Nitrate leaching in crop rotations with grass-clover as influenced by sward age, grazing, cutting and fertilizer regimes

Summary

Grazing animals have profound effects on pastoral systems including nutrient removal by grazing and redistribution through excreta. Generally, in grazed pastures the conversion of consumed N into product is low and a substantial quantity of N (>70%) is recycled through the direct deposition of animal excreta. This low N utilization by grazing animals reflects the relatively high concentrations of N required for metabolic functions and optimum growth of plants compared to that needed by the grazing ruminant for amino acid and protein synthesis (Haynes and Williams, 1993). Increasing the N concentration in grass, such as by increasing the rate of N fertiliser application, can result in a substantial N surplus (i.e. N inputs – N outputs in products). For example, N surpluses of 150 to 250 kg N ha-1 yr-1 occur in highly productive dairy farm systems in the Netherlands and northern Germany (Rotz et al., 2005). Grazing cattle return N in urine patches at rates of up to about 1000 kg N ha-1, which is far in excess of plant requirements (Haynes and Williams, 1993). Urine N is in highly mineralisable forms compared to dung N, and within 3-5 days is rapidly converted to plant-available N in soil. This can result in inorganic soil N under urine patches up to 10 times greater than under dung patches, and more than 30 times greater than areas unaffected by excreta (Afzal and Adams, 1992).