Guideline Index

Chapter 13: Using Dairy Effluent

13.3 Nutrients in manure; how much is there?

Understanding the characteristics and quantity of manure being generated by the farm’s dairy herd is the logical starting point for developing management strategies to utilise the resource and ultimately achieve production gains.

As the nutrient concentration in effluent is dependent on a range of site specific factors and is therefore variable from farm to farm (see Section 13.3.4 ), it is often more useful to start by estimating the total amount of each nutrient that is captured by the effluent system. The mass of those nutrients recoverable from the effluent system (i.e. in the sludge or in the liquid effluent after taking losses into account), and subsequently how much of each is available for plant uptake, can then be estimated.

While monitoring 43 dairy farms locatThe volume and nature of excreta produced by the cow will vary depending on dry matter intake and composition of the diet, with higher production cows producing more excreta compared to lower producing animals.ed across Australia, the Accounting for Nutrients project determined that almost half a kilogram of nitrogen is excreted daily by each cow – see Table 13.1. That is about 7 times the amount of phosphorus excreted. Therefore, for an average herd size of 250 cows over a 300-day lactation, 32.4 and 4.6 tonnes of nitrogen (N) and phosphorus (P) respectively are excreted around a dairy farm.

Table 13.1  Daily nutrient excretion by lactating cows from Accounting for Nutrients project; 43 farms over 5 visits.  Source: Gourley et al., 2010 .
Table 13.1 Daily nutrient excretion by lactating cows from Accounting for Nutrients project; 43 farms over 5 visits. Source: Gourley et al., 2010 .

The volume and nature of excreta produced by the cow will vary depending on dry matter intake and composition of the diet, with higher production cows producing more excreta compared to lower producing animals.

It is important to be aware that dietary nutrient content in excess of requirements is excreted.

Excess dietary potassium is excreted in urine, as is excess protein in the form of urea. Excess phosphorus, however, is excreted in faeces, with only negligible amounts appearing in the urine. Because of the excretion of excess dietary nutrients, it is often suggested that manure management should focus at the front of the cow, rather than the back end. Similarly, adding excess salt in various feed additives to stimulate appetite can compound problems with salinity management when reapplying effluent, or in areas with heavy stock movement.

More detailed techniques for estimating manure and nutrient excretion can be found in the Effluent and Manure Management Database for the Australian Dairy Industry.

13.3.1 How much of that nutrient enters the effluent system?

Most existing guidelines assume that 10% to 15% of the daily manure output generated by the dairy herd is deposited onto surfaces from which effluent is collected – see Table 13.2. Although that is a reasonable estimation for the holding yard at the dairy, industry trends towards feeding increasing levels of supplements or mixed rations on a feedpad suggest this assumption needs to be adjusted on some farms to avoid underestimating the volume of manure and nutrients to be handled.

Surveys with farmers utilising permanent feedpad systems indicate it is common for these facilities to accommodate the herd for a significant majority of the day depending of seasonal climatic conditions and farm activities. Effluent system design and nutrient management practices on these farms will be significantly different to more typical, predominantly grazing-based farms.

Table 13.2  Amount of nutrients expected to be deposited in different areas for a typical (grazing-based) 250 cow farm over a 300 day lactation.  Source: Gourley et al., 2010 .
Table 13.2 Amount of nutrients expected to be deposited in different areas for a typical (grazing-based) 250 cow farm over a 300 day lactation. Source: Gourley et al., 2010 .

13.3.2 Nutrient movement within the pond system

The fate of nutrients entering the effluent system is an important consideration in dairy effluent management. An understanding of whether nutrients are partitioned with solids in the sludge or remain in the liquid effluent is the key to effective nutrient management and allows farmers to fine-tune fertiliser decisions.

This knowledge also allows farmers to allocate some monetary value to the nutrients recovered during desludging to credit against synthetic fertiliser inputs, thereby justifying the additional cost associated with their reuse further from the dairy.

Sedimentation of settleable solids is a key treatment process in ponds treating dairy shed effluent, partitioning both solid material and nutrients to the sludge in primary anaerobic ponds – see Figure 13.1. However, the fraction of manure nutrients partitioned to sludge is lower than for separated solids for two reasons. Firstly, organically bound N and P tend to be concentrated in fine, poorly settleable manure particles, and secondly, a fraction of the organic nutrients that do settle to the sludge are mineralised and released back into the effluent (Fyfe 2013).

Volatilisation of ammonia-N from the pond surface is responsible for some loss of nitrogen from the effluent system. While some past rules of thumb suggested this loss may be as high as 50% of total N, recent research on a commercial dairy in NSW suggests this loss may be less than 30% (Fyfe 2013).

 

Figure 13.1  The proportions of nitrogen (N), phosphorus (P) and potassium (K) in the liquid (effluent) and sludge components of an anaerobic treatment pond and the proportion of N lost by ammonia volatilisation
Figure 13.1 The proportions of nitrogen (N), phosphorus (P) and potassium (K) in the liquid (effluent) and sludge components of an anaerobic treatment pond and the proportion of N lost by ammonia volatilisation

Potassium is highly soluble and non-reactive; therefore it is not prone to sedimentation or precipitation and is conserved through the effluent system and mostly recovered in the liquid effluent – see Table 13.3.

Table 13.3  Fate of nutrients in ponds, and the percentage of these nutrients available for plant uptake (shown in brackets).  Source: Fyfe, 2013
Table 13.3 Fate of nutrients in ponds, and the percentage of these nutrients available for plant uptake (shown in brackets). Source: Fyfe, 2013

13.3.3 Nutrient availability

Some of the nutrients in effluent will be in a form that is not immediately available for uptake by plants.

For example, the N contained in dairy ponds can be separated into five different pools as follows:

  1. Nitrate (usually negligible amounts),
  2. Exchangeable ammonium ions or other nitrogenous materials that can be readily converted to ammonium (which is plant-available),
  3. Organic N compounds which are potentially available for mineralisation,
  4. Microbial biomass, and
  5. Essentially unavailable N which is resistant to microbial attack and the mineralisation process.

While 1 and 2 are available for plant uptake, mineralisation of 3 and 4 is necessary before plant uptake can occur, and this takes time.

Depending on the source, effluent will contain varying proportions of N in each of the above five pools. For example, sludge from the primary pond in an effluent system contains a large proportion of relatively stabilised organic material, with varying resistance to microbial attack and hence N release. Sludge only contains small amounts of immediately plant-available ammonium ions with much of the organic material in the sludge requiring mineralisation to release plant-available N over time. Thus primary pond sludge should be considered a slow-release nutrient source.

Second pond effluent however, typically has a very low solids content and has therefore quite different characteristics to that of sludge. It usually has a high ammonia N content (typically 50 to 90% of total N in southern Victorian surveys) and comparatively low amounts in an organic N form. As a high proportion of the total-N is in readily plant available forms, the application of second pond effluent will give quick plant responses. Research by Ward (2010) indicates that for first pond sludge, N uptake by plants over three years totalled 70 to 83% of N applied. Between 40 and 50% of the N applied was taken up in the first year, 10 to 30% in the second and 5 to 12% in the third. For the second pond effluent, responses to the ammonium-N applied were limited to within five to six months of application.

13.3.4 Typical nutrient concentrations in ponds

Before effluent or sludge is spread on the reuse area, it is best practice to decide what application rate (usually ML/Ha for effluent, or t/Ha for manure solids) is your target so that you apply the appropriate amount of nutrient – see Section 13.5.3 . For this exercise, it is necessary to know the nutrient concentration (usually kg/ML or mg/kg) in the material to be applied.

A large number of dairy effluent ponds have been sampled across Victoria, and to a lesser extent, other dairy regions in Australia. To provide some indication of potential nutrient content in effluent ponds, the following tables (13.4, 13.5 and 13.6) present average constituent concentrations for effluents from primary, secondary and single ponds, respectively. Note: The amount of nutrients is expressed in milligrams per litre (mg L-1) which is equivalent to kilograms per megalitre (kg/ML) of effluent. A megalitre is one million litres.

Table 13.4  Reported primary pond effluent characteristics (standard deviations in brackets ).
Table 13.4 Reported primary pond effluent characteristics (standard deviations in brackets ).

 

Table 13.5  Reported secondary pond effluent characteristics (standard deviations in brackets ).
Table 13.5 Reported secondary pond effluent characteristics (standard deviations in brackets ).

 

Table 13.6  Reported single pond effluent characteristics (standard deviations in brackets ).
Table 13.6 Reported single pond effluent characteristics (standard deviations in brackets ).

 

As expected, results varied widely with significant ranges reported across farms (cross-sectional sampling). This variation is due to factors such as:

  • the age of effluent in ponds,
  • desludging frequency,
  • time of year sample was taken,
  • presence of any solid separation pre-pondage, and
  • the level of dilution caused by rainfall, runoff and varying amounts of washdown water.

For these reasons, it is always strongly recommended that farmers sample their own effluent or sludge and base their reuse activities on their own data.

Tables 13.4 and 13.5 generally show that there was less variation on individual farms monitored over time (longitudinal sampling). This suggests that without significant changes in operation or management, it is preferable to use the results of previous effluent analyses on a specific farm rather than rely on “typical” data. The variation over time for single ponds (Table 13.6) was higher than for other ponds because of the interaction of the filling and emptying cycle with settled solids.

A complicating factor is that nutrient concentrations in the primary pond will vary with depth. Table 13.7 shows the range in nutrient concentrations (kg/ML) at depth in the primary pond at DemoDAIRY (south-west Victoria) prior to (P) and after (A) agitation.

Table 13.7  Nutrient concentrations at depth (kg/ML) in the primary pond prior to and after agitation
Table 13.7 Nutrient concentrations at depth (kg/ML) in the primary pond prior to and after agitation
Source: Nutrient sampling conducted by John Kane (DPI Warrnambool) and Worldwide Organics Pty Ltd at the Terang Demo Dairy (2003).

The DemoDAIRY data shows that when unagitated, nutrient concentrations in the first pond increase with depth for N, P and S but not for K. Agitating (stirring) the pond did have an effect on mixing nutrients throughout the pond. Therefore, be aware that basing your application rate on the nutrient concentration at the top of the pond could mean that you will be out by a factor of over threefold if you use effluent from the bottom. You may be applying excessive amounts of nutrients to some areas and be at risk of burning crops and seedlings.

When emptying a first pond that has been collecting nutrients for a number of years, it is recommended that this effluent be applied to an established pasture or to paddocks prior to cultivation.

In other words, apply it before sowing a new crop or pasture. Consider taking samples of the agitated sludge as you are desludging the pond and use that data to improve the target application rate next time you desludge. If farm operational details and the desludging period remain close to the previous period, then the data will be useful.

There was no nutrient gradient at depth with the second (storage) pond at DemoDAIRY, which indicates that taking a sample at any level in a second pond will give a reasonably representative result.

13.3.4.1 Attributes of a primary effluent pond
  • Solids content variable – requires specialised extraction and spreading equipment
  • Very high in organic matter
  • Higher than 25mm per application could lead to potential risks
  • High concentrations of nutrients, especially N, P, Ca & Mg
  • Small proportion of nutrients in readily plant available forms
  • Most nutrients in various organic forms that require mineralisation over time before plant-available
  • Effectively a slow release, sustained release organic fertiliser
13.3.4.2 Attributes of a second effluent pond
  • Low solids – comparatively easy to pump and apply
  • A high proportion of nutrients in readily plant available forms, but nitrogen responses relatively short lived
  • Effectively salty irrigation water with large slugs of urea and potash
  • Well suited to:
    • Replacing potassium on hay/silage paddocks
    • Boosting growth of summer forage crops