The key research findings from the review are as follows:
• Even well managed grass/clover pastures remain deficient in nitrogen for much of the year. As a result, it has been shown that grass/clover pasture responds readily to the application of nitrogen fertiliser.
• The response to nitrogen fertiliser application is variable and depends on the rate, timing, carry-over effect of nitrogen fertiliser, and site conditions such as slope, aspect, soil fertility status and pasture composition.
• The best time of year to apply nitrogen fertiliser to grazed pasture depends on when extra feed is required by the animal, and the ability of the pasture to respond to the application of nitrogen.
• The response to late winter/early spring application of nitrogen fertiliser is generally higher and more reliable than nitrogen applied in autumn.
• The application of nitrogen fertiliser can negatively affect the clover content in grass/clover pasture due to reduced stolon number and length, less stolon branching, inhibited biological nitrogen gas fixation, and increased competition for moisture and nutrients by the grass component in the sward. Therefore, a critical management practice to maintain adequate clover is tight grazing to control pasture cover, particularly in the Spring.
• Nitrogen derived from the application of fertiliser can be lost directly or indirectly (nitrogen excreted from animals) from soils under grazed pasture through nitrate leaching, ammonia volatilisation and nitrous oxide from denitrification.
• Direct leaching from the application of nitrogen fertiliser is low if application rates are not excessive and are synchronised with active growth of the pasture.
• Urine excreted by animals is the main factor affecting nitrate leaching losses.
• Ammonia volatilisation losses from fertiliser are typically less than 15% of the nitrogen applied, while emissions of nitrous oxide from nitrogen fertiliser are approximately 4% of total agricultural emissions in New Zealand.
• New technologies such as coated nitrogen fertiliser products, plant growth regulators, advances in precision agriculture and digital technologies have potential to be applied to grazed pasture systems to optimise nitrogen fertiliser use and minimise environmental impacts.
The review concludes: “With increasing expectations from consumers and constraints by regulators on the impact of using nitrogen fertiliser, it is likely there will still need to be innovation in the future to develop new nitrogen fertiliser products and technologies to further minimise environmental impacts and improve the efficiency of nitrogen fertiliser use”.