Reviewing 50 years of NZ use

For several decades research has been done on establishing the scientific basis for nitrogen management and fertiliser application in ryegrass and clover pastures. However, with so much research going on across various locations over the years, it hasn't always been easy to step back and see the full picture.

That is what prompted AgResearch senior scientist Dr Col Gray, with the backing of the Fertiliser Association of New Zealand, to take a closer look at the research undertaken over the last 50 years. He set out to provide an overview of this research, to raise the awareness of key findings, and importantly to provide a pathway for decision makers to find and access the research on nitrogen fertiliser use and management. Identification of knowledge gaps for new avenues of enquiry was another focus.

The result of this review is now published in the New Zealand Journal of Agricultural Research. The full open access article, Nitrogen fertiliser use in grazed pasture-based systems in New Zealand: a summary, can be viewed at https://www.tandfonline.com/doi/full/10.1080/00288233.2023.2198719.

The key research findings from the paper 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 nitrogn 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".