The problem
Milk may be a great food but it is also a serious pollutant.
Polluting substances are rated by their biological oxygen demand (BOD) — the amount of oxygen consumed by bacteria in breaking down organic matter. If a watercourse is contaminated with these pollutants, so much oxygen is consumed that oxygen levels become too low to support aquatic life and deaths result. The higher the BOD, the more serious the pollution risk.
The table below outlines typical BOD for a number of pollutants.
| Pollutant | BOD-mg/L |
|---|---|
| Raw sewage | 300 |
| Dairy effluent | 3200 |
| Pig effluent | 11 500 |
| Milk | 30 000 |
| Silage effluent | 60 000 |
The figures in the table show why milk must be disposed of carefully — and why you need to be even more careful with silage effluent.
What are your options?
Restrict production
If pick-up is likely to be disrupted for a few days, you could consider cutting down feed to reduce milk production. In practice, however, there is a limit to what you can achieve without causing long-term problems. Reducing the amount of pasture offered and/or slightly reducing the amount of concentrates fed will reduce milk production. However, most interruptions to pick-up don't last long enough for these measures to be really practical.
Do not cut concentrates completely — you can run into problems when reintroducing them.
Restricting intake of water is effective but is less acceptable and should only be considered as a last resort. Be very careful with this in summer.
Disposal
Milk should be disposed of on-farm unless it can be carted to an acceptable disposal site. It should not be allowed to enter surface or groundwater — dispose of it carefully. If it is allowed to pond, it will smell. Spreading over pasture or cultivated ground is the best option.
Small quantities of milk
Dispose of it carefully.
Feed to calves or other livestock. Addition of citric acid at 2g/L will preserve milk for a few days, even in summer. If the bulk milk has been inadvertently contaminated with antibiotics, it can still be fed to livestock but any unnecessary intake of antibiotics by livestock is undesirable. The milk from individual cows treated with antibiotics should not be fed to livestock.
Large quantities of milk
Dispose of it carefully.
Feed to livestock, either yours or your neighbour's. Take care if it contains antibiotic residues.
Apply to pasture; use an effluent tanker or sprinkler to apply to freshly grazed pasture to maximise the time between grazings. In winter, use your driest paddocks to minimise run-off. If possible, dilute the milk 1:10 with water. Don't apply milk to pasture close to watercourses and spread it over as large an area as possible.
Dedicated pond or trench
Provided that you have suitable soil, use a front end loader to dig a trench capable of holding about two days milk — milk must not leak into groundwater. Make sure that it is far enough away from houses so that odour will not be a problem. The trench should be backfilled immediately after the last of the milk has been disposed of. If you need to dispose of more than two days milk, dig a second trench.
Direct to effluent storage pond
This is not an ideal solution since a large volume of milk may overwhelm the bacteria in the pond which decompose the organic matter. Ponds of suitable size and construction should be able to accept up to three days milk within a fortnight. More than this will most likely produce odour and a reduction in treatment efficiency. If very large amounts of milk are added to a pond, it may take months to recover and may smell. If at all possible, try to pump the milk straight from the collection sump to pasture, by-passing the storage pond.
Acknowledgement
The original version of this material was authored by Bill Russell, Ian Bell and Ernie Haggett.