Friday, March 29, 2024

Guidelines to cut phosphorus loss

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West Coast dairy farmers could cut phosphorus (P) losses by about 80% by adopting new farm dairy effluent irrigation guidelines designed for farmers in high rainfall areas. At the same time they could make some significant financial savings by capturing nutrients. AgResearch scientist Dr Seth Laurenson reported findings from studies carried out in the Lake Brunner catchment at the Fertilizer and Lime Research Centre workshop at Massey University in February. Low-rate application of farm dairy effluent to a maximum depth of 10mm at least 48 hours after rainfall can dramatically cut P run-off into waterways compared with direct pond discharges.
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Laurenson said low rate meant 3.6-4mm/hour and sticking to that, as well as the depth limits, was essential to achieve good capture of nutrients in soil.

The difference between guidelines for the West Coast areas and most of the country is that effluent irrigation can be carried out when soil moisture deficits are lower than those recommended elsewhere, which means irrigation can happen when soils are wetter. However, periods of heavy rain should be avoided Laurenson said West Coast farmers struggled with common guidelines because they had to contend with rainfall of 3.5m/year or more.

They also had high-risk soils in many cases and humping and hollowing of paddocks created slopes that could amplify the risk already created by poorly drained soils, because it increased the likelihood of run-off.

By applying effluent at a low rate and to a maximum depth of 10mm farmers could more safely use the temporary water-holding ability of the soil that’s available between field capacity and saturation. Laurenson said that gave a much narrower margin for error than farmers had in other areas, where the guidelines advised application of effluent to a depth less than the soil water deficit and not to go above field capacity.

Storage

If those national guidelines rules were applied to farmers in the Lake Brunner area a farmer with 450 cows would need storage of more than 300,000 cubic metres (m3) and even then, without a low-rate approach to irrigation they’d still get run-off in some circumstances.

The study carried out to determine the guidelines took measurements on three dairy farms through the 2011/12 season, recording pond discharges to determine the total nutrient loading.

On one farm an irrigation trial was also set up where 10mm of effluent was applied irrespective of the magnitude of the soil water deficit. Surface run-off was monitored for P concentration and volume. Based on the findings, annual P losses for a 450-cow farm with 1240 square metres (m2) of yard and 3450mm/year rainfall were estimated to drop from 545kg P if direct discharge was used to 115kg when the irrigation guidelines were followed.

The volume of annual farm dairy effluent loss dropped from 40,000m3/year to 16,410m3.

While it didn’t cut out P losses altogether, adoption of the guideline across the catchments would be expected to create significant benefits to the region’s waterways, particularly P-sensitive water bodies such as Lake Brunner.

Farmers would also benefit from the guidelines in that they’d be able to keep required storage areas to a more practical size and they’d capture the nutrient value from the effluent.

That value was estimated to be about $32/cow/year, or $14,400 for a 450-cow farm.

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