NUTRIENTS BALANCES IN BEEF CATTLE PRODUCTION SYSTEMS AND THEIR IMPLICATIONS FOR THE ENVIRONMENT

the main inputs and outputs of nitrogen (n) and phosphorus (p) in soils and swards under typical southern chilean conditions were determined between 2004-2005 and 2006-2007 to establish soil and field gate n and p balances under different immediate stocking rates (63 and 191 anim ha-1 d-1) and field slope treatments (4 and 12%). treatments received 67.5 and 40 kg n and p ha-1 yr-1 as inorganic fertilizer. the main n and p input to the soil balances was n mineralization (304 kg ha-1 yr-1) and inorganic fertilizer (62 and 69% for n and p, respectively). the main output for all treatments was plant uptake (95% and 100% on average for n and p, respectively). the n and p apparent use efficiency was high (>95%). the main n and p input to field gate balances was inorganic fertiliser (>95%>), while animal production had a minor effect on n and p export. the soil balances ranged from -303 up to +58 and from -3 up to +20 kg of n and p ha-1 yr-1. field n and p gate budgets ranged between -309 up to +58 and from -8 up to +20 kg of n and p ha-1 yr-1, respectively, indicating a potential soil degradation in areas with low nutrient inputs in inorganic fertiliser and an overaccumulation in areas with low dry matter production of the grassland, which in turn can lead to environmental constrains. this should be especially considered for dairy systems, which are more intensively managed.