Successful weed control trials on lettuce in western Fresno County suggest that a Section 24(c) registration for Kerb herbicide applied through sprinklers will be sought for the fall 2006 season.

Kurt Hembree, Fresno County farm advisor, says chemigation with Kerb demands a degree of care in timing and procedures but after his two seasons of trials “we are pretty convinced it works well.”

At a recent vegetable production meeting at the University of California's West Side Research and Education Center at Five Points, Hembree outlined the history behind the idea.

He said a breakthrough was achieved with the concept 2003 in Arizona after extensive tests by Yuma County agent Barry Tickes. That year the practice got a Section 24(c) registration in Arizona. The same registrations were subsequently granted for Riverside and Imperial counties, which have similar conditions.

Kerb chemigation

Tickes worked with chemigation of Kerb as an alternative after aerial applications of it were halted in the aftermath of the Sept. 11, 2001, terrorist attack in New York City. He found that control, particularly on shepherd's purse, with Kerb through sprinklers was much improved over the aerial treatments.

Kerb currently is registered for ground and aerial application in Fresno County, and researchers have probed the chemigation concept after the increased use of sprinkler irrigation. Kerb dates back many years to when it was applied to furrow irrigated fields.

“With furrows,” Hembree said, “subbing of water kept the herbicide in place. But when growers started switching to sprinklers, the efficacy declined, and we wondered if sprinkler irrigation was driving it down in the soil profile.”

Several attempts to track leaching of the material remain inconclusive, but regardless of what is happening to it in the soil profile, Hembree and other weed control specialists in California took note of Tickes' success with chemigation.

During the past two seasons, Hembree has worked with the California Lettuce Research Board and Kerb's manufacturer, Dow AgroSciences, to refine the method in his county. Data from his trials is being submitted for the registration.

“Basically,” he said, “you put on the first irrigation and then you come back so many days later with the Kerb chemigation. The number of days later varies with the season, the field, and growing conditions.”

Weed seed factor

He said a new registration, which may also be sought in the future for Monterey County, would focus on the point from when weed seeds have sprouted until they have emerged, the period they are most sensitive to the herbicide. It would allow for rates of 2 to 2.5 pounds per acre instead of the ground rate of up to 4 pounds.

Tests during the past two years on the amount of water to apply in the chemigation found the optimum range to be 0.15 to 0.25 inch. Depending on nozzle size and pressure, Hembree said, the running time is about one hour to 90 minutes. To “set” the material for exposure to emerging weeds without driving it lower, about 0.4 inch of additional water works best.

For best results, Hembree said the steps have to be closely followed. In lettuce planted in August and September, the routine worked well with chemigation three or four days after the first irrigation. The lettuce crop emerged at five to six days, and the weeds were caught at a susceptible state.

“Having adequate amounts of pipe available for the staggered planting dates and delivering the proper amounts of water are critical for treating the right field at the right time. Scheduling can be difficult, but it can be done. The diamond pattern for sprinklers like they use in Arizona gave a good overlap of coverage and worked well for us.

“You also want to use spherical tanks with continuous agitation so the material doesn't settle out like it will in cylindrical tanks. The recommendation is three gallons of water for each pound of Kerb.”

For winter-planted fields, the chemigation likely will need to be made perhaps up to seven to 10 days later than for fall-planted fields during periods of cooler temperatures and slower weed germination.

Chemigation timing

The key for timing the chemigation, Hembree added, is to act on the state of weed germination, not follow a mind-set of so many days. “If it's cold and wet, you have to wait until they begin to sprout.”

He said the cost of the chemigation treatment is $5 to $10 per acre greater but the additional control is worth it.

Hembree said if the chemigation registration is granted, he would be working with Dow AgroSciences officials to hold workshop training for growers and PCAs on the fine points of the practice. The Section 24(c) registration for Fresno County would seek to include endive, escarole, and radicchio also.

In another presentation during the meeting at Five Points, Shannon Mueller, Fresno County farm advisor, reported on her trials with mini seedless watermelons at Five Points, Parlier, and Hollister.

Her colleague in Fresno, Richard Molinar, and Aziz Baameur, Santa Clara County farm advisor, collaborated in the trials on yields, sweetness, and other fruit characteristics of several sterile triploid, “personal” varieties.

These melons, weighing three to seven pounds each, have been on the market for the past two years as an alternative to traditional seeded melons weighing 18 to 35 pounds and “icebox size” seedless melons weighing three to 12 pounds.

Mueller said marketers for the mini melons promote their thinner rind, which implies more edible flesh. “But one of the concerns we have,” she said, “is the thinner rind also makes it more susceptible to damage during picking, handling, and transport.”

Rind thicknesses ranged from one-quarter inch to one-half inch, and the Petite Perfection variety had the thinnest rind, while the Valdoria had the thickest.

In informal taste testing at a field day, she said, they found that the orange-red color of the small melons tended to be less appealing than the darker red of others and that higher sugar levels did not necessarily correlate with taste preferences.

A portion of the trials was devoted to a preliminary evaluation of plant spacing of mini melon transplants, and the results showed no difference in melon size with five spacings, ranging from 12 to 48 inches. Melons tended to weigh about five pounds each.