Influence of nutrition on nitrogen and phosphorus excretion of livestock - Model calculations on the basis of a literature review

The objective of the present study was to quantify the influence of feeding on the nitrogen and phosphorus excretion of livestock by means of model calculations on the basis of published literature data. As the ecological principles on an agricultural farm, where nutrients are moving in a cycle between soil - plant - animal - soil, have to be taken into account the excretion was estimated not only per anima but also per unit area. In milk production the excretion of slurry and nitrogen increases with increasing feed intake, milk yield and the content of milk protein. The amount of slurry per unit forage area decreases with increasing forage quality, as the number of cows which can be fed per hectare decreases because of the lower yield under Alpine growing conditions and the higher forage intake. Depending on milk yield and forage quality nitrogen excretions of between 90 and 180 kg N per hectare of forage can be expected. By comparison the amount of N excreted in specialized bull fattening systems is up 2.5 times higher because their diet consists of forage maize which has higher yields. The N excretion per hectare forage maize increases with increasing daily gains. In pig production there are three possibilities of reducing N excretion as follows: (a) feeding exactly according to their protein requirements (b) optimizing the amino acid composition of the feed protein and (c) by the use of bacterially fermentable cell wall carbohydrates. The amount of slurry per unit area depends on the method of production, the yield of grain and the proportion of purchased feedstuffs and is generally between 70 and 180 kg N per hectare. In poultry production, because of the higher proportion of protein concentrates, the excretion of N is about 2.5 times higher. The principals governing the excretion of P are similar to those of N. The amount of P excreted per cow per year increases with increasing milk yield from 8.9 to 10.8 and to 12.7 kg at milk yields of 4000, 6000 and 8000 kg, respectively where P is supplied according to requirements. Similarly to milk production, P excretion increases with improved performance due to a higher proportion of purchased feedstuffs also in the case of bull fattening. In addition to level of supply, anima performance and stocking rate the utilization of P in the different feed sources has a substantia impact on excretions. At a P utilization of 50 % and mean daily gains of 700 g the P excretions per fattening pig are 750 g. If the P utilization is improved by 10 % the P excretions are reduced by 18.9 %. The results show that nutrient returns to the soil from animal production correspond approximately to removals by plants under balanced situations of production (fertilization, stocking rate, purchase of concentrates). Increased crop yields and improved animal performance brought about by purchased feedstuffs lead to excessive and therefore problematical nutrient returns.