth rate and leaf area ratio with increasing ages. The relative growth rate presented values of 0.097 and 0.048 g g-1 day-1, with 10 and 31 days, accordingly. The growth ages provided changes on gas exchange and growth index of Brachiaria grass, however, these characteristics are little Influenced by irrigation depth under the conditions of this study.ResumoPurpose with this study assessing gas exchanges and growth rates in the manned capim-brachiary under five irrigation slides (3,84; 4,39; 6,19; 8,62 and 10.46 mm day-1) and four ages, in a fully casualised design in the fitting of subdivided parcels, with irrigation slides being studied on the parcels and ages (10, 17, 24 and 31 days) on the sub-parcels, with four repetitions. Interaction between irrigation slides and ages was found in the variables: foliar transpiration, leaf temperature, photosynthesis rate and foliar weight ratio. A root grid response with a minimum point for the internal CO2 concentration and specific foliar area (0.0055 m² sheet g-1 sheet) was checked depending on age progression. The internal CO2 concentration was estimated at 220,09 and 176.20 ppm at 10 and 31 days of growth, respectively, with a minimum value (173.68 ppm) at 27 days. A square response was found with a peak point (25.04 g m-2 sheet day – 1) for the net assimilation rate with the increase in ages. A linear descending response to relative growth rate and foliar area ratio with increased age has been observed. The relative growth rate showed values of 0,097 and 0.048 g g-1 day-1 with 10 and 31 days of growth respectively. Growth ages bring about changes in gas exchanges and growth rates of capim-brachiary, but these variables are little influenced by irrigation pelaslâmings under the conditions of this study.