Nutrient Loss Research at Lake Brunner

AgResearch, on behalf of DairyNZ, has estimated the reduction in P loss that could be achieved by various management options on dairy farms within the Inchbonnie catchment, Lake Brunner. The lake is particularly sensitive to increases in soluble P from rivers and steams.

Options included:

• Optimising soil Olsen P (ie. Decrease on many properties)

• Using low solubility P fertiliser

• Land irrigation of dairy and yard effluent

• Fencing of streams

• Construction of herd shelters

Five farms were chosen and the farmers involved each chose three options to be modeled for their property. They then chose two options to use.

The most cost effective options were the first two. AgResearch estimated that P losses could be reduced by around 20% on a typical farm.

Low-rate irrigation of effluent onto humped and hollowed land is a better option than direct discharge to surface waters in this high rainfall area. However, the instantaneous rate of application should be less than the soil infiltration rate so as to limit direct loss of P in surface run-off.

The important messages for effluent irrigation are:

• Nutrients are of concern in this lake system

• Current guidelines don’t work on the West Coast due to high rainfall

• The framework (national irrigation guidelines) needed adapting for this region

• Huge gains in applying effluent to land and not to lakes

• Low rate and low depth important

• Still avoid wet periods

The West Coast is a high rainfall area and so there is considerable potential for nutrient runoff from farms. Most rivers on the Coast have a relatively short run to the sea so the amount of P lost to them is not especially significant from an environmental point of view. However, in the Lake Brunner catchment many streams flow into the lake, and because of its 3m annual rainfall and the use of mole drains there is a direct and rapid link between farms, streams and the lake. Consequently P is accumulating in the lakewater and much of it is slowly incorporated into the lake sediment.

Currently, algal growth is limited by the relatively low P levels but if those levels continue to rise, a point will eventually be reached where ready availability of P will lead to rapid algal growth. This will create anoxic conditions just above the lakebed and P will be released from the sediment, making the situation much worse.

Phosphorus finds its way into Lake Brunner through:

• Direct runoff when rain falls after the application of phosphate fertilisers

• Leaching from soils when Olsen P levels are too high for the soil to retain P

• Direct discharge of manure from animals when cross or stand in waterways

• Runoff from stock races, stand off pads, paddocks

• Discharge of treated effluent from ponds

A study funded by DairyNZ and carried out by AgResearch has modelled a number of on-farm methods for reducing P loss to waterways and assessed their effectiveness and costs.

Recent work done by AgResearch investigated cost effective and practical methods to reduce P loss on dairy farms.

Two projects were carried out by AgResearch. The first identified on-farm management strategies that farmers in the Inchbonnie catchment of Lake Brunner could use to reduce nutrient loss. Farmers chose strategies that best suited their farms that were determined through discussions with the farmer. These included:

• Optimising soil P levels

• Use of low-solubility P fertiliser

• Fencing streams and drains and grass buffer strips

• Restricted grazing

• Effluent pond storage and low rate land application

• Constructed natural wetlands

• Herd shelters/standoff pads

Five farms were chosen and farmers involved chose three mitigation strategies that they thought would suit their property and management system. AgResearch estimated the P reduction of each strategy using OVERSEER® and the cost using Farmax DairyPro, expressing cost effectiveness in terms of the cost per kg of P retained. The farmers then each chose two of their two preferred options to implement and the combined reduction in P estimated. Generally they selected the most cost-effective, but in some instances the chosen mitigation strategy was the one that best suited the farming management system.

Estimated reduction in P loss ranged from 12 to 37%, reflecting the different options chosen by the farmers and also differences in farm systems. The average was 21%, and AgResearch believes that this figure could be achieved by other farms in the catchment simply by optimising soil P levels and applying low solubility P when necessary.

Application of dairy effluent to land could also have a significant effect on reducing P loss and retaining useful nutrients. The AgResearch soil and landscape risk framework for Farm Dairy Effluent (FDE) management has been developed for most areas of New Zealand. However, in such a high rainfall area the amounts of dairy effluent are large and there are seldom soil moisture deficits large enough to allow safe application onto land. Our work focused on low rate application of effluent to land as an alternative to direct discharging to rivers.

This project also quantified nutrient losses in the direct pond discharges of effluent to streams throughout the year, and assessed the suitability and practicality of a low-rate effluent application to land.

Losses from three monitored discharging pond systems showed an average loss of nutrients valued at around $30 per cow indicating both a financial loss and considerable risk to Lake Brunner water quality.

The trial applied effluent to land during the lactation season as an alternative to discharging to rivers. Effluent was applied to humped and hollowed land, and nutrients losses from these plots were measured and compared with losses from control plots during the irrigation event and subsequent rainfall. Application to land was effective in reducing N and P nutrients losses by around 90% and K losses by half. Losses from these plots were only slightly higher than from the controls that had no effluent applied, indicating that effluent application to land was a very effective way of mitigating losses.

AgResearch says that the frequency of application could be increased without risk.

However, to avoid surface runoff the rate of application should always be less than the instantaneous infiltration rate of the soil. Irrigation systems with high instantaneous application rates are unsuitable for high risk soils in sensitive catchments where irrigation cannot be deferred.

Based on this study, a modified version of the AgResearch soil and landscape risk framework for Farm Dairy Effluent (FDE) management has been developed for the West Coast. These modifications allow effluent to be applied to soils in excess of the soil water deficit. However the depth and rate of application is still important.

The recommendations are that:

• Apply a depth of FDE no greater than 10 mm and 50% of plant available water on any given day

• Ensure that the instantaneous rate of FDE application is less than the soil infiltration rate (if no soil water deficit exists)

• Do not commence application when soils are draining following rainfall.

AgResearch estimates that if these recommendations are followed the loss of P, the nutrient of greatest ecological concern in this environment, can be reduced by approximately 80%.

Ideally, covered yards and stand-off and feed pads should also be considered to minimise the rainfall contribution to total effluent volumes.