Forages for Reduced Nitrate Leaching, Bill and Shirley Wright (Canterbury)

Production of grass-based pasture and lucerne was strongly affected by temperature and soil moisture across the four years monitored.

• Production of the grass pasture was affected more by the drought in summer 2015-16 than lucerne with its deep tap root.
• Lucerne was less productive in the wetter 2016-17 and 2018-19 seasons than the grass pasture.

Growth of grass-based and lucerne pastures on Highlands during 4 years to June 2019.

• Diverse pastures were only transient under sheep and beef grazing and rapidly transformed into grass-dominant swards.
• Plantain did not persist under current sheep and beef grazing management and therefore could not be expected to decrease N leaching without substantial changes to current grazing practices.

Plantain was incorporated into diverse mixtures on the farm following discoveries about plantain’s potential to reduce N leaching. Pasture production and proportion of plantain in the mix were measured for the 2017-18 and 2018-19 seasons. These young mixed pastures were very productive in comparison with the older grass or lucerne pastures as shown in the table below.

Total dry matter production (kg/ha) assessed using pasture cages from 12^th Sept 2017 to 26^th Sept 2018 (2017-18), and 26^th Sept 2018 to 17^th June 2019 (2018-19).

As shown in the figure below the amount of plantain in the pasture mixes was very low and was never more than 13% of the herbage available in the first season, by 2018 there was very little plantain observed, and none in 2019.

The changes in proportion of plant species in a mixed pasture sown in autumn 2016.

The use of grazing maize was shown to be more successful on this farm, with its high risk of summer-autumn drought, than growing plantain pastures which have a much lower herbage yield.

The soils and climate on this farm are suitable to grow maize to be used as a forage crop in late summer and autumn. Grazing maize suits this dryland farm very well because feed can be carried from spring and early summer into periods where there is a risk of drought. There is a risk of crop failure, some crops were frosted shortly after sowing but recovered, and in the most recent year a crop was drowned out shortly after sowing. Despite this risk, the crop has given substantial yields in some years, as shown in the table below.

Maize production assessed pre-grazing over five years.

Grazing of maize is not easily represented in the Overseer model. Maize is a low protein feed meaning that the N excreted in the urine is likely to be lower than with pasture.

However, the high crop yields meant that a large number of cattle were required to graze the crop which is likely to leave a significant amount of nitrate in the soil.

On this farm it was best to use the crop in early autumn before the stems resist grazing later in autumn. This also allowed Italian ryegrass or pasture to be sown after grazing, which then functions as a catch crop taking up soil mineral N, reducing risk of N leaching.

Grazing maize

Earlier sowing of catch crops tended to have a yield advantage and greater cost effectiveness. It is worth considering earlier sowing of catch crops wherever and whenever possible.

Catch crops were shown to reduce the risk of N leaching through their ability to take up N remaining in the soil following winter grazing of fodder crops and appear to be the most appropriate tool to reduce the high N loss risk associated with winter grazing of fodder crops.

A catch crop trial was carried out following a 2018-grazed crop of kale. Catch crops, sowing dates and cultivation are summarised in the table below. There were also control plots which were kept fallow. At the start of November plots were split in half and SustaiN fertiliser was applied to one half of each plot. Final harvest was taken when crops were ready to be taken for whole crop silage, this was 21/12/2018 for triticale, ryecorn and wheat, and 7/1/2019 for the remaining crops.

• Light cultivation assisted seedling establishment and ultimate production from oats. Direct drilled oats that did not receive additional N fertiliser during the growing period were one third less productive.
• Triticale was most productive when additional fertiliser was applied during the growing period.
• For those which received no added N, oats with some tillage were very productive compared to other crops.
• Average levels of soil mineral N in the soil profile were slightly lower at the final harvest (30-40 kg N/ha) than they had been at sowing (40-60 kg N/ha). This indicates that catch crops have the potential to reduce leaching.
• The season the trial was run was dry prior to sowing and very wet later which meant pugging was minimal. Heavy soils have great ability to hold on to N.

Total dry matter production (kg/ha) assessed using quadrats within plots on three occasions during the growing season for different species and type and timing of sowing, and application of N in the growing season.

Catch crop trial 2018.