Digging into the data: first-year effluent trial update

Demonstrating the potential profitability of using dairy effluent on ryegrass pasture, this three-year trial aims to provide farmers with reliable, real-world data on how pastures respond to liquid effluent.  This includes nutrients and any irrigation potential on the shoulders of the season. 

In late August, Dairy Australia hosted a field day at the trial site to inspect the performance of ryegrass with and without effluent application. The day brought together farmers, service providers, and industry stakeholders to observe and discuss the potential benefits of using effluent as a nutrient or irrigation source. 

Sown in late April – the first year of a three-year project - the trial comprises four treatments to allow direct comparison of nutrient strategies: nil nutrient application, effluent application only, effluent plus post-harvest nitrogen applications and a treatment providing the equivalent nutrients as granular fertiliser.  

The fertiliser treatment applied the same amounts of nutrients as a single 10mm application of effluent in May (30kg/ha of N, 4kg/ha of P, 67kg.hs of K and 1.7kg/ha of S) 

Dan Parnell, WA Dairy Care Project Manager for Dairy Australia and Rob McFerran from Department of Water and Environmental Regulation guided attendees through the trial site, covering soil temperature trends, agronomic events, visible differences in pasture health, and early dry matter (DM) results.

"The trial hopes to provide data on how effluent can save money on fertiliser and also test any in pasture production benefits from small irrigation events on the shoulders of the season,” Mr Parnell said.  

Also inspected on the day was a local trial of broadleaf herbicides to control barley grass, which garnered high levels of interest from attending farmers in particular. 

Early September was time for the third harvest of the trial plots, with Dairy Australia’s WA extension advisor Indi Brockman and Dan joining forces with Department of Water and Environmental Regulation Rob and Niamh Rain to get the job done.  A centre strip was cut from each plot, weighed for total fresh mass, and sub-sampled for dry matter content and tissue testing 

The site was mown down to a perfect 5cm residual with the ride on and the team admired their handiwork surrounded by every farmer’s favourite smell - fresh cut grass. 

	24 June	27 July	8 September	Total DM (kg/ha)
Nil	551	351	249	1150
Effluent Only	901	497	249	1647
Effluent plus Nitrogen	945	1047	1403	3394
Fertiliser Plus	951	1175	1387	3513
Table 1: Cumulative Dry Matter (kg DM/ha)

“We’ve only applied the one 10mm application in autumn and there was some encouraging response in the first two harvests, with effluent growing an extra 0.5t/ha compared to the nil plots,” Mr Parnell said.   

“Since then, we have seen strong responses to nitrogen in both the effluent plus N and fertiliser Plus N treatments.”  

The trial continues through 2026 and 2027, with ongoing data collection on pasture yield, nutrient uptake, and environmental outcomes. Future field days will be held to share insights as the seasons progress. This project is only possible with the support of Western Dairy and the Department of Water and Environmental Regulation through the Healthy Estuaries WA program. Our sincere thanks go to Grant and Nikki Evans for hosting the trial on their property. 

For more information on how Dairy Australia supports dairy farmers with designing and maintaining an effective effluent system that improves farm and nutrient management, visit Effluent Management or reach out to your local team.