Phosphorus Management for Sustainable Dairy Production International Conference: Steps to Sustainable Livestock John Bailey Sustainable Agri-Food Sciences Division (SAFSD)

 BUT, this limit may need to be reduced and extended to all dairy farms to lower P losses to water-bodies and prevent eutrophication problems getting worse  However, further restrictions on farm P imports might critically lower soil Olsen-P concentrations thereby limiting grass production and P content and undermining the P nutrition of high-yielding dairy cattle Background  Under the Nitrates Action Program for Northern Ireland, derogated (dairy) farms are currently restricted to a maximum farm phosphorus (P) surplus of 10 kg P/ha/yr

Objective To determine what level of P surplus is required to optimize soil P, and grass production, and thus minimizing risk of P loss to water, whilst ensuring an adequate concentration of P in dairy cow diets

Methods  Relationships between farm P surpluses, soil Olsen-P, and the P concentrations in dairy cows diets, were explored on 8 dairy farms broadly representative of intensive dairy production systems in Northern Ireland: Herd size: cows Milk yields: l/cow/yr Concentrate use: kg/cow/yr  Farm P imports, exports and surpluses were monitored over a 4-year period and average annual P surpluses calculated for each farm  All fields on each farm were soil sampled and average (area-weighted) soil Olsen-P concentrations per farm determined  Representative silage and grazed fields were herbage sampled 3-times per year and P concentrations determined in both these and samples of concentrate feeds collect twice annually

Methods  Average concentrations of P in dairy cow (whole) diets, i.e. in grazed grass, conserved forage and concentrate feeds, were estimated over a 4-year period based on the following information and assumptions: Known amounts of concentrates fed to cows per annum – from farm records Assumption that 70% of concentrates are fed during the confinement period and 30% during the summer grazing period Concentrations of P in the dry matter (DM) of silage grass, grazed grass and feed concentrates - as analysed Estimated intakes of grass and silage DM by cows based on energy requirements for milk production and maintenance, minus the energy supplied by concentrates (FIM, 2004), and assuming an average weight of 650 kg for high-yielding dairy cows

Results  Average soil Olsen-P concentrations for farms were significantly correlated with average farm P surpluses for the 4-year period  The relationship indicated that the mid-point of the target Olsen-P 2+ range (20-25 mg P/l), associated with optimum grass production and minimum risk of P loss to water, could be achieved with a P surplus of 5 kg P/ha/yr Farm P surplus (kg P/ha/yr) Soil Olsen-P (mg P/l) 5 kg P/ha Average farm Soil (Olsen) P versus Mean farm P surplus Mid point index 2+

Results  Mean concentrations of P in grazed swards and silage swards (averaged over the growing season) were significantly correlated with average farm P surpluses  The relationships indicated that a P surplus of 5 kg P/ha/yr was more than sufficient to maintain concentrations of P in both types of swards above the critical concentration of 2.5 g P/kg DM (De Wit et al., 1963) Herbage P (mg P/kg DM) Farm P surplus (kg P/ha/yr) 2.97 g P/kg DM 2.66 g P/kg DM Grazed Swards Silage Swards

Results  A significant correlation was also found between farm P surpluses and P concentrations in dairy cow (whole) diets  Optimal dietary P concentration for high- yielding cows (3.6 g P/kg DM) (Ferris et al, 2010) was associated with a P surplus of just 2.7 kg P/ha/yr Farm P surplus (kg P/ha/yr) Farm P surplus versus P concentration in dairy cow whole diets Dietary P (g P/kg DM) 2.7 kg P/ha Optimum 3.6 g P/kg DM

 A farm P surplus of 5 kg P/ha/yr appears to be sustainable target for intensive dairy farms in Northern Ireland:  It maintains grassland within the optimum soil Olsen-P range (Index 2+), thus minimising risk of P loss to water, and neither compromises grass production nor dairy cow nutrition Conclusion

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