Nitrogen cycling in Brachiaria pastures:: the key to understanding the process of pasture decline

In the tropical regions of Brazil there are at least 80 million ha (Mha) of pastures planted to grasses introduced from Africa, principally Brachiaria spp. It is estimated at least half of these pastures are degraded, that is, support very low stocking rates, show low plant cover, are invaded by non-palatable native species and often densely populated with termite mounds. The main causes of the process of pasture decline are lack of maintenance fertilisation and excessively high animal stocking rates. In this study the effect of increasing stocking rate on the fluxes of nitrogen (N) though the animal (forage consumption, production of faeces and urine) and through the plant (growth, senescence) pathways were studied on Brachiaria humidicola pastures grazed by Zebu beef cattle in the Atlantic forest region of the south of Bahia (Brazil). As stocking rate increased from 2 to 4 animals ha(-1), live weight gain per animal decreased from 153 to 120 kg per animal per year but overall weight gamper hectare increased from 305 to 360 kg per year. Nitrogen exported in the animal weight gain only increased from 7.3 to 8.6 kg N ha(-1), but the pathways of N cycling were radically changed. Increasing the stocking rate from 2 to 4 animals ha(-1), increased total N consumed by the animal from 94 to 158 kg ha(-1) per year, and that deposited in plant litter decreased from 170 to 105 kg ha(-1). This resulted in increases in N deposited as urine and dung in the paddocks from 50 to 90 and 37 to 59 kg ha(-1), respectively. A large proportion of these excretions were deposited in rest areas and around drinking troughs where the grass was so trampled that it could not take advantage of this N, and other nutrients. Data from complementary studies showed that N losses from urine could be between 35 and 80%, being much higher in areas without vegetation. We conclude that pasture decline is hastened by increasing stocking rates because of these losses of N and the decrease in N and other nutrients available for grass growth. (C) 2004 Elsevier B.V. All rights reserved.