Eutrophication from Agricultural Sources - Field Plot Study

The Impact of the Grazing Animal on Phosphorus, Nitrogen, Potassium and Suspended Solids Loss from Grazed Pastures - ERTDI Report 68 (Tunney et al.)

Summary: Final Report from the ERTDI-funded project: 2000-LS-2.1.2-M2 - H. Tunney et al

Published: 2007

ISBN: 1-84095-220-2

Pages: 47

Filesize: 3,178 KB

Format: pdf


Ninety per cent of the 4.2 million ha of Irish farmland is grassland. In the past, Phosphorus (P) deficiency limited grassland production in Ireland, and this was corrected by chemical fertiliser use in the 1960s and 1970s. This led to increased intensification of grassland with a doubling of grass yield and of grazing animal numbers – from approximately 3 million to over 6 million livestock units. There is little information available on relative contribution of increased chemical fertiliser use compared to increased grazing animal numbers on P loss to water. The main objective of this study was to assess nutrient loss, particularly phosphorus, in overland flow from cut and grazed grassland plots with a range of soil test phosphorus levels.

Nutrient concentrations and loads from six grazed and cut field plots were studied between September 2000 and March 2004, at Teagasc, Johnstown Castle, Wexford. Overland flow volume and flow proportional overland flow samples were collected and analysed for P and nitrogen (N) fractions. A subset of samples was also analysed for potassium (K) and suspended solids (SS).

Overland flow water samples were analysed for total reactive phosphorus (TRP) in 2000–2001 and for dissolved reactive phosphorus (DRP), total dissolved phosphorus (TDP), and total phosphorus (TP) in 2002–2004. There were significant variations in P concentrations over the seasons and between the six field plots. Concentrations of DRP in overland flow varied form under 0.005 mg/l to over 3 mg/l. The estimated annual DRP loads in overland flow from the plots ranged from 0.1 to 1.2 kg/ha/year. There was a significant linear relationship between soil test P and mean annual DRP concentrations in overland flow. There was more than a ten-fold difference in mean annual DRP concentrations in overland flow between plots with the lowest and highest soil test P levels. This compared to a maximum of 66% increase in DRP concentrations between cut and grazed plots that may be attributable to the presence of the grazing animal.

A significant correlation was found between the three P fractions measured And TDP fractions represented 86% of total P. There was a significant correlation between total dissolved N (TDN) and total nitrogen (TN) concentrations and also between TP and TN concentrations in overland flow water; the latter was about five times higher than the former.

The highest phosphorus concentrations and loads occurred in autumn when overland flow started, after an extended summer dry period (autumn/winter wash-out effect). The difference in mean P concentrations in overland flow between cut and grazed plots, which was most evident on the high soil test phosphorus plots, were also generally highest in autumn/winter. In contrast, there was generally no difference between cut and grazed plots in January and February when concentrations were lowest and before the beginning of the grazing season.

Three factors influencing phosphorus concentrations in overland flow were identified in this work:

  • the highest concentrations were from plots with the highest soil test phosphorus and visa versa;
  • a seasonal phosphorus cycle with high phosphorus concentrations in autumn/winter when overland flow commenced after the summer and decreasing phosphorus concentrations over the following two months and the lowest levels of phosphorus concentrations at the start of the year;
  • a relatively small increase in phosphorus concentrations with grazing compared to cutting treatments on some occasions.

Whether cut or grazed, grassland can have an increased potential for P loss in overland flow compared to other land use. This is because of high surplus P inputs into intensive grassland systems in fertiliser and purchased feeds and the accumulation of the applied fertiliser and animal manure P at or near the soil surface. In contrast to what occurs with tillage soils, this can lead to very high soil P levels in the top few centimetres of grassland soils that are available for release to overland flow water. Therefore, special care is necessary in fertilising and managing grassland soils in order to minimise the risks of P loss to water.

The conclusion that the grazing animal has a limited impact on P losses compared to other factors is in broad agreement with US studies. A five-year study (Tunney et al., 2000) indicated that cattle grazing did not have a significant cumulative effect on P in runoff at the whole pasture scale.

The principal conclusion from this study is that while the grazing animal does influence P concentrations in overland flow, this impact is minor when compared to the other factors (such as soil phosphorus levels) which determine P loss from grassland under standard management regimes.