The homogeneous topography of Argentina's flooding pampas conceals a substantial amount of spatial and temporal ecosystem heterogeneity. Differences in soils, grassland botanical composition and plant growth regimes that occur down to individual paddocks influence livestock grazing patterns and, predictably, affect the productivity of cattle ranches in the region. Over 40 years of ecological research have greatly improved understanding of the structural and functional heterogeneity of this ecosystem. This better understanding has led to the development of grazing management strategies that help ranchers optimize secondary production by achieving a more efficient use of vegetation. As a result, cattle ranches are rapidly increasing profitability by integrating grass-fed yearling finishing programmes with the traditional cow-calf operations of the region.

Two experiments were carried out on a tall fescue sward in two periods of spring 1994 and on a tall wheatgrass sward in autumn 2001 and spring 2003 to analyse the effect of sward surface height on herbage mass, leaf area index and leaf tissue flows under continuous grazing. The experiment on tall fescue was conducted without the application of fertilizer and the experiment with tall wheatgrass received 20 kg P ha )1 and a total of 100 kg N ha )1 in two equal dressings applied in March (autumn) and end of July (midwinter).Growth and senescence rates per unit area increased with increasing sward surface height of swards of both species. Maximum estimated lamina growth rates were 28 and 23 kg DM ha )1 d )1 for the tall fescue in early and late spring, respectively, and 25 and 36 kg DM ha )1 d )1 for tall wheatgrass in autumn and spring respectively. In the tall fescue sward, predicted average proportions of the current growth that were lost to senescence in early and late spring were around 0AE40 for the sward surface heights of 30-80 mm, and increased to around 0AE60 for sward surface heights over 130 mm. In the tall wheatgrass sward the corresponding values during spring increased from around 0AE40 to 0AE70 for sward surface heights between 80 and 130 mm. During autumn, senescence losses exceeded growth at sward surface heights above 90 mm. These results show the low efficiency of extensively managed grazing systems when compared with the high-input systems based on perennial ryegrass.

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