Environmental Impacts of dairying in Canterbury

1. Environmental Impacts of dairying in Canterbury Ross Redpath Royal society teacher fellow 2003

2. Implications of increased dairy farmingon Central Canterbury Plains • Land in sheep grazing (and cropping), is being converted to dairy farming • Dryland areas are being converted to irrigation • Increased use of nitrogen fertilisers • Change of grazing animal from sheep to cows • Stock density increasing to about 3.3 cows/ha • Increased dry matter production from pasture

3. Issues for groundwater quality • Increased soil fertility implies larger pool of soil nitrogen for conversion to mineral forms (nitrate) • Irrigation increases soil-water drainage, which can increase leaching of this additional nitrate to groundwater • Removal of groundwater for irrigation decreases the ability to dilute contaminants, such as nitrate which are leached into the aquifer

4. Soil organic matter Relevant parts of the nitrogen cycle Pasture Fertiliser & legumes NH4+ NO2- NO3- Leaching

5. The journey through the unsaturated soil zone • Excess soil water, from rainfall or irrigation, trickles down through the sands and gravels towards the groundwater surface • Movement is highly intermittent, because it is climatically driven • The average velocity of groundwater is a few metres per year

6. Nitrate leaching from dairy farming • The main factor is the level of milk production per hectare • Pasture has a lower C:N ratio than that required for milk production and body maintenance • The excess nitrogen is excreted in the urine • Urine patches contain more nitrogen than the pasture can use, and the excess is leached

7. Typical soil nitrate-N leachate levels

8. 3.3Dairy cows/ha Feed Urine Dung Ammonia loss Denitrification Fertiliser Pasture/soil ~ 5000 kgN/ha Leaching Nitrogen system for dairy production(approximate N gains/losses - kg/ha/y) 70 Milk & meat 400 220 110 100 200 30

9. Key ideas about leaching from agricultural soil • High production agriculture requires readily-available mineral nitrogen in the root zone • This nitrogen, usually in the nitrate form, is dissolved in the soil water • The nitrate concentration in the soil water is relatively constant for a given level of pasture/milk production

10. Other pastoral farming issues • Appropriate application of nitrogen fertiliser to pasture is not a significant contributor to nitrate leaching • Properly managed application of animal effluent onto pasture is not a significant contributor to nitrate leaching • Other grazing animals have less effect than cattle because the urine applications are smaller

11. Summary • Past agricultural production has already contaminated some of our groundwater • Highly productive agriculture generates soil water drainage which is contaminated with nitrate to at least the MAV(Minimum allowable value) • Good agricultural management can minimise the rate of contamination • We rely on dilution with high quality groundwater to avoid ‘pollution’ (i.e.values > MAV)