title: Effect of varying the phosphorus content of dairy cow diets on losses of phosphorus in overland flow following surface applications of manure.
abstract: The increasing use of concentrate feedstuffs within Northern Ireland dairy systems has resulted in significant farm gate phosphorus (P) surpluses, and these have contributed to increased soil P levels and risk of P loss to overland flow. However, the P content of feed concentrates can be lowered without compromising animal performance. This study focuses on P losses from grassland and evaluates how adjusting the P content of manure impacts on the P composition and concentration in overland flow. Dairy cows were offered diets containing 5.3 to 3.0 g P kg(-1) dry matter (DM) and produced manures with a range of P contents. Manure was applied at a rate of 50 m3 ha(-1) to 0.5-m2 grassland plots, and simulated rainfall (40 mm h(-1)) was applied repeatedly 2, 9, 28, and 49 d after during the summer, winter, and spring. Decreasing the P content in the diet, from the highest to the lowest P treatment (43%), produced a proportionately greater reduction in manure TP content (63%), but reductions were not exclusively in the water-soluble fraction. Following surface applications of manure, P concentrations in overland flow increased in all seasons (P < or = 0.001), while the greatest impact of varying the manure P content was most evident during the first simulated overland flow event. When diet P content was reduced from 5.4 to 3.0 g P kg(-1) DM, a statistically significant reduction in runoff P concentration was observed in all seasons. Elevated P concentrations in overland flow were observed for 28 d in spring and 9 d in summer and winter. The large drop in P concentrations between simulated rainfall events on Day 2 and Day 9 suggests that increasing the time interval between manure application and the generation of overland flow has a greater impact on P losses than does varying the dietary P content.
authors.affiliation: School of Biological Sciences, Queen's Univ, Belfast. sharon.orourke@ucd.ie
authors.forename: S M
authors.initials: SM
authors.lastname: O'Rourke
authors.forename: R H
authors.initials: RH
authors.lastname: Foy
authors.forename: C J
authors.initials: CJ
authors.lastname: Watson
authors.forename: C P
authors.initials: CP
authors.lastname: Ferris
authors.forename: A
authors.initials: A
authors.lastname: Gordon
mesh_terms: D000821:Animal Feed / Q000032:analysis*; D000818:Animals; D002417:Cattle; D003612:Dairying; D004032:Diet / Q000662:veterinary*; D005260:Female; D008372:Manure; D010758:Phosphorus / Q000737:chemistry*; D011891:Rain; D012621:Seasons; D013997:Time Factors; D014872:Water Movements; D014874:Water Pollutants, Chemical / Q000737:chemistry
chemical_list: D008372:Manure; D014874:Water Pollutants, Chemical; D010758:Phosphorus