Growth and senescence of Urochloa brizantha under Brazilian Cerrado conditions

The growth and senescence of Urochloa brizantha cv. Marandu were analyzed under the Brazilian Cerrado conditions. The seeds were sowed in November at the beginning of the rainy season. Samples of tillers were collected. The culms, inflorescences, along with green and dead leaves were removed to obtain leaf area and dry matter measurements. The maximum leaf area index (LAI = 5.80 m2 m-2) was achieved at 97 days after emergence (DAE), the maximum above ground total dry matter (TDM = 22.8 Mg ha-1) at 151 days, and the maximum crop growth rate (CGR = 0.260 Mg ha-1) at 70 days. Leaf senescence began at 55 DAE, with less than 0.001 Mg ha-1 of dead leaves dry matter (DLDM), and reached 1.753 Mg ha-1 at 151 DAE. Between 117 and 151 DAE, in the final cycle of the plants, there was lodging of the canopy by wind. This lodging had favored the sprouting of new tillers and a sudden increase in net assimilation rate (NAR). The LAI values did not show such expressive increase during the same period, suggesting that NAR may contribute to crop growth more than LAI due to increased light penetration after the occurrence of the lodging. In this paper an equation that estimates the net photosynthetic rate from the net assimilatory rate was presented. Key words: Leaf area, dry matter, growth rate, net assimilation rate, pasture grass