Eficiência da uréia com inibidor de urease no cultivo irrigado da forrageira Tithonia diversifolia

Tithonia diversifolia stands out as a fodder plant due to the high production of biomass, high nutritional value, good acceptance by the animals, rusticity and to present perennial life cycle. However, there are few studies on the nitrogen demand of this forage, as well as the use of urea with urease inhibitors in irrigated systems, in order to increase the efficiency of nitrogen utilization in agricultural crops. The objective of this study was to evaluate the efficiency of urea utilization with the N- (n-butyl) thiophosphoricotriamide (NBPT®) urease inhibitor in irrigated T. diversifolia. A randomized complete block design with four replications was used in a 2 x 5 factorial scheme, the first factor being urea associated or not with NBPT® urease inhibitor and the second one at doses 0, 50, 100, 150 and 200 kg ha-1 of N. The efficiency of using urea with or without inhibitor was characterized by the evaluation of losses of N by volatilization and by the production capacity of T. diversifolia. N soil losses were measured at 3, 6, 9, 12, 15 and 18 days after application of the treatments. The dry mass, chlorophyll a, chlorophyll a, total chlorophyll and photosynthetic rate were characterized in the plants. The highest losses of N-NH3 were observed on the 6th day after application, in the treatments without urease inhibitor and in the highest doses of urea. For the photosynthetic rate at the dose of 135.43 kg ha-1 of N in the treatment with NBPT®, the highest photosynthetic rate of 42.36 cmol CO2 m-² s -1 at 60 days after fertilization was obtained. With the increase of the dose of N in the soil, an increase in chlorophyll content was obtained with the use of NBPT®, at 153.50 kg ha-1 of N, at 60 days the maximum content of chlorophyll a, and at dose of 176.16 and 147.00 kg ha-1 of N the maximum chlorophyll content and total. The use of NBPT® reduces the loss of N-NH3 to less than 5% of the applied doses. Fertilization with urea at a dose of 50 kg ha-1 of N with NBPT® presents higher agronomic efficiency and lower cost per kg or biomass produced compared to the other doses of fertilizer applied. The use of the inhibitor promotes a positive response in the chlorophyll content, photosynthetic rate and, consequently, higher growth and productivity of T. diversifolia.