Abstract
A four-treatment (UG-UG, UG-G, LG-G and HG-G) experiment (involving sixteen plots: 3·0 × 7·0 m) examined the effect of early spring grazing intensity on particulate phosphorus (PP) losses in surface run-off. Ten dairy cows fitted with manure collection ‘bags’ grazed during two short-term grazing events, Grazing-1 (23 February) and Grazing-2 (6 April). During Grazing-1, two treatments remained ungrazed (UG-), while treatments LG- and HG- were lightly grazed and heavily grazed respectively. At Grazing-2, three treatments were grazed to a similar intensity (-G), while one remained ungrazed (-UG). Run-off was generated at two and 16 days after Grazing-1 and Grazing-2 using rainfall simulators (40 mm h−1) and analysed for a range of P fractions. Grazing had no effect on either dissolved reactive phosphorus (DRP) concentrations or dissolved unreactive P concentrations (mean, 0·15 and 0·16 mg L−1 respectively) in run-off. However, PP concentrations increased (P < 0·05) following Grazing-1 (0·39, 0·53 and 0·72 mg L−1 with UG-, LG- and HG- respectively, Day-2 Rainfall event), with these effects still evident following Grazing-2, especially with HG-G (3·25 mg L−1). The risk of PP loss in run-off can be substantially reduced by removing cows from pastures before significant damage to the soil takes place.
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