Phytosociological Survey of Weeds on Degraded and Well-Managed Pastures: Agronomical and Ecological Implications

The objective of this research was to carry out a survey of weeds in pastures in the Middle Valley of Paraíba do Sul, Rio de Janeiro State, Brazil, in order to subsidize weed management and pasture recovery. Weed identification and plant count were carried out in pastures with four levels of degradation, classified as low (N1), moderate (N2), strong (N3) and very strong (N4), with five replications. Thirty-nine weed species were identified and distributed into16 botanical families. Poaceae, Asteraceae and Fabaceae were the most relevant families. The number and density of weeds increased as the level of degradation decreased. The relative importance of weed species varied with the level of degraded pasture. The main weeds found in N1 were Melinis minutiflora, Desmodium incanum, Croton lundianus, Andropogon bicornis, and Imperata brasiliensis; in N2: Paspalum notatum, Melinis minutiflora, Imperata brasiliensis, Sida rhombifolia, and Desmodium incanum; in N3: Paspalum notatum, Melinis minutiflora, Sida rhombifolia, Eupatorium maximilianii, and Imperata brasiliensis; in N4: Paspalum notatum, Melinis minutiflora, Cynodon dactilon, Eupatorium maximilianii, and Imperata brasiliensis. The similarity index was high, showing the homogeneity of weeds among areas. The predominant species, considering all areas, were in increasing order of importance: Cynodon dactilon, Melinis minutiflora and Paspalum notatum. Decision-making about applying control measures could be marked out when the plant density reached out or exceed the average of 3.58 plants m-2.