Nitrogen fertilizer replacement value of dairy soiled water in grass swards as affected by timing and rate of application

Dairy soiled water (DSW) is a dilute mixture of cow faeces, urine, milk and detergents produced from the washing of milking plant and holding areas, and contains nutrients. Its N fertilizer replacement value (NFRV), compared to inorganic fertilizer (IF), was determined in a plot experiment on two grassland soils (well-drained and poorly drained) in SW Ireland over a year with 4 N rates (0, 15, 22 and 30 kg N/ha), nine application timings and four replicates. Herbage was harvested at 4 and 8 weeks after each application to determine the total 8-week DM yield. While caution is required, as results are based on only two sites and one year, DSW increased herbage production and showed potential to substitute for inorganic fertilizer N with surprisingly high NFRVs of 72 to 90%, potentially offering cost savings to farmers, improved N use efficiencies and reduced risk of nutrient loss to the environment. High NFRV may be due to factors such as the even spread and infiltration of DSW, low rates of ammonia emission or a stimulatory effect on the release of soil N. Yield increased significantly (p < .001) with N rate from approximately 1,200 kg DM ha(-1) at 0 kg N/ha to approximately 1,700 kg DM ha(-1) at 20-30 kg N/ha but application beyond approximately 20 kg N/ha was associated with increasingly diminished yield response. Yield response was greatest from March to July and poor response in winter suggests a greater risk of N loss during this period.