DINÂMICA DE CRESCIMENTO DA AVEIA PRETA SOB DIFERENTES DOSES DE NITROGÊNIO E AJUSTE DE MODELO MATEMÁTICO DE RENDIMENTO POTENCIAL EM FUNÇÃO DE PARÂMETROS CLIMÁTICOS

Abstract

Aiming at studying the growing dynamics of black oats (Avena strigosa Lam.) under several levels of nitrogen (O; 75; 150; 225 and 300 kg /ha), and to validate a mathematical model of potential yield of forage based on degree-days and incident solar radiation, a field experiment was carried out at the experimental station of Federal University of Paraná - UFPR, municipality of Pinhais, PR, Brazil. Sowing was made on April 19, 1999, with 60 kg of seeds/ha, in rows spaced by 17 cm. The experiment was irrigated and kept free of weeds and pests. Nitrogen fertilization was made with urea split in three applications at 24, 54 and 84 days after plant emergence. Forage production was evaluated after four cuttings, with three during vegetative growth and one in the reproductive stage. Every week, the following evaluations were made: dry matter and nitrogen content (% N) of the upper part; leaf blade / culm ratio, and leaf area index (LAI). Every two weeks leaves were measured and tillering were counted on selected plants. Photosynthetic Active Radiation (PAR) data were collected with PAR sensors installed in the treatments, stored in a CR-10 datalogger (Campbell Scientific), and PAR incident, reflected and transmitted. There was a significant response to nitrogen dosage and a significant interaction of dosage with cutting time. There was lower productivity in the absence of nitrogen in all cutting times evaluated, with an accumulated production of 3422.6 kg dry matter / ha. The treatment with 300 kg N/ha provided 9953.6 kg / ha dry matter and was superior the all the others during the reproductive phase of the crop. During the vegetative phase the maximum productivity of 7268.7 kg dry matter / ha was obtained with 225 kg N/ha. Nitrogen modified all morphogenic traits evaluated: leaf area index, tillering, leaf blade / culm ratio, leaf length, leaf elongation rate and senescence. The evolution of leaf area index in the first two cuttings was related to both tillering increment and leaf length, but in the third cutting the evolution was mainly due to leaf expansion. The black oat philocron of the cultivar IAPAR 61 was approximately 80 degree-days for the main tiller independent of the nitrogen dosage. The submodels to estimate the evolution of LAI based on degree-days, and the absorption efficiency (Ea) based on the extinction coefficient (K) and on the submodel of LAI, were suitable to estimate the absorved photosynthetically active radiation (PARa) and for growth analysis of biomass based on radiation, but they differed for the phase of plant establishment and re-growth. Potential growth of black oats IAPAR 61, under non-limiting conditions, can be estimated by a simple model with two climatic variables: accumulated global solar radiation and degree-days, easily available from weather stations.