Inland Elite Dairy Network

There is increased interest in utilising waste from intensive industries such as dairies as a substitute for fertilisers for conventional farms. This practice potentially provides dual benefits in terms of reducing the level of nutrient concentration around intensive animal production enterprises as well as reducing the reliance on synthetic fertilisers. Composting waste products prior to applying them on paddocks is seen as a useful way to homogenize and break down the particle size to make spreading more even and efficient. There is also a view that the composting process creates an environment conducive to microbial growth that may ultimately convey benefits to plant growth. The few experiments that have been conducted using organic soil ameliorants tend to assess the impact of very high rates (often 100 tDM/ha) of product applied. It is clearly not feasible for farmers to use rates of this magnitude in conventional broadacre applications.Dairy farms produce a lot of organic waste products through a build-up of manure and fodder residues, particularly when hand feeding through the drier periods. Local dairy farmers had concerns about spreading straight manure on paddocks and potential weed burdens from hay brought off-farm.

This project part-funded a field experiment conducted on a Riverina dairy farm to measure the benefits to plant growth and soil health on applying commercial rates of organic ‘humified’ dairy compost. The project was a collaboration between the Inland Elite Dairy Group and the EH Graham Centre for Agricultural Innovation (principal researcher: Richard Hayes).

The project measured the environmental, production and economic value of recycling the humified compost back to the dairy farm soil.A local dairy farmer had an on-farm composting facility in which solid waste from the dairy was mixed in windrows with clay, organic material (straw or shredded paper) and a liquid ‘humifying’ agent as recommended by a commercial consultancy.

A replicated field experiment was conducted to measure differences in the impact of this composed product at application rates 0, 0.5, 2.5 and 5.0 t/ha of dry matter.

Compost was applied in November 2008 shortly before the sowing of a millet crop. Changes in plant productivity, species composition and herbage mineral content as well as changes in soil properties were measured. Measurements ceased at the end of the first growing season, in April 2009.

Plant response

There were no clear differences in plant growth in response to compost application. Herbage yield was unaffected by compost application. Mean total herbage yield was 10.1 t/ha across all treatments. There was also no difference in botanical composition of the forage crop between compost treatments. Summer grass weeds comprised a mean of 50% of total biomass across all treatments. There was also no difference in mineral uptake by millet due to the application of compost.

Soil health

The compost was of high quality in terms of physical appearance and chemical consistency. However, no significant differences were measured in key soil chemistry analyses and plant properties in the first crop after compost application. Available phosphorus (Colwell) was 110 ppm averaged across all compost treatments, and pH was 5.5.

Soil biology was also shown to remain unchanged due to compost application. There was also no correlation between soil biology and mineral uptake of plants.

A number of factors may have contributed to the absence of response to applied compost.
•The background fertility of the dairy farm is relatively high and plant nutrient uptake was apparently adequate. Additional nutrient application in compost may have been in excess of plant requirements.
•The compost was applied at relatively low rates and the addition of clay and other organic matter to the nutrient-rich dairy waste (manure) reduced the nutrient concentration in the compost.
•The composting process can tie up minerals in larger chemical compounds, making them unavailable to plants in the short term.

Despite reports of increased soil biological activity through addition of organic products such as compost, there were no differences measured in soil biological parameters 5 months after application.

The project showed that the practical constraints of spreading biological products in broadacre enterprises are also important to consider. The compost used in this experiment was quite uniform relative to other organic products such as raw manure. Even at the high application rate of 5 t/ha, no significant differences in key soil physico-chemical and biological properties were measured. Depending on soil properties, available nutrient concentrations and seasonal conditions, much higher rates may be required to obtain significant crop and soil responses. Many supporters of composting promote compost as a ‘living organism’ in terms of its rich microbial populations. Drying compost prior to application would kill many of the beneficial organisms. Compost commonly contains a relatively high moisture content when spread– in this case 33% moisture, which adds to the application costs.

The results suggest that composted dairy waste may have a role in ‘higher value’ products, such as horticulture or domestic/urban markets. In these more intensive markets the higher application rates are more likely to be economically viable.