Environmental Impacts Environmental Impacts of dairying in Canterburyof dairying in Canterbury

Ross RedpathRoss RedpathRoyal society teacher fellow 2003Royal society teacher fellow 2003

Implications of increased dairy farmingImplications of increased dairy farmingon Central Canterbury Plainson Central Canterbury Plains

Land in sheep grazing (and cropping), is Land in sheep grazing (and cropping), is being converted to dairy farmingbeing converted to dairy farming

Dryland areas are being converted to Dryland areas are being converted to irrigationirrigation

Increased use of nitrogen fertilisersIncreased use of nitrogen fertilisers Change of grazing animal from sheep to Change of grazing animal from sheep to

cowscows Stock density increasing to about 3.3 Stock density increasing to about 3.3

cows/hacows/ha Increased dry matter production from Increased dry matter production from

pasture pasture

Issues for groundwater qualityIssues for groundwater quality Increased soil fertility implies larger Increased soil fertility implies larger

pool of soil nitrogen for conversion to pool of soil nitrogen for conversion to mineral forms (nitrate)mineral forms (nitrate)

Irrigation increases soil-water drainage, Irrigation increases soil-water drainage, which can increase leaching of this which can increase leaching of this additional nitrate to groundwateradditional nitrate to groundwater

Removal of groundwater for irrigation Removal of groundwater for irrigation decreases the ability to dilute decreases the ability to dilute contaminants, such as nitrate which are contaminants, such as nitrate which are leached into the aquiferleached into the aquifer

Relevant parts of the Relevant parts of the nitrogen cyclenitrogen cycle

Fertiliser & legumes

Soil organic matter

NH4+

Pasture

NO2-

NO3-

Leaching

The journey through the The journey through the unsaturated soil zoneunsaturated soil zone

Excess soil water, from rainfall or Excess soil water, from rainfall or irrigation, trickles down through the irrigation, trickles down through the sands and gravels towards the sands and gravels towards the groundwater surfacegroundwater surface

Movement is highly intermittent, Movement is highly intermittent, because it is climatically drivenbecause it is climatically driven

The average velocity of groundwater The average velocity of groundwater is a few metres per yearis a few metres per year

Nitrate leaching from Nitrate leaching from dairy farmingdairy farming

The main factor is the level of milk The main factor is the level of milk production per hectareproduction per hectare

Pasture has a lower C:N ratio than that Pasture has a lower C:N ratio than that required for milk production and body required for milk production and body maintenancemaintenance

The excess nitrogen is excreted in the The excess nitrogen is excreted in the urineurine

Urine patches contain more nitrogen Urine patches contain more nitrogen than the pasture can use, and the than the pasture can use, and the excess is leachedexcess is leached

Typical soil nitrate-N Typical soil nitrate-N leachate levelsleachate levels

Land use Main cause ofnitrate loss

Nitrate-N inleachate (g/m3)

Horticulture Inefficient fertiliser use 30

Arable farming Cultivation 24

Dairy farming Urine from cows 12

Light grazing Animal urine 8

Nitrogen system for dairy productionNitrogen system for dairy production(approximate N gains/losses - kg/ha/y)(approximate N gains/losses - kg/ha/y)

Milk& meat

Pasture/soil~ 5000 kgN/ha

3.3Dairy cows/ha

Feed Urine Dung

Leaching

Ammonia lossDenitrificationFertiliser

200

30

400 220 110

70

100

Key ideas about leaching Key ideas about leaching from agricultural soilfrom agricultural soil

High production agriculture requires High production agriculture requires readily-available mineral nitrogen in readily-available mineral nitrogen in the root zonethe root zone

This nitrogen, usually in the nitrate This nitrogen, usually in the nitrate form, is dissolved in the soil waterform, is dissolved in the soil water

The nitrate concentration in the soil The nitrate concentration in the soil water is relatively constant for a water is relatively constant for a given level of pasture/milk given level of pasture/milk production production

Other pastoral farming issuesOther pastoral farming issues AppropriateAppropriate application of nitrogen application of nitrogen

fertiliser to pasture is not a significant fertiliser to pasture is not a significant contributor to nitrate leachingcontributor to nitrate leaching

Properly managedProperly managed application of application of animal effluent onto pasture is not a animal effluent onto pasture is not a significant contributor to nitrate significant contributor to nitrate leachingleaching

Other grazing animals have less effect Other grazing animals have less effect than cattle because the urine than cattle because the urine applications are smallerapplications are smaller

SummarySummary Past agricultural production has already Past agricultural production has already

contaminated some of our groundwatercontaminated some of our groundwater Highly productive agriculture generates Highly productive agriculture generates

soil water drainage which is soil water drainage which is contaminated with nitrate to at least the contaminated with nitrate to at least the MAV(Minimum allowable value)MAV(Minimum allowable value)

Good agricultural management can Good agricultural management can minimise the rate of contaminationminimise the rate of contamination

We rely on dilution with high quality We rely on dilution with high quality groundwater to avoid ‘pollution’ groundwater to avoid ‘pollution’ (i.e.values > MAV) (i.e.values > MAV)