Ecophysiology and Management Response of the Subtropical Grasslands of Southern South America

The subtropical grasslands of Southern South America (campos) are located approximately in the range of 24° S to 37° S latitude. These grasslands are dominated by C3 and C4 grasses, and in a lesser extent to herbaceous plants, shrubs and dispersed trees, with some exceptions in particular regions. Previous to the introduction of large herbivores, at the beginning of the XVII century, like cattle and horses, shrubs and tall grasses dominated the vegetation. Then, associated with greater grazing pressure and partially due to fire, the original campos are maintained, actually in a pseudo-climax herbaceous phase. These native communities are highly stable, adapted to long periods of water stress and flooding. Campos are generally grazed at continuos stocking all year around by sheep, cattle and horses. With the exceptions of the secondary plant successions, the summer growing species (C4) dominate the campos vegetation, having a high relative frequency (75%). Continuos grazing and coarse summer grasses contribute to decrease the frequency of winter species (C3). There is a high spatial and temporal variation in the native communities related to the type of soil, texture, water capacity, topography, altitude, management, etc., which determinate the different proportions of vegetation types and botanical compositions. The temporal variation is mainly associated with climatic factors, principally rainfall, affecting the variability of the annual dry matter production of campos vegetation (CV = 40 to 50%). The range of annual forage production of campos vegetation varies between 2.5 (shallow and low fertility soils) to 7.0 ton DM ha-1 (deep and fertile soils). Most of the annual forage production is mainly concentrated in spring and summer, reaching 70-80% and 60-70% in shallow and deep soils, respectively. Grazing management, and particularly the stocking rate utilised, is the principal factor determining changes in the botanical composition in this type of vegetation. In general, the use stocking rates greater than 1 stock unit ha-1, for longer periods, causes reductions in forage production, associated with increases and decreases in the frequency of productive species and weeds/shrubs/forbs, respectively. Pasture degradation is even greater, when high stocking rates are applied in conjunction with the use of high sheep/cattle ratios (e.g. 5:1). The use of rotational grazing systems enhances pasture production by 12% as well as pasture utilisation. When continuos stocking is applied, it is difficult to achieve pasture utilisation greater than 50%. Avoiding overgrazing in degraded vegetation, it is possible to obtain a recovery in pasture conditions, while the degradation caused by continuous cropping is unrecoverable. Animal production is negatively affected when herbage mass or sward height are lower than 1000 kg DM ha-1 or 5-6 cm, respectively. Animal selectivity allows sheep and cattle to have a greater crude protein (33 and 22% respectively) and metabolisable energy (15 and 11% respectively) levels than those obtained in the pasture offered. The use application of low quantities of nitrogen and phosphate on native vegetation increase forage production and nutritive value by 50%, enhancing the contribution of winter species (C3) in the sward. Legume introduction by overseeding increases native pasture production by 60-100% and pasture nutritive value by 50-100%, particularly during winter. This response is associated with the contribution of the introduced species and additionally by the increments in proportions of the native winter species (C3), resulting in a higher plan community biodiversity. Improved pastures permit to obtain 4 to 5 times greater secondary production than those unimproved. The generated scientific knowledge has contributed to apply better pasture management practices, which resulted in biological and economical benefits to the farmer community and the whole society in the longer time, with special care of animal and plant communities biodiversity and water conservation for human and animal use. Both animals and plants will be for a long time our main source of food and fibre in the world, hence conditioning our actions and behaviour in the manner that we conserve our natural resources for the future generations