Estimating nitrogen excretion and deposition in Australian grazing dairy systems for improved nutrient management

Sharon R Aarons1, Cameron JP Gourley1, Mark Powell2, Murray C Hannah1

1 Agriculture Research and Development, Department of Economic Development, Jobs, Transport and Resources, Ellinbank Dairy Centre, 1301 Hazeldean Road, Ellinbank, Victoria 3821, Australia website, [email protected]

2 US Dairy Forage Research Center, USDA Agricultural Research Service, 1925 Linden Drive West, University of Wisconsin, Madison, WI 53706, USA

Abstract

Current nutrient management approaches in Australian dairy systems largely target the application of fertiliser nutrients. However, increasing animal densities and greater reliance on purchased feeds means that nutrient inputs in feeds have increased.  Consequently, the role of grazing animals in nutrient flows and deposition needs to be accounted for in dairy industry nutrient management plans.  However, quantifying nutrient intakes and therefore nutrient excretion is difficult, due to challenges in estimating pasture dry matter intake by grazing cattle.  To quantify N fluxes through grazing dairy cows we modified an animal performance method for estimating annual dry matter intake to calculate daily N intake and excretion.  Using the excretion data, we estimated N loading rates to specific locations visited by the lactating herds within the dairy farms.  The results indicated that these herds received a mean of 52% of their energy requirements from supplementary feeds despite the grazing base of the dairy systems.  Calculated annual N flows through the lactating herds were 60% of total N inputs onto these farms. Mean N intakes (545 g/cow/day) were well in excess of recommended levels resulting in excretion on average of 433 g N /cow/day in these systems.  The resulting deposition of excreted N to pasture paddocks was not uniform, with 30% more N returned to paddocks that were generally closer to the dairyshed.  The smallest mean annual load of excreted N was deposited in the dairyshed and yards.  However, this N load is typically applied as effluent to paddocks closest to the dairyshed which further exacerbates N accumulation and potential losses in these parts of dairy farms.  These results demonstrate that quantifying excreta N loads and spatial nutrient distribution by grazing dairy cows is required for improved N management in grazing system dairy farms.