The property, Karetu Farm, is 145ha effective, irrigated by a centre pivot and fixed grid system, with 20ha of leased land. Production is budgeted at 465kg milksolids (MS)/cow from 490 crossbred cows or 225,873kg MS. The stocking rate is 3.4 cows/ha. Three dry corners only receive effluent irrigation and are used to grow 6ha of kale and 3ha of lucerne. The farm imports 203 tonnes of barley, fed in the dairy, and 335t balage during the milking season and grazes the whole herd off over winter.
Other factors included in the Overseer programme were rainfall of 650mm/year, the soil type of Waimakariri (silty sand, with stones) and pugging happening only occasionally. The Olsen P was 22, Quick Test potassium eight (low leaching potential) and organic sulphur eight. There were no supplements made onfarm from the ryegrass/white clover pastures. The farm applied 400kg/ha of 15% sulphur superphosphate in September and 125kg/ha ammonium sulphate (ammo 35) in August. There was 90kg/ha urea/month applied from October to April, the same period when irrigation was carried out and 12-24mm of effluent was applied to 30% of farm.
The first step was to establish the nitrogen leaching potential under the present farming system, which was prepared by Mat Cullen, Fonterra and Peter Brice of Ravensdown.
The initial run indicated that the farm was leaching 33kg N/ha. As the suggested goal should be less than 20kg N/ha, there were discussions to identify several strategies. Those chosen were expected to have the least effect on the farming system and not involve capital spending.
The proposed changes included:
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Having 20% of the herd grazed off the farm in April and August and 30% grazed off in May.
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Changing the proportion of clover in the paddocks in Overseer from medium to very low, due to clover root weevil.
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Changing nitrogen applications, with a reduction on the milking platform from 90kg/ha to 70kg/ha, only applied until March. Ammo 36 was reduced from 125kg/ha to 80kg/ha and applied from August to September. The end result was a drop from 291kg to 221kg N/ha which reduced nitrogen leaching to 22kg N/ha.
Financial modelling using data from 2012-13 compared the existing situation with these changes using the Farmax model. Expense categories were consolidated except for those that differed, and depreciation was not included.
Production was predicted to drop by about 9000kg MS with the changes, with Farmax taking into account the lower milk prices in 2012-13 and Fonterra’s payment schedule over that year.
Caution is needed when looking at the results of a model for one specific location as this farm had a lower stocking rate than many in Canterbury and a relatively large area of ineffective land which diluted the nitrogen leached per ha. Clover root weevil damage pasture may only be temporary, as pastures were recovering.
A major concern with modelling with Overseer has to do with the farm’s subsoils, as the industry standard is to label all soils as ‘medium’. If the subsoil is changed to ‘light’, the nitrogen leaching moved to 55kg N/ha/year under the present system and 43kg N/ha/year under the proposed changes. With the change to light subsoils the leaching from the kale and lucerne blocks increased markedly. If these areas were replaced with ryegrass/white clover pasture, leaching would be reduced by 4kg N/ha/year across the farm in both scenarios.
The farm used relatively cheap surface water for irrigation and was operated by a contract milker who paid some expenses, leading to a different level of income and outgoings compared with an owner-operated farm.
Overseer analysis indicated that relatively minor changes could reduce nitrogen leaching markedly, ignoring the light subsoil issue. Farmax modelling indicated the changes would have reduced operating profit by about $10,000 during the 2012-13 season.
But it’s likely capital intensive changes would be needed if subsoil categories were changed and farmers couldn’t meet targets. One quoted was for $3500/cow for a free-stall shed to house the cattle over the shoulder months of April, May and August as well as providing winter housing. The interest cost at 6% for such a shelter would be more than 40c/kg MS. There would likely need to be more machinery but this could be offset by increased cow productivity or extending lactations.
Like all models, Overseer and Farmax are not perfect – it’s impossible to develop a model that caters to all farming systems perfectly accurately. But these tools can be used to identify the problem, allow the testing of different scenarios and may give farmers a way forward in their attempts to minimise environmental effects while operating successful businesses.
Marv Pangborn is a Lincoln University agricultural management lecturer and farmer and Sally Peel is a Master of Commerce Agriculture graduate, now employed by DairyNZ.
