SIMULATION OF DEFOLIATION EFFECTS ON PRIMARY PRODUCTION OF A WARM-SEASON, SEMIARID PERENNIAL-SPECIES GRASSLAND

A simulation study was conducted on the production potential under grazing of a perennial C4-species grassland. A model of plant growth governed by soil moisture availability and prevailing weather conditions was adapted to account for perenniality, C4 photosynthesis, intra-seasonal regrowth, and defoliation effects. Model behaviour was validated against a set of field data obtained from the Nacunan experimental site, Mendoza Province, Argentina. Primary production, defoliation effects and potential forage utilization as a consequence of different stocking rates and grazing systems were simulated for two consecutive years. The model indicated that where animals were grazed continuously, primary production was only a slightly reduced by relatively heavy grazing. This occurred because of the particular growth pattern of the vegetation which is characterized by pulses with relatively high growth rates over short periods. These growth rates are considerably higher than potential defoliation rates, even at relatively high animal densities. The effect of an intensive, short duration rotational grazing system compared to continuous grazing had only a small effect on the simulated primary production and on the total amount of utilized green herbage, even though these variables varied widely between subdivisions within a grazing rotation. It is concluded that the growth pattern of the grassland vegetation, determined mainly by the characteristics of the prevailing weather in the region, limits the utilization of the green leaf to about 11-14% of the annual herbage production when animals have to be maintained throughout the year on the grassland. The rest of the available forage must be consumed as dry feed. As a result, the primary productivity of such grasslands appears to have low sensitivity to fairly large variations in stocking rate and grazing management. These results explain some of the inconclusive and conflicting results of grazing trials, and so have significant implications for research policy and grazing management in semiarid environments similar to those studied in the region.