The effect of early spring grazing and dairy cow grazing intensity on particulate phosphorus losses in surface run‐off

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.