White clover (WC) (Trifolium repens L.) is a useful component of European grasslands due to: (i) its capacity to convert dinitrogen (N2) gas to plant-available nitrogen (N) in the soil via biological nitrogen fixation (BNF); (ii) its tolerance of grazing; and (iii) its high nutritive value for ruminant livestock. Its relative importance has declined in recent decades in line with the intensification of ruminant production systems that increasingly rely on maize silage and intensively fertilized grass leys. There are many challenges in managing WC on farms. These include: (i) maintaining the ideal balance between the grass and WC in grassland; (ii) low and inconsistent dry matter (DM) productivity; (iii) difficulties with ensilage due to the low herbage DM and sugar concentrations; and (iv) increased risk of bloat. However, the cost of fertilizer N has increased substantially since the late 1990s, particularly relative to the farm-gate price received for milk, beef and sheep meat. This price:cost squeeze has generated renewed interest in the use of WC on farms. Furthermore, under legislation stemming from the Nitrates Directive, permissible stocking densities and rates of fertilizer N input are lower than previously in many European countries, and the lower productivity of WC-rich grassland is not as much of an obstacle to adoption on farms as it has been in the past. As well as the capacity that WC has to improve herbage nutritive value, the main advantage of WC-based systems stems from the replacement or reduction of fertilizer N input by BNF and the contribution that this makes to farm profitability and environmental performance. Although WC-rich grassland has lower productivity, lower fertilizer N costs can largely close the gap in farm profitability between WC-based and more intensively managed systems. There is generally less N circulating within lower stocked WC-based systems, resulting in lower N losses to water and lower ammonia and methane emissions to the atmosphere; losses that are often closely related to stocking density. WC has additional advantages when it comes to the other greenhouse gases: nitrous oxide and carbon dioxide. Direct emissions of nitrous oxide are lower from WC-rich grassland than from N-fertilized grassland at the same level of productivity and substantially lower than intensively fertilized grassland. Emissions of carbon dioxide associated with the manufacture, transport and application of nitrogenous fertilizers are avoided by the use of WC. Using life cycle assessment, studies have shown that WC-based systems have between 11% and 26% lower carbon footprint per litre of milk than N fertilized systems; the largest difference was with more intensive systems reliant on high input of fertilizer N. Escalating fertilizer N costs have improved the profitability of using WC in pasture-based systems in recent years. From the perspective of the overall future sustainability of pasture-based ruminant production, WC-based systems offer economic competitiveness, lower energy dependency and lower environmental impact.
|Title of host publication||Legumes in Cropping Systems|
|Number of pages||18|
|Publication status||Published - 01 Jan 2017|