Eutrophication from Agricultural Sources: The Impact of the Grazing Animal on Phosphorous Loss from Grazed Pasture

ERTDI Report 66 - Final Report - Tunney et al

Summary: Final Report for the ERTDI funded-project 2000-LS-2.1.2-M2 - This report is a synthesis of the findings from the 3 sub-projects.

Over 90% of the 4.2 million ha of farmland in Ireland is under grass. Most of this is grazed and approximately 25% of this area is cut at least once per year. Little information is available on the impact of the grazing animal on phosphorus (P) loss from grassland soils to water. The main objectives of this project then were to:

• Review existing information on P loss from grazed grassland.
• Measure the P and nitrogen (N) loss from grazed and cut grassland on a number of soils.
• Identify the most important factors influencing P loss.
• Investigate physical, chemical and biological processes that affect P fluxes.
• Propose remedial measures to reduce P loss from grassland.

The project was carried out in three parts:

• Field-plot study – The Impact of the Grazing Animal on Phosphorus, Nitrogen, Potassium and Suspended Solids Loss from Grazed Pasture. The nutrient concentrations and nutrient loads from grazed and cut field plots (0.24 to 1.54 ha each) were studied in this experiment which started from September 2000 and finished into March 2004, at Teagasc, Johnstown Castle, Co. Wexford.
• Small-plot study – The Impact of the Grazing Animal on Nutrient Losses to Water. The small-plot (1.5 m x 15 m each) studies assessed the impact of the grazing animal on nutrient losses to water. These were carried out at four sites over two years (spring 2002 to spring 2004). Two of the study sites were located at Teagasc, Johnstown Castle, Co. Wexford and two at Teagasc, Grange, Co. Meath.
• Phosphorus Dynamics in a Grazed Grassland Ecosystem – This study investigated the pools and fluxes of P in grazed grassland (mainly in 1.5 m2 field plots and laboratory studies, from August 2002 to July 2004).

This report is a synthesis of the main conclusions and recommendations from these three studies. The final reports from the three individual subprojects are available from the Environmental Protection Agency (EPA) and are published on the EPA website (www.epa.ie) at:

• Tunney, H. (in press). Eutrophication from Agricultural Sources – Field Plot Study – The Impact of the Grazing Animal on Phosphorus, Nitrogen, Potassium and Suspended Solids Loss from Grazed Pasture, ERTDI Report 68, EPA, Ireland.
• Kurz, I. and O’Reilly, C. (2006). Eutrophication from Agricultural Sources – Small Plot Study – The Impact of the Grazing Animal on Nutrient Losses to Water,  ERTDI Report Series No. 41, EPA, Ireland.
• Bourke, D., Jeffrey, D., Dowding, P., Tunney, H. and Kurz, I. (in preparation). Eutrophication from Agricultural Sources – Phosphorus Dynamics in a Grazed Grassland Ecosystem, EPA, Ireland.

The principal conclusion from this study is that the grazing animal can have a significant influence on total P concentrations in overland flow but not on soluble P concentrations. This impact is minor when compared to the other factors (i.e. effect of soil P level) that determine P loss from grassland under standard management conditions. The wide variation in P concentrations between the six field plots was related predominantly to factors (i.e. accumulated surplus applied P reflected in soil test P) other than the presence or absence of grazing animals.