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Summary of the 2020 Year

Executive summary

Reducing Sediment Loss From Winter Crops – Science Report, May 2021 Update Update

Brendon Malcolm, Nathan Arnold, Lochie MacGillivray, Paul Muir, Noel Smith, Dave Horne5 Plant & Food Research: 1Lincoln, 2Hawkes Bay; 3AgFirst, Hawkes Bay; 4On Farm Research, Poukawa, Hawkes Bay; 5Massey University, Palmerston North

March 2021

Sediment losses to waterways can have major environmental impacts on fresh water quality, and winter grazing in hills country livestock productions systems has been identified as a significant contributor. The aim of the project/trial work is to test the effectiveness of catch crops on the mitigation of sediment losses from these systems.

Three trial sites were set up across the Hawkes Bay and Manawatu regions. One of the sites (Poukawa; aspect 28–30 degrees) consisted of a fully replicated trial with runoff collections equipment essentially capable of collecting and measuring runoff in real-time. The catch crop treatments tested were oats and Italian ryegrass, and were compared to a fallow control. The other two hill country sites (one at Flag Range and the other at Marton) used a more simplified sediment trap system, and compared fallow soil with Italian ryegrass. Overall, soil runoff losses were notably low compared with previous studies. This was likely due to the lack of excessively large rainfall events, as well as a relatively low level of pugging as a result of dry ground conditions at grazing.

At the Poukawa site there was evidence that establishing a catch crop reduced the amount of soil/sediment runoff by up to 38%. This was attributed to protection against surface damage by rain drops during heavy rainfall events, under an established crop canopy.

Establishing catch crops under the cool winter conditions was shown to be more productive than a base simulation where catch crops were not included, when using crop data from the Poukawa site. Oat and Italian ryegrass yields at Poukawa reached 1.9 and 2.4 t DM/ha, respectively in year one and 3.6t and 2.4t again in year two.

For a typical summer dry eastern North Island property of 520 ha there was a modelled net annual benefit in year 1 of $6,894 compared to the base scenario where ground was left fallow. This increased to $29,692 in year two.

For this model farm, Overseer modelling indicated a reduction in N lost from the system in the catch crop scenario compared to the base scenario, with total farm losses reduced by 456 kg N/yr.

Results are likely to be highly variable between years. Therefore, we recommend that the trials are repeated at all three sites in 2020.

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