1 HOW EVENLY ARE N AND P LOADS FROM ANIMAL EXCRETA DISTRIBUTED ACROSS THE FARM LANDSCAPE? Innocent Rugoho * , Sharon R Aarons and Cameron J P Gourley Agriculture Research and Development Division, Ellinbank Centre, Department of Economic Development, Jobs, Transport and Resources, Ellinbank, Victoria 3821 AUSTRALIA * Corresponding author: E-mail: innocent.rugoho@ecodev.vic.gov.au Abstract Nitrogen (N) and phosphorus (P) are key nutrients which drive production in grazing dairy farms. However, if not used efficiently, N and P deficiency can limit production while excesses can degrade water and air quality. Pasture and fertiliser management generally assume that nutrient loads are relatively uniform across a farm landscape. However, there is evidence that cows spend different times in different areas on the farm and uneven nutrient load distribution is poorly understood. The purpose of this paper is to outline key components of N and P flows and transformations within grazing-based dairy operations, and present data on N and P use efficiency and varying nutrient distribution from dairy cows at a grazing scale. We recommend that a greater understanding of animal movements and expected patterns of within-farm nutrient distribution is required to improve N and P management decisions on dairy farms. Keywords: nutrient loading, animal movement, distribution pattern. The ongoing trend for increased intensification is occurring in grazing-based, as well as confinement-based dairy farms, worldwide. Grazing-based dairy farms are increasingly reliant on imported feed and fertiliser, with consequent greater N and P flows, transformations and losses, and a decreasing dependence on N inputs from N 2 fixation by pasture legumes. Across a range of grazing-based dairy systems in various geographic locations, whole-farm level N and P exports in products are commonly 20 to 35% of total farm inputs (Gourley et al., 2012a). While it is important to understand how much N and P is imported and subsequently removed in product on dairy farms, the ability to quantify within-farm flows and transformations, as well as the spatial and temporal distribution of N and P, is essential to identify opportunities for improved management. While flow pathways of N and P on to and out of a dairy farm are generally easy to calculate from readily available data at the farm scale, within-farm N and P cycling processes are more difficult to quantify and may be different between contrasting systems (Rotz et al., 2005). The relationships between N and P intakes and that in milk and manure, have received considerable attention in confinement dairy systems (Powell et al., 2010), but relatively little information is available within grazing systems. This is likely due to the difficulty in accurately determining pasture N and P intakes from preferential grazing and species selectivity by the grazing animal and also variations in pasture quality and mineral contents due to climatic and seasonal conditions (Gourley et al., 2012a).