Transfers of phosphorus within three dairy farming systems receiving varying inputs in feeds and fertilizers

Inputs of phosphorus (P) above requirements for production on dairy farms l ead to surplus P with increased risk of P transfer in land run-off to surfa ce waters causing eutrophication. The impact of reducing surplus P inputs i n purchased feeds and fertilizers on milk and forage production was investi gated in a comparison of three dairy farm systems on chalkland soils in sou thern England over a 3-year period. In accordance with current commercial p ractice, no attempt was made to regulate P inputs in system 1, which accumu lated an average annual surplus of 23 kg P ha(-1). Progressive reductions i n purchased feed and/or fertilizer inputs into systems 2 and 3 decreased su rplus P to 17 and 3 kg ha(-1), respectively, without apparently limiting ei ther milk or herbage dry matter production. The estimated reduction in faec al P output from system 3 cows fed a low P diet compared to system 1 cows f ed a high P diet was 26%. Milk P concentrations significantly (P < 0.001) i ncreased in systems 2 and 3 which included maize in the diet. Output of P i n milk and meat products, as a proportion of the total dietary P inputs, in creased from 28% in system 1 to 36% in system 3. Surplus P was greatest in continuous maize fields receiving both dairy manure and starter P fertilize r. Withholding P fertilizer in system 3 did not reduce P offtake in cut her bage on soils of moderate P fertility. Total annual losses of P in storm ru n-off and leaching were no greater than annual inputs of P from the atmosph ere (0.5 kg ha(-1)). The results indicate there is scope to reduce surplus P on commercial dairy farms without sacrificing production targets at least in the short term. Purchased feeds are the largest of the P inputs on inte nsive dairy farms, yet these are rarely quantified on commercial holdings.