Establishment of Aries Grass Submitted to Interspecific Competition with Weeds.

The objective of this study was to evaluate interspecific competition between Panicum maximum cv. Aries and weeds during the establishment period at Pinhais in Paraná state, Brazil (25° 26' 41'' S, 49° 11' 33'' W), where the climate is characterized as subtropical. The shading levels were related to weed competition, measured as light interception by weeds above the forage canopy: (i) without weeds present (0% shading), (ii) clipping weeds when 40% shading occurred, (iii) clipping weeds when 70% shading occurred, and (iv) no clipping (60% shading over time), all arranged without and with nitrogen (200 kg/ha broadcast) in a 4 x 2 factorial. Stature (extended leaf height), tiller dynamics and dry matter per plant of P. maximum cv. Aries were measured. For weed data, we measured the shading they caused above the forage canopy and the relative presence of each species. The measurements were made every 7 d for 42 d. Productivity of the forage grass was greatest in the absence of weed competition. Nineteen weed species were present and the relative importance over time was greater for Raphanus raphanistrum L. and Bidens pilosa L., which shaded the forage species. Shading increased plant stature but decreased number of tillers and dry mass of Aries, while nitrogen increased all three variables. The combined effect of nitrogen and shading increased plant stature, but decreased the number of tillers and dry matter production of the forage, acting as a growth promoter of weeds, i.e., increasing the relative intensity of interspecific competition. Although differences in average shading percentages were small between the clipped and not clipped treatments, the variability in shading over time introduced by clipping resulted in different morphological plant responses compared with the more constant shading present in the not clipped treatment.