Irrigating dairy effluent to pastures
If managed carefully, irrigation of dairy effluent to pastures and crops can make good use of the nutrients, organic matter and water. It can also help to minimise the pollution of streams and groundwater.
Dairy effluent contains many nutrients and can be used on a range of pasture and crops, which has economic benefits.
Applying dairy effluent to pastures can increase pasture growth considerably according to a number of Australian and overseas studies.
Victorian research has shown large and valuable responses can be obtained for both pasture and forage crops.
The response varies depending on the time of year and is due to both the nutrient and water content of the dairy effluent.
Protection of surface and ground water
Dairy effluent must be managed in such a way that it remains on the farm and does not contaminate waterways or sub surface (ground) water. This is enforceable by the Environment Protection Authority (EPA) under the State Environment Protection Policy (SEPP).
When applying liquid and solid dairy effluent appropriate buffer distances from surface waters should be maintained to avoid contamination.
Effluent should be applied to pastures or crops at the right time and in amounts that the plants can use the water and nutrients. If too much is applied at one time or if applications are repeated too often in the one place, there is a high risk of runoff to streams or groundwater.
These risks are increased during periods of waterlogging.
Ponds are commonly used to avoid the need to irrigate effluent under these conditions. If ponds are not suited to your farm and direct irrigation is the only option, then careful irrigation over greater areas than normal is required to minimise runoff or leaching of nutrients.
What is in dairy effluent
Dairy shed effluent largely consists of the manure, urine and washdown water.
Other things that can enter the effluent system include:
- gravel from the cow's hooves
- soil particles
- cow hair
Depending on the irrigation system used the manure, urine and water might need to be separated from any other contaminate before storage and irrigation.
Dairy effluent is a valuable source of nutrients. Levels of nutrients contained in dairy effluent are generally lower than that of fresh manure, due to many factors.
- amount of water used in the dairy
- rainfall on the pond catchment
- type of effluent system (direct application versus pond systems)
- pond cleaning frequency
- cow's diet.
A number of effluent systems have been sampled since 1996 around Victoria to get an idea of their nutrient content (Table 1). The nutrients are expressed in milligrams per litre (mg/l) or its equivalent, kilograms per megalitre (million litres or 1000 cubic metres) of effluent.
Table 1 Nutrient content of second effluent ponds in Gippsland
mg/l or kg/ML
mg/l or kg/ML
mg/l or kg/ML
43 - 2100
13 - 1400
67 - 3900
Source: B. Bradshaw
Availability of nutrients
Nitrogen availability to plants is generally less with effluent than with artificial fertilisers because some of the nutrients are bound up in the organic matter.
In liquid effluent a high proportion of nitrogen will be in plant available forms.
Sludge often has a high proportion of nitrogen in organic (slow release) forms. Some of this nitrogen will be made available for plant use into the future.
Potassium has high availability and is a potential problem since it can replace magnesium in pastures making them more prone to cause grass tetany in cattle. This is only likely to be a problem if effluent is applied in the tetany season or restricted to a small part of the farm.
Loss of nutrients
Nutrient content of stored effluent is less than directly applied effluent. Some nitrogen is lost to the atmosphere whereas phosphorus and potassium and some of the nitrogen settle out in the sludge.
The nutrient content of effluent varies widely and is dependent on many factors. The most accurate method of determining the nutrient value of your effluent is to take a representative sample and have it tested. This will then allow you to calculate appropriate application rates.
The following will assist in calculating appropriate application rates:
- Generally, nutrients concentrations in effluent results are reported as milligram/litre (mg/L). Where mg/L is the same as kilograms in one million litres (kg/ML). It is useful to use kg/ML as we are used to applying kg of nutrients per hectare and ponds are often measured in megalitres.
Taking values from Table 1 as an example:
- One megalitre of water applied to 1 hectare is equivalent to 100m of rain or irrigation per hectare. So in this example for every 100mm per hectare of effluent applied you will apply 200kg of N, 52kg of P and 395kg of K.
These rates of nitrogen and potassium are high and the water equivalent being applied is 100mm. This high application rate would probably result in runoff or leaching from many soil types, which could potentially lead to pollution of waterways or ground water.
If applying 25mm/ha of effluent you would be applying 50kg of N/ha, 13kg of P/ha and 99kg of K/ha. This is around the maximum amount of nitrogen that should be applied in one application and will also provide a moderate amount of phosphorus and a large amount of potassium.
How much to apply
The main aims in applying effluent to your pastures or crops are to:
- maximise the use of nutrients by the plants without causing nutrient overload or waterlogging
- minimise the loss of nutrients by runoff to streams and drains or leaching through the soil to groundwater.
Avoid applying effluent to the same area year after year. This can lead to overloading the soil with some nutrients such as potassium.
Care also needs to be taken with salts which may be contained in the effluent.
So, unless you test your effluent and monitor the soil where it is applied on a regular basis, it is recommended that you rotate your applications around at least 3 or 4 areas, moving each year.
Applying effluent to lower fertility paddocks will increase the fertility in these areas. Normal fertiliser applications needed to be altered in paddocks were effluent has recently been applied.
Application of effluent will need to match herd grazing rotations to ensure paddock withholding periods don't interrupt feeding schedules.
Best time to apply effluent to pasture or crops
Effluent from your storage pond (second of 2 ponds or your single pond) should be applied over summer and autumn, so that the pond starts each wet season almost empty.
Applying effluent during the wetter months runs the risk of runoff to streams or leaching to groundwater when soils are saturated. Aim to empty the pond before soils become saturated.
Apply effluent when pasture or crops are actively growing, so that they can use the nutrients. Applying to recently grazed pasture will allow the withholding period to be incorporated into the grazing rotation.
Effluent can also be applied after silage is cut to promote regrowth and also replace lost nutrients.
Effluent can also be applied to forage crops. It is best to apply effluent when the plants are in the rapid growth phase.
Effluent should not be used to germinate seedlings or young plants due to high nitrogen and potassium, as well as the salt concentrations.
Sludge will also need to be removed from your first pond every 2 or 3 years. This can be applied to paddocks about to be cultivated for a crop, or sprayed thinly on pasture at similar times to the effluent. Vacuum tankers and can be useful for this purpose.
Salts in dairy effluent
The EC levels (an indication of the amount of salts) in the samples analysed in table 1 averaged 3250 uS/cm and had a range of 1500 to 9300uS/cm. These are relatively high and above the recommended upper limit for irrigation which are 1500 to 2000uS/cm. If bore water is used for washdown then salt levels can be high.
Recycling effluent concentrates the salts even higher and more care needs to be taken in irrigating. If salt levels are high then it is better to apply small amounts of effluent over a large area and empty the ponds more often.
In some situations where effluent water contains a high proportion of sodium compared to calcium and magnesium — a relationship called the sodium adsorption ratio (SAR) — continued application on the same area may eventually cause soil structure and drainage problems.
The average SAR in the given samples is about 2.4, which is below the level of concern of about 6. Some ponds contained high SAR levels and that effluent should not be applied to pastures in excessive amounts. Where SAR levels are above 6 it is best to seek advice before irrigating.
Animal health considerations
Diseases such as Johne's Disease, Salmonellosis. Leptospirosis, mastitis and Enzootic Bovine Leucosis can be contained in raw effluent and milk.
For more information see Dairy effluent and animal health.
Worm eggs, coccidial eggs, clostridial organisms and tetanus spores are also passed in manure.
In most cases the period of time before application to pasture and the dilution effect of the water — tends to greatly minimise the risk of these problems occurring in the herd.
To reduce any risks the following actions should be carried out:
- Young stock under 12 months old should not graze treated areas and drains from treated areas must not flow into areas where young stock are being kept to minimise the risk of infection with Johne's Disease.
- Do not graze areas where effluent has been applied for at least 3 weeks.
- Graze areas to receive effluent just prior to application to allow increased sunlight penetration to kill organisms and to allow the full rotation length before area is re-grazed.
- Spread effluent during the warmer months to reduce survival chances of offending disease organisms.
Contact your vet if you have concerns about any specific animal health problems associated with applying dairy shed effluent to pastures or crops.
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