Nutrient limits are increasingly a fact of life for farmers throughout the country as regional councils implement plans to address water quality. The actual limits and how those are managed depend on the region or catchment you farm in. Nitrogen and phosphorus are the key nutrients that need to be managed to ensure water quality is maintained.
With a low milk payout forecast this season, many farms will be considering changes onfarm to preserve cash flow. These changes could affect nitrogen losses to water. Some regional councils require average nitrogen losses over several seasons to be within set limits, so changes in farm practice this season could affect your freedom to operate in subsequent seasons.
Nitrogen losses to water are determined by the quantity of nitrogen that washes through soils, below the plant root zone and through into ground-water or drains. Things which influence this are:
• The amount of nitrogen introduced on to the farm, through fertiliser and supplementary feed, or fixed by clover.
• The quantity of nitrogen exported from the farm as products such as; milk, meat, animals or silage
• The number and type of animals grazing onfarm through the season, as animals redistribute nitrogen across the farm. A cow urinating concentrates the equivalent of 500-1000 kg/ha of nitrogen in a single spot. Conversely, infrastructure, such as standoff pads, allows this to be captured and reapplied over a larger area.
• The soil type and annual drainage, as soils vary greatly in the quantity of water and nutrients they can store.
• The number of days available for plants to grow and use any nitrogen in the soil before it is vulnerable to drainage below the rooting zone.
• Reducing the use of purchased supplementary feed and adjusting the stocking rate down to match the feed supply is one response to reduce expenses with a lower payout. This reduces the intensity of farming through autumn and should reduce nitrogen losses.
Lincoln University Dairy Farm adopted this approach last year, to reduce nitrogen losses rather than for financial reasons. Nitrogen fertiliser was applied as planned into the autumn but no supplements were used after mid-February and empty and cull cows were dried off and sold earlier than usual to remove their feed demand from the milking platform.
The impact of this approach on the farm operation was:
• From March, cow numbers were reduced by 10-15% from the previous year.
• Feed inputs (drymatter/cow) were about 10% lower overall.
• Milk production was 8% lower overall.
• Overall farm profit was reduced by $500/ ha, at an $8.40 payout.
• Nitrogen losses to water were reduced by 15-20% from what was predicted had the farm continued buying in grass silage to support all cows milking through autumn.
Lower supplements brought in meant less nitrogen coming on to the farm, and early removal of empty and cull cows lowered the autumn stocking rate. Both these changes reduced nitrogen losses but the reduction was greater because the changes were made in autumn. Farming practices in autumn have a greater impact on nitrogen losses than in summer, because there is less time for growing plants to use any nitrogen before it drains through the soil with late autumn or winter rain.
Another response to the lower payout is to put some of the milking platform into crop and winter some cows on the platform instead of sending them out to external grazing.
This approach risks significantly increasing nitrogen losses. Nitrogen applied by cows through urine in winter is vulnerable to washing quickly through wet soils. Low plant growth rates also result in less uptake of nitrogen by plants.
Other analysis by Lincoln University Dairy Farm showed that nitrogen losses from their winter cow grazing block were equivalent to 25% of the losses on the milking platform. This was based on most cows wintered off for 8-10 weeks. If those cows were wintered on, a substantial increase in nitrogen losses was likely. However the size of the increase depended on a range of factors such as overall changes in stocking rate, milk production and supplement use.
Another Canterbury dairy farm, on free draining soils, considered wintering half their dry cows at home through June and July. This involved putting 10% of the milking platform into forage rape in January, and buying additional supplements to support autumn feed demand. This scenario, modelled using Overseer, increased their nitrogen losses by 16%. However, if they culled the bottom 10% of the milking herd in February, the nitrogen loss was reduced to a 10% increase, but with a 3% drop in milk production.
There are three things you need to think about as you get ready for autumn:
• Understand what the nutrient limits are in your region or catchment. Regional councils are at various stages of putting the rules in place and limits vary.
• In some regions, nutrient limits are averaged over several years. If you achieve losses less than the limit this year, then you may be allowed losses greater than the limit next year. As long as you are within the limit on average this could allow you to push production harder when payout is high and pull back on production when payout is low.
• If you are planning to adjust your farming practice this autumn and winter, check with your farm consultant or nutrient advisor to find out how this might affect your nutrient losses.
