A land of tranquility and historical significance lies in the vast valley below Picacho Peak. The towering spire juts 3,300 feet above sea level and shadows the low desert floor in south-central Arizona.

Picacho means ‘big peak’ in Spanish. Experts believe the dark red-colored jutted peak, located between Phoenix and Tucson, is a tilted, eroded rock left behind by a lava flow.

Just a stone’s throw from this natural Arizona rock marvel, agricultural experts of sorts (specialists) are making strides in field studies on a commercial pecan farm in the community of Red Rock to make pecan operations more profitable.

Precision ag a platform

Specialized precision agriculture equipment is being used as a platform for multiple research projects – in this case to advance weed control and reduce herbicide use in pecans, and improve usable light in tree canopies to maximize pecan crops and tree health.

The researchers agree that the science learned here could be used in other commercial tree orchards and row crop agriculture.

The specialists include: Bill McCloskey, University of Arizona (UA) weed scientist; Richard Heerema, New Mexico State University (NMSU) pecan specialist; and Pedro Andrade, UA precision agriculture specialist.

The local farmer cooperator in the trial is Herb Kai. Managing the 510-acre pecan farm is Barry Armstrong.

The three-year project is funded in part through a $60,000 grant from the Arizona Department of Agriculture through the USDA specialty crops block program.

Precision ag tools

On a perfect spring day in early March, Andrade unloaded a Kubota diesel ATV from a trailer and drove the 4X4 into a seven-year-old commercial pecan orchard.

The ATV is outfitted with active spectral sensors with optical filters from Trimble (WeedSeeker) and Holland Scientific (ACS-430). These sensors shine light on the ground to measure light reflectance to determine exact weed locations and the approximate size or weed density. 

A second type of sensor from the company Apogee is a two-inch-long infrared thermometer which measures the surface (ground or vegetation) temperature. McCloskey says a weed is present when a sensor finds a sudden decrease in temperature on the orchard floor.

Also included on the rig is a 'ruggedized' field computer for data acquisition and actuator control. A state-of-the-art global positioning system (GPS) now available commercially – the Trimble CFX-750 – is included.

It gathers GPS coordinates inside a leaf- and limb-laden canopy, feeds positioning information to the field computer for herbicide application control to precisely spray herbicides directly on weeds, not the ground. GPS data is also used to geo-reference light intensity information generated as the platform moves inside the orchards.

Reducing herbicide use

McCloskey is testing various herbicides (active ingredients), application strategies, and spray timing for weeds.

“My goal in this project is to develop a specific list of recommendations based on real world data to help reduce the weed population in fields, herbicide treatments, and field passes,” McCloskey said.

Once in place, the weed specialist says, “The field would only need spot spraying - perhaps a dozen plants on one-half acre - which could help growers reduce expenses.”

The most worrisome weed in Arizona pecan orchards is junglerice, McCloskey says. It’s a tough weed to control and could become resistant to herbicides in Arizona in the future as it already has in other countries. He urges growers to use weed control tactics and herbicide mechanisms of action (MOA) for effective long-term weed management.

Preventing herbicide-weed resistance

In a UA greenhouse about three years ago, McCloskey confirmed the first case of weed resistance to herbicide in the Grand Canyon State – Palmer amaranth and Roundup. Several additional cases have been confirmed since.

McCloskey urges growers to use multiple herbicide active ingredients with different MOAs for weed control in all crops. He notes that not a single new herbicide mechanism of action has been discovered in the last two decades, though several products used in other crops are gaining registrations against weeds in pecan orchards.

Uncontrolled weeds in pecan orchards under 10-years old, McCloskey says, can substantially stunt tree growth, delay the onset of harvest, and reduce yields. The cross sectional trunk area of young trees can be cut in half by a small amount of weed competition during the growing season.

A second Arizona pecan field trial site is located in Sahuarita under the management of FICO. There are two other weed control study sites in Cochise County.

The best canopy for optimum yield, quality

UA engineers Pedro Andrade and John Heun integrated specialized instrumentation to the front section of the Kubota ATV to measure the distribution of light hitting the ground as the ATV moves inside the orchard.

The mechanical structure in front includes a 25-foot-wide array of light bars manufactured by Decagon. In this line of work, Andrade is collaborating with NMSU’s Heerema. Together, they are conducting field trials on the Arizona sites and in New Mexico’s famed pecan belt, the Mesilla Valley.

The New Mexico field trial is a hedging study examining the frequency of mechanically pruning pecan trees - topping and siding – and how its frequency impacts light interception and fruit production.

In Arizona, Andrade and Heerema’s work focuses on how the direction of mechanical pruning in rows impacts light interception and yield.

Optimal pruning

In the Mesilla Valley, their goal is to determine optimal pruning regimens for pecan trees to allow the most usable sunlight to penetrate the tree canopy to maximize photosynthesis to produce and maintain a healthy pecan tree with high nut yield and quality.

The light bars measure usable penetrable light through the tree canopy called photosynthetically active radiation (PAR – usable sunlight) which drives photosynthesis to create carbohydrates to produce fruit.

“If the canopy keeps all of the light at the top and not enough makes its way into the canopy, the canopy will not receive sufficient light to produce pecan fruit,” Heerema said.

In this case, the survival rate of shoots would decline. A large part of the canopy would die causing the orchard to never reach its full potential.

“We’re trying to determine what percentage PAR interception is optimal for production to maintain high yields, plus maintain a larger crop in alternate bearing years,” Heerema said.

Recommended PAR

Heerema guestimates that 60 percent PAR is required in the canopy to help produce a good crop.

“We want to produce higher yields in bearing and non-bearing years.”

Using precision agriculture means collecting huge data sets. In a June 20 field trial, Heerema says the equipment collected about 20,000 data points.

Heerema noted, “We’re working our way through huge datasets to achieve these recommendations.” It’s a slow process.

A federal grant by the USDA-NIFA Specialty Crops Initiative (SCRI) is financially supporting the experimental work of Andrade and Heerema. This grant provides an excellent opportunity to collaborate with other universities, including University of California Davis, Washington State, and Oregon State.

Using the same collaborative approach, Andrade and Heerema could generate this same type of data in a proposed pecan study in cooperation with UA Extension Specialist Jim Walworth to study the impact of soil-applied zinc on tree growth.

Back to the Red Rock trial, pecan grower Barry Armstrong is anxious for the specialists to complete their studies so he can implement the findings.

“As a grower, this will save us time and money,” Armstrong said. “The fewer chemicals we spray is a good deal for everyone.”

McCloskey added, “It’s a win-win.”