Saturday, March 30, 2024

Irrigation makes its mark

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When the country was in the grips of a drought two years ago, the pivot on Andrew Mirfin’s West Coast property showed the stark difference to the dryland part of the farm, producing 4.5 tonnes of drymatter (DM)/hectare more.
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His farm on the outskirts of Ikamatua in the Grey Valley now straddles 430ha effective after adding a neighbouring 100ha farm this season and it’s been one of the four monitor farms on the West Coast for the past six seasons.

Mirfin installed a 180-degree Rainer Irrigation pivot with 13 spans two years ago to irrigate 70ha near the dairy, at a cost of $400,000, and reckons it has already paid for itself.

The Grey Valley is one of the driest areas on the West Coast, with about 1600mm of rain falling on Mirfin’s farm each year and it can quickly disappear in some of the stonier ground that has just a few centimetres of topsoil. February is usually the driest month in the area, but weather is an unpredictable factor in the farming equation and last year the Coast had a month without rain through September and October. That was enough for Mirfin to turn the irrigation on briefly to freshen pasture and ensure growth continued.

But it was in the 2012-13 season, which held most of the country in the grip of a drought, that the newly-installed pivot proved its worth. DairyNZ has worked out the figures for the monitor farm and that year the irrigated pasture grew 4.5t DM/ ha more than the unirrigated paddocks, adding up to 315,000kg DM across the 70ha.

DairyNZ senior scientist Dawn Dalley says that assuming 78% of that extra feed was utilised, an extra 19,733kg milksolids (MS) was produced and at Westland Milk Products’ payout of $5.76 that season, it added up to an extra $113,662 in Mirfin’s pocket.

Because he paid for the irrigation system out of cashflow rather than borrowing and paying interest, Dalley says the opportunity cost of capital meant he benefited by $98,000 that gave him a return on investment of 24.7% excluding extra running costs, compared with putting that money in the bank and getting $15,000 in interest.

Today, Mirfin milks 1100 crossbred cows, leaning toward Friesian, after buying and developing more land. In the 2012-13 season, the figures worked out by DairyNZ show the cost of imported supplements and grazing, if the irrigation hadn’t been in place, would have been 34.7c/kg DM to replace the 315,000kg DM, and that would have taken $109,305 out of Mirfin’s pocket.

Taking the extreme drought out of the equation and looking at the irrigation in a normal year, Dalley says the pivot would provide an estimated 1.5t DM/ ha more than the dryland paddocks and going through the same workings would produce 6577kg MS for $37,885 on the same payout. That worked out at a return on investment of 5.7% excluding additional running costs.

‘It’s all about timing and you don’t want to be turning it on when you think you need it but you don’t.’

That’s all good for Mirfin, who says the area under the pivot grew 19.5t of drymatter last season compared with parts of the farm, especially some of the areas that have recently been developed from bush, that produce only 11t. In the past two years with the irrigation – and other factors – he has produced 500kg MS/cow while cutting down on some of the costs. He used to grow about 12ha of turnips for summer but now has the extra grass and needs fewer supplements.

Freight on supplements from Ashburton would normally cost him $45/t. He still feeds an average 700kg of wheat a cow through the season, depending on how much grass is grown, and 60-70kg/cow of molasses. If it gets dry he adds soya meal for extra protein. It all depends on price though and in the past 12 months he has used palm kernel more because it has been cheap.

His farm working expenses are still up there at $4.60/kg MS but that includes the costs of setting up the new block with fertiliser and other expenses before he was producing milk off it.

The cost of running the pivot is relatively cheap at $120 a day because it is only lifting water about 12m from the neighbouring Little Grey River that feeds into the Grey River, and he’s only running it for about 40 days in a normal year. He also runs it at about 40% of its maximum speed to deliver 7mm of water every 24 hours.

“And that keeps up when it’s stinking dry.”

He’s now contemplating the next stage of irrigation once funds allow, and already has the maps and mainline drawn up. It will be cheaper than the initial pivot because much of the setup such as the intake from the river is already in place. Like the initial pivot, it will travel 180 degrees because of barriers restricting the full circle.

Most of the season there’s enough rainfall to keep the grass growing and last year his irrigation was used less than 40 days of the entire season, while the drought the year before was about 65 days. But the figures show the benefit of having that water available at crucial times.

It also means he can graze the irrigated paddocks down to a residual of 1600kg DM/ha because they grow faster. He can graze them every 16-18 days through a dry period compared with the dryland paddocks that are usually on a 23- to 24- day round, which stretches out to 30 days as it gets drier.

In a couple of paddocks under the pivot he has planted the perennial ryegrasses Expo and Extreme to make the most of the irrigation and that helped to grow 100kg DM/ha a day in the drought.

Since erecting the pivot he has installed two Aquaflex soil moisture sensors; one with a strip at a depth of 150mm while the other has two strips to check moisture at 150mm and 300mm.

They check ground temperature as well as soil moisture, so when he decided to irrigate in October after 32 days without rain he knew the soil temperature was marginal for grass growth at 8-9C.

The information feeds to the house so he knows on any day what the soil moisture and temperature are on the irrigated section of the farm.

“It’s all about timing and you don’t want to be turning it on when you think you need it but you don’t. And otherwise you would be out there digging holes.”

He doesn’t put the effluent through the pivot, spreading it through a travelling irrigator instead. Down the track it will all go through the same system and put nutrients on to the irrigated paddocks as well as water.

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