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Should precision agriculture target high-cost crop inputs?

Manila, Ark., cotton producer David Wildy isn't saying he's on the right track with his precision agriculture program; 10 years of experimentation has taught him better. But he's starting to realize that targeting his variable-rate applications toward high-cost inputs such as seed and fertilizer have the potential to save a lot of money.

Wildy, speaking at InfoAg Mid-South 2007 in Starkville, Miss., currently uses a Veris mapping cart to detect soil differences and aerial imagery to capture plant biomass variability, and he has implemented numerous studies on variable-rate applications for plant growth regulator, defoliant and seed, while looking into on-off applications for nematicides and insecticides.

Wildy wasn't long out of the gate in 1996 when grid sampling detected tremendous variation in phosphorus, potassium and pH on his farm. “The pH ranged from 4.7 to 7.0, which meant that our composite samples on a lot of fields were showing no need for lime.”

In fact, of 21 grid-sampled fields, only five would have called for lime when using a composite sampling method. “But our grid sampling called for lime on parts of all 21 fields. We put lime on those portions of the fields, spending $2.33 an acre. Putting out a flat rate on the five fields would have cost us $7.11 per acre. We also saved about $5 an acre with variable-rate fertilization versus a flat rate.”

Some low-cost inputs don't show as much of a cost benefit for Wildy, especially when he includes the cost of gathering precision information. “When I first started doing precision agriculture, I thought that variable-rate applications of plant growth regulator would really be a salvation for us because our cotton is so variable in growth. We did make cotton a little more uniform, but data for the last two years indicates that we aren't saving anything.”

Wildy says PGRs are so inexpensive today that there's not as much of a potential for savings. In fact, the savings hardly pay for the cost of the imagery that Wildy uses to establish his management zones, although the imagery can be used for other purposes.

There is more potential for savings with boll openers and defoliants, Wildy notes, primarily because they're more expensive. “The benefit is going to be field specific. In some years we have weaker areas of a field that almost defoliate themselves and certainly there we can have some savings. But in other years, when the weaker areas still have leaves, it takes a boll opener to get it open. So the rates needed vary much from year to year and field to field. It's not always cut and dry.”

Wildy stresses that growers and consultants should avoid variable-rate applications of insecticides, which can occur when an insecticide is piggybacked with a variable-rate application of PGRs.

“I agree with entomologists that the best way to build resistance is with sub-lethal doses of insecticide. I don't think we're doing the right thing by putting out variable-rate insecticides. The applications need to be either on or off. There is some equipment coming that will allow us to do some injection of insecticide and that will be helpful.”

Insecticide termination by zone offers another possibility for savings, according to Wildy. For example when fields are plant-mapped for COTMAN, field corners in a center pivot-irrigated field as well as other areas under the pivot often exhibit different growth curves, meaning they will be safe from insects at different times.

“Even if you have a plant bug infestation that is field-wide, if you can identify it by zones, you can possibly eliminate zones that are safe. This has to be done through an aerial image. Whether we have the time or the manpower to scout it is something we're going to have to work out.”

Wildy believes the greatest potential for savings is with variable seeding. “We're paying for a whole lot of crop upfront in seed cost. It's the highest input item we have. But with the variability we have across the field, we certainly know that we can't have the correct population everywhere.

“A bag of seed today of B2RF (Bollgard II/Roundup Ready Flex) will cost you about $102. We found that if we cut our seeding rate by half a seed per foot from 3.5 seeds per foot to 3 seeds per foot over my entire farm, it reduces my seed costs by $110,000.

“Are we going to use precision agriculture to save a dollar or two per acre on plant growth regulator or should we be looking at the highest input items? We need to see if we're going to hurt our yields by going to 3 or 2.5 seeds per foot? If we maintain yields, there are a lot of dollars there.”

Wildy's research compared yield performance at 2, 3 and 4 seeds per foot across several zones. “Three seeds per foot had the best yield fieldwide, but we think we can refine that even more by looking at different rates in different zones.

“If you divide the field in three zones and you have 2, 3 and 4 seeds per foot in each of the Veris zones and image zones, 2 seeds per foot had the best return per acre across all zones. If I plant 2 seeds per foot instead of 4, that's over $220,000 I can save.”

Wildy notes that some tweaking may be required to reap savings from targeting high-cost inputs. Recently he studied on-off applications of Telone fumigant based on a threshold of 250 nematodes per sample. “We increased yield 53 pounds per acre with the application, but by the time we figured in the sampling costs and bought the Telone, we lost $16 an acre. So the landlord appreciated it, but it didn't work out well for the grower.”

Wildy believes there is potential for savings when variable-rate applications are based on soil type information gathered by a Veris mapping cart, according to studies he conducted with the University of Arkansas in 2005 and 2006.

On a 160-acre block, the mapping cart, which correlates soil electrical conductivity to soil type, was used to draw soil zones so Telone could be applied to the sandier portions of the field.

Wildy put in three treatments and each treatment had 10 replications. “If you're going to do research, be sure to quantify your results. You can't just observe that a certain treatment made more than it did last year. You can't even compare one side of the pivot to the other side. You have to put in replications to help you decide if it's working or not, because you're spending a lot of money doing it.”

Wildy put out a check, a flat rate of 3 gallons of Telone and site-specific applications of Telone. The site-specific zones yielded an average of 93 pounds over the check. The flat rate of 3 gallons per acre had a 113-pound increase over the check.

“The flat rate produced our best yield, but our best economic value came from the site-specific application. On our flat rate treatment, we had an increase in net returns of $2,600 on the field compared to our site-specific treatment of $3,500. It looks interesting enough that I want to look at it some more.”

In fact, Wildy and his crew are building a site-specific nematicide applicator that rides on a Paratill. “This spring, we plan to apply nematicide on about 300 acres, with soil variability data gathered from the Veris rig.”

Wildy stresses that he still has a lot to learn about the technology. “To me we are just on the cutting edge, just now seeing the tip of the iceberg of all the things we're going to have to figure out.”

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