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Ingestion and excretion of nitrogen and phosphorus by beef cattle under contrasting grazing intensities


R. J. Orr, Rothamsted Research, North Wyke, Okehampton, Devon EX20 2SB, UK.


There is increased global interest in the environmental impacts of farming, including the need to prevent the contamination of soil, water and air with excessive amounts of nitrogen (N) and phosphorus (P) in grazed systems. Reduction in grazing intensity has frequently been recommended to meet biodiversity and production goals in sustainable grazing systems. The objective of this experiment was to measure N and P ingestion and excretion by beef cattle grazing semi-natural pastures at two grazing intensities (Moderate or Lenient). The cattle grazing at Moderate grazing intensity had significantly more defaecations each day than those grazing at Lenient intensity (9·5 vs. 7·5) and tended to have more urinations (7·0 vs. 5·8). For the Moderate and Lenient treatments, respectively, 113 vs. 76 g N d−1 was excreted compared with 136 vs. 94 g N d−1 ingested; 12 vs. 8 g P d−1 was excreted compared with 13 vs. 10 g P d−1 ingested and urine N comprised 0·51 and 0·52 of the total N excreted each day. In improved, intensively managed grassland systems, urine N comprises a much higher proportion (approximately 0·70–0·85) of the daily total N excreted. The lower level found here is likely to impact on potential volatilization, denitrification and leaching losses, and these aspects should be examined further to see the extent to which semi-improved grasslands containing increased plant diversity compared with improved grasslands can deliver higher resource protection, as well as enhanced grassland faunal diversity and abundance.