Frequency of defoliation of native and naturalized species of the Flooding Pampas (Argentina)

This study describes the effect of herbage mass of species on defoliation patterns in a multispecies temperate-subhumid natural grassland of the Flooding Pampas at low, medium and high sward herbage mass [761 +/- 8.7, 989 +/- 137 and 1591 +/- 44.0 kg green dry matter, (DM) ha(-1) respectively] maintained by continuous grazing of cattle at variable stocking rates. Four native and naturalized warm-season species were studied: the grasses Cynodon dactylon, Leersia hexandra and Paspalum dilatatum and the legume Lotus tenuis. Sward state of the four, species was described in terms of sward surface height, herbage biomass and the number of individual grass tillers and legume stems per unit area. The defoliation pattern was monitored twice-weekly during an experimental period of 3 weeks on permanently marked individual plants. Proportions of individual plants defoliated and the ranking of species selected differed among sward treatments and species. The results showed that the density of the species was a major factor-determining the pattern of species defoliation and suggest that, in continuously grazed mixtures, the pattern of defoliation might be more dependent on the abundance of a species in the plant community than on species preferences. The results show the importance of considering this structural characteristic of swards in understanding the process of species selection by large herbivores in multispecies vegetation.