Dairy farmers in New Zealand are encouraged to adopt a range of management strategies, both well established and emerging, to reduce nitrogen (N) losses to waterways.
In most regions the OverseerFM nutrient budgeting software (hereafter referred to as Overseer) is the tool of choice in the assessment of N losses for both regulatory and monitoring purposes.
Overseer can be used to assess the impact of improved farm practices on N leaching and runoff from individual farms.
Here is a summary of a paper published in Grasslands of the basic overview of the management strategy duration-controlled grazing. There are other papers which cover this study in more detail so let us know if this is something you would be interested in.
Key Points:
Duration-controlled grazing resulted in a 36% reduction in total N loss.
There was good agreement between the Overseer predictions of N leaching and values measured at the trial site.
Farmers can use duration-controlled grazing as a management strategy to reduce N loss that will translate Overseer.
A 3-year dairy system field trial at Massey University was run from 2008 to 2010. N losses in leaching and runoff under duration controlled grazing were compared with those under standard grazing management.
Duration-controlled grazing is a practice which involves removing cows from the pasture either after a set interval, or, when a target pasture residual has been reached.
This system has a number of potential benefits. Duration-controlled grazing can reduce deposition of urine and dung on soil, thereby reducing nutrient losses. It results in shorter grazing intervals, therefore, damage to wet soils and pasture can be reduced.
With the use of stand-off areas, supplementary feeding can improve feed utilisation.
The trial site was located on flat land, <3% slope, with an average rainfall of 1000mm/year.
The pasture was ryegrass and white clover based on a Tokomaru silt loam soil which is classified as a Fragic Perch-gley Pallic Soil.
The trial plots were 850 m2/plot, each with an individual mole and pipe drain system. Within each grazing plot there was also a 50m2 surface runoff sub-plot to measure the nutrients lost by runoff.
Treatments:
Duration-controlled grazing (4 hours per grazing).
Standard grazing (7 hours per day-grazing, 13 hours per night-grazing).
The duration controlled grazing cows were allowed to graze for 4 hours after each milking resulting in 2 grazing events per day at 4 hours each (8 hours of grazing total).
The remaining time other than milking (~12 hours) was spent occupying a stand-off facility such as a freestall-barn pictured below.
The total nitrogen leaching losses across the 3 years are shown in Figure 1 below. The greatest N loss measured for both treatments was in 2011. The variation between the years can be explained by the difference in rainfall and drainage patterns.
The increase in rainfall often results in more drainage events occurring, therefore potentially more N loss occurring through drainage.
The annual rainfall was 1245, 1032 and 1220 mm/year for the 2008/2009, 2009/2010 and 2010/2011 years (September–September). Although the rainfall was similar in the first and third seasons, the third season had a more concentrated rainfall over the spring and winter months.
In the years with more drainage events the practice of duration-controlled grazing will show a greater response when compared with standard grazing than the years with less drainage occurring.
Even though there is seasonal variation, there is consistently a greater total N loss from the standard grazing system compared with the duration-controlled grazing treatment.
The surface runoff nitrogen was 3.2 kgN/ha for standard grazing and 2.9kgN/ha for duration controlled grazing. When the surface runoff values were added to the drainage nitrogen values, the overall total nitrogen loss for the standard treatment was 22kgN/ha/year compared with 14 kgN/ha/year for the duration-controlled grazing treatment
This is a reduction of 36% in total nitrogen under duration-controlled grazing compared with standard grazing on the same soil type.
The soil type on this farm is a poor draining soil type and these nitrogen leaching losses are typical for the soil type.
The standard and duration-controlled grazing scenarios were modelled in Overseer to compare the modelled values to the actual values measured.
The Overseer estimated nitrogen loss values were very close to those measured in the drainage water and runoff water as shown in Figure 2 below. The duration-controlled grazing losses were the same at 13 kgN/ha/year for both the overseer predicted (purple stripe) and measured N loss (solid purple). The Overseer modelled for the standard grazing was 1 kgN/ha/year greater (23kgN/ha/year, blue stripe) than the measured N loss (22kgN/ha/year, solid blue).
Farmers adopting duration-controlled grazing as a mitigation strategy will only be able to claim the reduction in the total nitrogen losses estimated by Overseer. There was good agreement between the Overseer predictions of N leaching and values measured at the trial site for both grazing treatments.
As there was agreement between the modelled and actual total N loss, duration-controlled grazing is a worthwhile management strategy to consider on farm to reduce N loss to water.
This means that the mitigation strategy of duration-controlled grazing can be used on farm and closely modelled in Overseer, thus the farm getting a tangible value to use for both regulatory and monitoring purposes.
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