Soil scientist questions accuracy of Overseer modelling

Former AgResearch scientist Samuel Dennis doesn't trust the numbers produced by Overseer farm nutrient budgeting software so he's working on new technology he says will give much more accurate results. Tony Benny reports.

Independent soil scientist Samuel Dennis with his farmer father Chris on the family farm at Glenroy, Canterbury.
Tony Benny

Independent soil scientist Samuel Dennis with his farmer father Chris on the family farm at Glenroy, Canterbury.

Samuel Dennis grew up on his parents' sheep and beef farm at Glenroy, in the foothills that rise from Canterbury Plain, inland from Christchurch, and today he and wife Sarah live with their six children on a corner of family property.

Until two years ago he worked in AgResearch's farm systems department. He studied soil science at Lincoln University, graduating with an honours degree and completing a PhD.

"When I was at school I had a very strong interest in science and I was studying things from a creation perspective at home and from an evolution perspective at school," Dennis, a committed Christian, says.

Chris Dennis uses "time-controlled grazing", also known as holistic farming, grazing stock together in large mobs in ...
Tony Benny

Chris Dennis uses "time-controlled grazing", also known as holistic farming, grazing stock together in large mobs in small paddocks and shifting them daily.

"That tension caused me to have to think critically and critical thinking is essential for science so that ended up leading me into a science career."

Read more:

* Accurate entry of Overseer data vital for Waikato farmers

Overseer expands for new demands

But the 10 hours a week spent commuting to Lincoln became too much for Dennis. "Our fifth child was on the way at the time and I realised I was just not getting enough time with the kids.

"We were wanting to home school and I thought, 'I'll go out on my own and make it work'," he laughs. "So that's what I did and I'm still here."

Since striking out on his own Dennis has done contract science work for large organisations like AgResearch, DairyNZ and Beef + Lamb NZ, as well as for individual farmers, doing Overseer nutrient budgets for environmental regulation compliance.

Ad Feedback

But he's become increasingly disenchanted with the way the computer modelling programme is being used.

"I'm really trying to pull back from Overseer modelling for individual farmers because the more I look into it and the more different ways of farming I see, I realise I just can't trust the numbers.

"I'm just not professionally comfortable with reporting numbers based on Overseer knowing they will be used to assess regulatory compliance, because it's just wrong in many cases."

Dennis says Overseer works well for what it was designed for, working out farm fertiliser recommendations, but not as a regulatory tool.

"Overseer was developed to work out fertiliser recommendations and in order to do that, it has to work out how much nutrient is being lost. You need that to work out how much fertiliser you're going to have to apply to replace it so it's an important number and on a statistical average basis, it's good enough.

"If you get your fertiliser recommendations slightly wrong, it doesn't matter because you're going to pick it up in your fertiliser tests a few years later and you'll correct it. There's no regulatory consequences of it."

But real farms are far more complicated than allowed for in Overseer, Dennis says.

"It can only account for the few things that have actually been researched in sufficient detail to put into a model. There's far more that's not in the model that's happening on a real farm than is in the model."

Dennis says one paddock can have several soil types but that, along with topography, variable pasture composition and tree lines, is among many factors that affect nutrient loss but for which Overseer isn't flexible enough.

"Most critically, changes in soil organic matter over time are almost completely ignored.

"Most of the research has been done in flat highly productive pastures, very little has been done in the high country. When you get up into the hills, when you get into higher rainfalls, anything on different soil types, anything that's away from the standard conditions that you tested, the model's accuracy is going to rapidly fall away."

On top of his doubts about the accuracy of the figures Overseer produces, Dennis finds the whole farm environment plan process frustrating.

"The farmer's only called you up because the council told him he had to and it takes a fair bit of time to do the work so you end up having to invoice them a fair amount of money.

"It's the most depressing thing to invoice someone for something they never actually wanted to do in the first place and doesn't actually deliver any real value to them whatsoever."

Dennis believes rather than computer modelling a far better way to assess farm nutrient loss would be by actual measurement, something until now deemed unaffordable and impractical but which emerging technology should make possible.

"There are definitely opportunities opening up with new technologies that aren't available yet and that's what I'm trying to work on. There are technological solutions in other industries that I can repurpose, put it that way."

Wary of saying too much in case his ideas are stolen, Dennis believe there's potential in a number of different approaches. "There's a couple of different methods that I'm in the process of designing and getting underway."

"I'll be looking for clients who have complex situations they believe aren't being accounted for who would be happy using experimental approaches to try to work out what their situation is while I refine the technology that I'm working on."

Meanwhile at home Samuel's father Chris has adopted holistic farming techniques, another of the variations not accounted for in Overseer, Samuel says. He prefers to call the system "time-controlled grazing", whereby large mobs are rotated through small paddocks with daily shifts and long recovery times, trampling down long pasture as they go and building up soil organic matter.

"You've got a lot more trash left behind and all of their urine and dung is going on top of this surface litter layer which is much deeper than your regular pastoral situation. A certain amount of the nitrogen is going to be used just in the litter layer before it even its the soil."

Under time-controlled grazing, pasture should develop bigger root systems and these should pick up more nitrogen before it can leach out, he argues, adding that urine is more evenly distributed and international research shows phosphate run off is reduced in this type of grazing management.

"But that's not accounted for in Overseer. It's an extreme example of a farm system where you can see all of these different things that just aren't accounted for in the model.

"On a different scale, a lot of that's true for any farm – there's a lot of stuff that any farmer would be doing that's not properly accounted for in the model."

 - Stuff

Ad Feedback
special offers
Ad Feedback