Morfogênese e produção de fitomassa de cultivares do gênero Cynodon submetidos à doses crescentes de nitrogênio

Abstract

We evaluated the flow of biomass and its components in two cultivars of Cynodon (Tifton 85 grass and vaquero grass) fertilized with increasing levels of nitrogen (N) (control – without nitrogen; 200; 400 and 600 mg dm-3) under greenhouse conditions, in a completely randomized design, with factorial arrangement. Forages were studied during three regrowth cycles, with results presented as mean values of the cycles. In the study of the effect of nitrogen, Tifton 85 grass had a positive response to increasing levels of N for the leaf elongation rate (LER). The vaquero grass showed a quadractic response for the LER with increasing levels of N. As for the stem elongation rate, there was an increasing linear response in both grasses according to increasing levels of N. The leaf senescence rate (LSR) of the Tifton 85 grass has been enhanced by nitrogen fertilization. The vaquero grass revealed a quadractic response for this variable with nitrogen fertilization, with a minimum value of 0.96 cm tiller-1 day-1 with nitrogen level of 42.5 mg dm-3. The phyllochron and the total number of leaves of both grasses have been influenced by nitrogen fertilization, with downward and upward linear responses, respectively, with increasing levels of nitrogen. For the final mean leaf length (MLL) of both forages, we verified an increasing response with increasing levels of nitrogen. Tifton 85 grass and vaquero grass presented similar LER in the absence of nitrogen, but higher values were observed for the fertilized Tifton 85 grass. A difference was detected between forages for culm elongation rate (CER) and LSR in all levels studied, with higher values for the vaquero grass. The Tifton 85 grass had greater phyllochron for all levels of nitrogen studied. The vaquero grass showed a greater NTF compared with Tifton 85 grass in all nitrogen levels examined. The Tifton 85 grass presented higher MLL for all levels of nitrogen studies. In relation to the effect of nitrogen fertilization, Tifton 85 grass and vaquero grass presented increasing responses to N levels for the biomass of total forage (BTF), of green forage (BGF), of green leaf blade (BGL), of green culm (BGC), of dry forage (BDF) and of tiller population density (TPD). There was a greater production of BTF, BGF, BGC and BDF for the vaquero grass in all N levels. A higher production of BGL was observed in the Tifton 85 grass in all evaluated levels of N. We verified a greater TPD of the vaquero grass for all levels of N. The Tifton 85 grass showed a higher BL/C ratio for all levels of N evaluated. The nitrogen fertilization provides positive responses of the biomass flow and forage production of Tifton 85 grass and vaquero grass under the conditions of the present study. The vaquero grass proved to be better than Tifton 85 grass in the characteristics stem elongation, leaf senescence, total number of leaves and biomass production (total forage, green forage and culm). However, the Tifton 85 grass is superior as for the leaf elongation, phyllochron, leaf length, and leaf biomass production in the nitrogen levels evaluated.