Kerb (Pronamide) is one of the three primary herbicides used in lettuce. The latest statistics available indicate it is used on almost 30 percent of the state's lettuce acreage treated with herbicide.

It is a mainstay in lettuce production in the low desert growing season from September to April. However, in recent years complaints about poor weed control with Kerb use have been increasing.

Tests conducted over the past two years have demonstrated that the cause for many of the cases of poor weed control was the leaching of Kerb below germinating weed seeds with sprinkler irrigation. Kerb works by inhibiting root growth of developing seedlings. It has postemergence activity on some very small weeds but it is important that roots and shoots absorb a lethal dose of the herbicide before they are well developed. The location of the herbicide at the time of weed seed germination and early development will greatly affect the level of control.

Most weed seeds that emerge with the crop are located within the top one-half inch of soil. It is a challenge to keep the herbicide in this region while applying enough water with sprinklers to germinate the crop and cool the soil. Kerb has commonly been applied after planting and prior to starting the sprinklers. One strategy to maintain the herbicide in the top one-half inch of soil has been to delay the application until after the sprinklers have been started but prior to the germination and early development of the weed seeds.

Germination factors

Several factors will influence weed seed germination after sufficient moisture is present. The most important of these are soil temperature and the temperatures at which the weeds that are present, germinate. As soil temperatures change from late summer, through the fall and winter and into the spring, the most effective time to apply Kerb likely will change.

A test was conducted to evaluate how long it takes for certain common weeds to germinate after moisture is present. This test was conducted in eight commercial lettuce fields from August to February in Roll, the Gila Valley, the Yuma Valley in Arizona and in Bard, Calif.

The test contained a winter annual grass, summer annual grass, two winter annual broadleaf weeds and three summer annual broadleaf weeds.

The time to germination will vary considerably even within the same field due to variations in soil and microclimate. Every year could be different and this test was intended to provide a general indication of germination times for various common weed seeds. The summer annual grass (barnyardgrass) took 24 hours to germinate in August and September, becoming longer in October (48 hours) to December (96 hours) with no germination in January. The winter annual grass (canarygrass) took 168 hours to germinate in September and dropped to 96 hours in December. The summer annual broadleaves took from 24 hours (purslane) to 96 hours (pigweed and nightshade) to germinate. The winter annual broadleaf weeds (lambsquarters and shepardspurse) took from 72 to 168 hours to germinate.

Time to apply

The weed seed germination test indicated that the best time to apply Kerb to minimize leaching and maximize the amount of herbicide in the zone where weed seed was germinating could be from 24 to 168 hours after the sprinklers had started.

Tests were conducted to determine the best time to apply Kerb after sprinkler irrigation had started. Two tests were conducted during the early season (August-September), two tests during the mid-season (October-November) and one during the late season (January-February).

All five tests were small plot tests conducted at the University of Arizona Yuma Valley Agriculture Center. Two pounds per acre of Kerb were applied either prior to starting the sprinklers or one to six days after they had been started. Each treatment was applied in a 20-gallon-per-acre spray volume and replicated four times.

For the early season tests, common purslane was planted into the plots to insure a uniform infestation of weeds. Wild mustard was planted for the mid and late-season tests. Evaluations were made by visually estimating percent weed control.

The effects of delaying Kerb applications to improve weed control will vary from season to season, year to year and field to field. It is possible, however, to draw some general conclusions:

  • Kerb is ineffective when applied prior to starting sprinklers except on some of the most sensitive weeds.

  • Kerb should be applied one to three days after starting sprinklers in early-season plantings (August - September).

  • The most effective time to apply Kerb during mid-season (October - November) varied from three to six days after the sprinklers were started.

  • The most effective time to apply Kerb in the late season (January) was five to six days after the sprinklers had started.

Increases crop injury chances

Any method that concentrates the herbicide in this region will increase the possibility of crop injury.

Lettuce can tolerate Kerb and other herbicides when growing conditions are favorable. Anything that puts the crop under stress during stand establishment lowers this tolerance. This includes heat, cold, salts, disease, insects, poor nutrition and weak or inappropriate varieties.

Primed and unprimed seed were compared in this year's test to evaluate this as a potential cause of crop injury. Priming did not cause increased injury in these tests. The only injury that has been observed has been from the treatments that were applied after the lettuce had emerged and when conditions were not favorable for rapid growth.

Delaying Kerb applications requires that aerial application be made. Aerial applications are generally less precise than ground applications and overlaps and skips are more likely. Where overlaps occur, a 2x rate is applied and injury more likely.