Rethinking GPS

BY TOM BUTTON

It's hard to imagine anything more modern than GPS yield monitor systems. Start with the fancy computer chips that power the yield monitors, add in the satellite systems - the same ones that steered U.S. warplanes in Desert Storm and Fox - and you'd expect to have a guaranteed route to efficient farming for the 21st century.

But four years after yield monitors hit the headlines in Ontario, the farmers with the most GPS experience are scaling back their expectations for the new technology. The $7,000 to $10,000 systems may still pay for themselves, but in drips and drabs of $100 to $200, not in overnight yield gains or in vastly improved fertilizer efficiency.

Other satellite-based systems have probably been oversold, too. Most tellingly, farmers who've spent $10 to $23 an acre on grid sampling may have little if anything to show for their investment.

"So far, what it amounts to is about $7,000 worth of entertainment," says Rick McCracken, a west Middlesex county farmer who outfitted his combine with an AgLeader yield monitor and CSI beacon receiver for his 1995 crops.

"Yes, it's useful, but not like I thought it was going to be," McCracken says.

Like others, McCracken wanted the GPS system so he could map the yield variability across his fields and then take corrective measures to raise the lower yielding areas up to, or at least close to, the field average.

True to its promise, the system has produced yield maps. And, as McCracken expected, those maps do show there's lots of variability in his fields. Unfortunately, they also show that there's little if anything that McCracken can do to improve his harvests in low-yielding areas.

That's because it turns out low yields have more to do with topography than with fertility, McCracken explains. When he lays his yield maps over top of a map of his field elevations, he gets a near perfect match. Yields are best in low areas and on side slopes, and worst on the knolls. Conversely, when he lays his yield maps over his fertility maps, he gets a useless hodgepodge.

OMAFRA soils specialist Doug Aspinall has been studying GPS systems on 40 Ontario farms - including McCracken's - over the past four years and says McCracken's findings are typical.

"The biggest cause of yield variability in the field is a combination of elevation and moisture availability," Aspinall says. "Yes, we're seeing lots of variability in fertility, but that isn't what's causing the yield differences."

Aspinall had thought that yield maps would be the key to lifting the output of low yielding areas. Once the low yielding areas were identified, he explains, it would be relatively simple to crosscheck with a fertility map and then design a variable rate fertility program to lift those yields.

"It isn't going to work," Aspinall says. He and McCracken point to McCracken's farm, where fields are manured from the operation's 15,000 laying hens. McCracken says that since his grandfather's days the family has known yields were poorest on sandy knolls. So for three generations the family has boosted manure rates in those areas.

As a result, the knolls now are higher in fertility than the side slopes and even the hollows. Yet the knolls continue to produce yields that are at least 30 per cent below the field average. "It's the opposite of what might have been expected," Aspinall says. "Obviously it's a situation that we aren't going to improve by adding more fertility."

Yields are already reaching expected maximums on the side slopes, even though fertility is below the field average. Again, adding more fertility won't help.

Is there any use for fertility mapping? "Maybe in some specific situations," Aspinall says. "For most farms, though, I have serious doubts."

Other researchers are getting the same answers. At the federal agriculture department's Ottawa experimental farm, corn scientist Lianne Dwyer is examining the lack of correlation between yields and grid-sampled soil analyses. "Only about 30 per cent of variations in yield can be explained by regressing yield against soil variables," Dwyer says.

At Ridgetown College, soil scientist Ivan O'Halloran shares the growing skepticism. O'Halloran believes that ongoing research exposes grid sampling as essentially useless.

"Grid sampling just isn't paying off, especially for field crops like corn, soybeans and small grains," O'Halloran says.

"There has been and continues to be a tremendous push to conduct grid sampling," O'Halloran continues. "The message given to farmers is that if they want to be in site-specific farming, grid sampling is the first step. This is definitely not true."

O'Halloran cites the fact that yield maps rarely correspond with fertility maps. So if growers fertilize according to the maps, they'll likely be wasting more money, he argues. Most important, however, is that without yield records, it's virtually impossible for farmers to figure out they're on the losing end.

In a typical situation, a field that has been rarely soil-tested may get grid sampled, O'Halloran explains. Results show combinations of high and very high P and K readings across the field. The farmer then edges back on fertility rates in the very high areas and, when yields continue to be good, gives the credit to grid sampling. In reality, O'Halloran says, "variable applications always appear to work because every location in the field is still being over-fertilized." It would have made more sense, he says, for the farmer to save the money from grid sampling, do a normal soil test and set fertilizer rates according to realistic recommendations.

Stratford Agri-Analysis is among Ontario's top grid samplers with a fleet of 12 trucks and 250,000 acres already grid sampled. Unlike some other grid samplers, Stratford Agri-Analysis doesn't sell fertilizer, says company agronomist Gary Roberts.

"We aren't making money by pushing more fertilizer," Roberts says. "Instead, we're pushing grid sampling because it's good for our customers."

Roberts says he agrees that in tightly controlled research trials it may look like variable-rate fertilizing may not pay. He says, however, that the research ignores on-farm risk management.

Few farmers are willing to take a chance on cutting back fertilizer rates, because they're concerned about yield potential, Roberts explains. Yet grid sampled maps, costing $10 per acre, can convince growers that portions of their fields are so high there's no need to continue high fertilizer rates.

"There's never only one thing going on in a field...real life is much more complicated than that," Roberts says. Farmers know that fertility isn't likely to be the sole factor behind yield: Issues such as elevation, compaction and moisture also play key roles.

But if fertility is the cause of 30 per cent of yield variability, as studies suggest, at least it's a factor growers can have some control over, Roberts says. "It may be the difference between profit and loss."

McCracken adds that yield monitors help in unexpected ways. For instance, he gets a $7 per tonne premium if he can move wheat straight from the combine to Strathroy Flour Mills. To do that, wheat must be dry. "I hadn't realized how quickly wheat moisture goes up and down....We can see a point difference in as little as 15 minutes," he explains. "With on-the-go moisture testing, we have a much better shot at that market."

His yield mapping program also helped identify a pocket of muck soil that needs manganese for better wheat yields. "The benefits are there, but they come in small increments," McCracken says. "We thought the focus was going to be on making improvements to our fertility program....As it turns out, we aren't heading in that direction at all."

Look beyond yield for benefits of GPS

North Huron county farmer Bruce Shillinglaw has five years' experience with GPS systems and sells AgLeader yield monitors. Shillinglaw sides with McCracken and O'Halloran. "Plain and simple, our fertility mapping doesn't correlate with our yield maps," Shillinglaw says.

Farmers can't expect to pay for their yield monitors through lower fertilizer bills, he says, or through higher yields from variable rate fertilizer application.

Shillinglaw says most yield variability is related to elevation, slope and moisture, not fertility. And even if fertility were key, there's little farmers could do to manage it. "Our ability to sample our soils simply isn't adequate," he explains. Shillinglaw has conducted tests using different companies to sample his fields, and compared them to soil samples he's taken from the same bucket and sent to different labs for analysis. "When you see how variable the results are, you wonder how they could ever have come from the same field, let alone the same sample."

But Shillinglaw says yield mapping will still pay for itself. Monitors show growers the effects of weed escapes, soil conditions, and stand uniformity. "There's no question it makes you a better manager," he says.

For example: Yield monitors give growers a chance to conduct more on-farm research. It's relatively simple, for instance, to change corn populations for a strip up the field and then measure the consequences in the fall.

© copyright 1999 Agricultural Publishing Company Limited.


back