FOR many years the line of thought with fertilising native pastures has been that it altered the composition of species to the detriment of natural biodiversity, and provided a limited production response, making worthwhile returns difficult.
However, research conducted by the Department of Primary Industries (DPI) at Cooma and funded by the local Catchment Management Authority (CMA) has shown not only can fertilising occur without damaging the frequency of native species, but it can also generate a profit.
DPI district livestock officer Doug Alcock said the impetus for the project was research from the CSIRO which suggested landscapes should have a mix of ecological communities, including unfertilised native pastures on about 40 per cent of the landscape.
It suggested fertiliser encouraged the invasion of the native pastures by annuals and introduced species.
"However, for the farm to be sustainable as a whole, the farmer needs to make money from it," Mr Alcock said.
"The problem with unfertilised native pastures was, not only were they less profitable, but production such as meat removed minerals such as phosphorous reducing the overall long-term productivity."
DPI's research aimed to investigate the impact of fertilising on productivity, as well as how pasture composition was affected.
In its paper called "Why fertilise native pastures?" presented by Mr Alcock at last year's Grasslands Society of NSW conference in Wagga Wagga, it was shown profitability of native pastures could be improved by fertilising, but return on investment was largely due to how graziers matched stocking rates to changing pasture production.
Mr Alcock said the study was conducted at two sites, Berridale and Bungarby, each of which had nine five-hectare paddocks arranged as three replicates of treatments.
The treatments were nil, low or high fertiliser.
The treatments weren't fixed, the aim instead being to reach desired phosphorus (P) and sulphur levels.
Once target fertiliser rates were reached, fertiliser input was lowered to maintenance rates.
Each paddock was set stocked with Merino wethers at the historical stocking rate of the original paddock.
Pasture species frequency was recorded, as were sheep weights, fat scores, fleece weights and fibre diameter.
It found the most efficient practice was to 'trickle' in the P at 125 kilograms a year until target P levels were reached, thereafter only applying P every second year.
Mr Alcock said the high rate (of 250kg/ha/year) didn't reach the target P levels any faster and had a minimal impact on improving carrying capacity when compared to the 125kg/ha rate.
The important feedback for farmers was the sheep on both the fertilised plots had higher body weight and fat score compared to the unfertilised pasture.
It wasn't until 2009 they had reached adjusted stocking rates (through increasing the rates on the fertilised blocks) where the sheep on the fertilised plots had the same weight and fat score as those on the nil fertiliser treatments.
Mr Alcock said the fertilised plots in these cases could have had their stocking rates increased at a faster rate, which would have also improved the return on inputs from an earlier stage.
This also highlighted the importance of understanding how to manage the improved pasture production and that an investment in pasture should also include an investment in livestock.
At Berridale the stocking rates for each treatment in the first year were 2.4 wethers/ha.
By 2009 stocking rates had been adjusted to the changed carrying capacity: nil fertiliser, 2 wethers/ha; low increase rate of fertiliser (125kg/yr), 4 wethers/ha, and; high fertiliser increase (250kg/ha), 4.8 wethers/ha.
Because the target soil P level was no different for each of the fertilised plots the report concluded the low increase rate was a lower risk option that was as profitable as the higher initial fertiliser rates.
Mr Alcock said because the target P level was only 27ppm Colwell with a low phosphorous buffer index, both fertiliser rates reach the target level after just two fertiliser applications.
It was reported the liveweights and fatness of sheep on the fertilised pastures increased immediately after the first application of fertiliser relative to the unfertilised treatments, supporting the idea the stocking rates could have been raised sooner.
It also appeared impacts of fertiliser on native species frequency were minor, with much bigger between year variation driven by seasonal conditions, indicating strategic rest of pastures can offset risk of destabilising pasture composition.