Elephant grass Carajás cv. mass production under different levels of nitrogen and cutting frequency

Studies on fertilization and harvest frequency with the hybrid Caraj?s (Pennisetum purpureum x Pennisetum glaucum) are still scarce. This grass is important mainly because of its cultivation through seeds instead grafting, which has high cost for acquisition and planting. The aim of this study was to evaluate the elephant grass Caraj?s cv. mass production in function of different harvest frequencies and levels of nitrogen in southern of the Piau? state, Brazil. Randomized block design with four repetitions, in factorial scheme 3 x 2 was used, where treatments were composed by three harvest frequencies (60, 75 and 90 days) and two nitrogen levels (100 and 200 kg N ha^-1) using urea. These variables were analyzed: percentage of dry matter (%DM), dry mass of forage at harvest (DMFH) and dry mass of total forage (DMTF). The data were submitted to variance analysis and the averages were compared by the Tukey test (P<0.05). Significant differences were observed in interaction between the levels of nitrogen and harvest frequencies for all variables. For %DM the highest value was obtained in the level 100 kg N h^-1 for the frequency 90 days, with average of 32.95%, providing increase of 11.62 and 11.52% of DM between the highest and the lowest values, comparing to the harvest frequencies 60 and 75 days, respectively. For DMFH in level 200 kg N ha^-1 in harvest frequency of 75 days, the lowest average (7.84 t ha^-1) can be observed. The harvest frequency of 90 days provided an increase in DMFH of just 3.33% compared to that of 60 days. The highest total dry mass of the elephant grass Caraj?s cv. was found for the level of 200 kg N ha-1 for the harvest frequencies of 60 and 90 days. The crop growth rate after harvest was stimulated by nitrogen fertilization, which probably promoted a fast recovery of leaf area index and higher tillering. This situation resulted in a higher interception of sunlight, faster grass growth rate and higher accumulation of forage mass. The highest nitrogen level and the highest time for harvest provided better results for the elephant grass Caraj?s cv. mass production in southern region of the state of Piau?.