FRNL New Knowledge Register

A meta-analysis of published data and crop sequence simulations with APSIM identified options to maximise the N use efficiency of forage and conserved feed crops grown in sequence with arable crops. This included crops to mop up early spring mineral N loads following winter grazing of crops and early autumn sown crops to mop up residual N from fertiliser and N mineralisation following summer crops. Multi-year field trials in Canterbury and Waikato validated and demonstrated the most promising crop sequences identified. Results informed the crop management guidelines for fodder beet and catch crops. K EY RESULTS OF THE WORK UNDERTAKEN • New FRNL field data and modelling concepts were used to develop two new biophysical models within the modern APSIM Next Generation (APSIM-X) platform. Specifically, Fodder Beet and Oats APSIM-X models are now officially released by the APSIM initiative and freely available through their repository. • The new APSIM-X models were used to investigate FRNL relevant questions. Key new insights using these models include: o Winter sown catch crops after fodder beet, a new usage of catch crops, shows potential for reduction in N leaching but results are more impacted by weather variability than conventional autumn-sown catch crops. o The ranges of catch crop effectiveness to reduce N leaching differs across NZ climate zones and soils, so spatial analyses are relevant. o Best management (e.g. early sowing) enhances catch crops effectiveness in winter grazing systems, as for autumn-sown conventional systems, although the maximum effectiveness achieved is lower for winter-sown crops. o An estimate of catch crop effectiveness for four regions, four sowing dates and three rainfall categories was produced using the new model capability and showcased in the catch crop management guidelines. • Autumn and winter-sown cereal catch crops can successfully establish under field conditions and reduce the risk of N leaching in Waikato, Canterbury and Southland study sites. Establishment success differed annually and locally due to weather conditions and the previous crop in the rotation. • The earlier the catch crop is established, the greater the potential to reduce N leaching, regardless of the time of year (even in winter). Positive economic and environmental returns were found in most years across all field trial sites but values depend on inter-annual variability (namely rainfall). • Key mechanisms by which catch crops reduced the risk of N leaching were plant-N uptake and water use. Nevertheless, other processes may also be important, such as microbial immobilisation from root exudates (carbon source), and reduced soil temperatures, slowing mineralisation and nitrification rates. These require further investigation. • In simulated urine patches winter-sown catch crops can produce yields of up to 12 t DM/ha at green- chop silage maturity (approximately mid-November) in all regions with potential to increase annual biomass production. In Southland, yields accumulate marginally slower than in the other regions, due to cooler temperatures. • Catch crops can remove significant amounts of soil mineral N. For example, approximately 40 kg N/ha was measured in above-ground biomass by early September, 140 kg N/ha by mid-October and >200 kg N/ha at green-chop silage maturity in November, on average in Canterbury. • Most of winter-sown catch crop biomass accumulates during October/November. At this time, most plant N uptake has already occurred and the crop progressively dilutes the N concentration. • Financial analysis has shown that oat catch crop yields of 6.1–10.9 t DM/ha, in the context of a real farm system, can result in returns of $855–1913/ha for standing feed (assuming $0.25/kg DM).

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