Preemergence Herbicide Combinations for Onion Weed Control Study

K. Umeda, G. Gal, B. Strickland

Abstract

DCPA (DacthalŽ) at 9.0 lb AI/A, the commercial standard herbicide, slightly reduced the number of plants and shortened the plant height compared to the untreated check. Preemergence (PREE) herbicide treatments that caused injury as height reduction similar to Dacthal included pendimethalin (ProwlŽ), propachlor (RamrodŽ), metolachlor (DualŽ), dimethenamid (FrontierŽ), ethofumesate (NortronŽ), and benefin (BalanŽ). Visual observations did not indicate significant stand reduction or crop injury compared to plant counts. Treatments that caused minimal visible crop injury (<10%) were Ramrod, Balan and lower rates of Frontier and Nortron. Moderate to acceptable injury (15%) was observed on onions treated by Prowl, Dual, Frontier, and Nortron. Treatments that significantly reduced crop stand were Prefar at 6.0 lb AI/A, lactofen (CobraŽ), thiazopyr (VisorŽ), and some combinations of the three herbicides. Combination treatments that caused marginally acceptable injury included Prowl plus Dual, Nortron plus Prowl, Prefar plus Nortron, Nortron plus Frontier, Ramrod plus Frontier, Ramrod plus Nortron, and Ramrod plus Balan. Early weed control ratings showed that Prowl at 0.5 and 0.75 lb AI/A, Nortron, Cobra, Visor applied alone gave acceptable control (>85%) of London rocket (Sisymbrium irio), sowthistle (Sonchus oleraceus), and sweetclover (Melilotus officinalis). Prowl at 0.75 lb AI/A and Cobra gave season-long control of all weeds. Early weed control ratings of combination herbicide treatments demonstrated that Prowl or Nortron combined with other herbicides gave acceptable weed control. Prowl at 0.50 lb AI/A plus Ramrod, Nortron, or Dual at the lower rates slightly improved weed control compared to each of the herbicides applied alone.

Introduction

Dry bulb onion production practices will be significantly challenged when Dacthal herbicide no longer becomes available for PREE weed control at planting time. Dacthal has for many years provided safe and effective broadspectrum weed control during the period to establish an onion crop. Onions are generally a very slow crop to germinate and grow and weed competition can severely reduce the crop stand, quality, and delay crop maturity. Fall planted onions have few summer annual weeds [purslane (Portulaca oleracea) and pigweeds (Amaranthus spp.)] and several winter annual weeds that compete during the long growing season from October to May. A sufficient number of effective postemergence herbicides are available but most require that the onions be at the 2-leaf stage of growth to ensure crop safety. Replacing Dacthal is a critical necessity so that onions can emerge and progress through the emergence and seedling stage so that postmergence (POST) herbicides and mechanical cultivation can be used. This test was conducted to evaluate several potential PREE applied herbicides alone and in combinations to determine efficacy and crop safety.

Materials and Methods

A small plot field test was conducted at the University of Arizona Maricopa Agricultural Center, Maricopa, AZ. The field was prepared per typical cultural tillage operations and then conventional 40-inch beds were listed and shaped. Onion cv. Henry’s Special was planted two seedlines per bed on 23 November 1997. The herbicide plots consisted of two beds and measured 25 ft long and treatments were replicated three times in a randomized complete block design. The PREE herbicide treatments were applied using a CO2 backpack sprayer equipped with a hand-held boom with four flat fan 8002 nozzle tips. The herbicides were delivered in 22 gpa water that was pressurized at 35 psi. The soil was dry immediately after planting and at 74 F. The weather conditions at the time of application on 24 Nov were skies with high scattered clouds, air temperature at 82 F, and slight breeze at less than 5 mph. The onions were furrowed irrigated immediately after herbicides were applied. Water was applied as necessary throughout the season. At intervals during the growing season, crop measurements and visual observations were recorded for weed control and crop injury.

Results and Conclusions

All PREE herbicide treatments caused injury to the onions relative to the untreated check (Table 1.). Dacthal at 9.0 lb AI/A, the commercial standard herbicide, slightly reduced the number of plants and shortened the plant height compared to the untreated check. Visual observations did not indicate significant stand reduction or crop injury compared to plant counts. Herbicide treatments that caused injury as height reduction similar to Dacthal included Prowl, Ramrod, Dual, Frontier, Nortron, and Balan. Treatments that caused minimal visible crop injury (<10%) were Ramrod, Balan and lower rates of Frontier and Nortron. Moderate to acceptable injury (15%) was observed on onions treated by Prowl, Dual, Frontier, and Nortron. Treatments that significantly reduced crop stand were Prefar at 6.0 lb AI/A, Cobra, Visor, and some combinations of the three herbicides. Combination treatments that caused marginally acceptable injury included Prowl plus Dual, Nortron plus Prowl, Prefar plus Nortron, Nortron plus Frontier, Ramrod plus Frontier, Ramrod plus Nortron, and Ramrod plus Balan.

During the period of onion stand establishment, few herbicide treatments provided very good weed control. Early weed control ratings showed that Prowl at 0.5 and 0.75 lb AI/A, Nortron, Cobra, Visor applied alone gave acceptable control (>85%) of London rocket, sowthistle, and sweetclover (Table 2). Near the end of the season, Prowl at 0.75 lb AI/A and Cobra continued to give better than 85% control of all weeds. Early weed control ratings of combination herbicide treatments demonstrated that Prowl or Nortron combined with other herbicides gave acceptable weed control. Prefar or Ramrod combined with other herbicides did not offer acceptable levels of season-long weed control. Combinations of herbicides did not provide improved season-long weed control compared to herbicides used alone. Prowl alone at 0.75 lb AI/A controlled weeds season-long and addition of other herbicides did not increase weed control efficacy. The addition of Prefar or Cobra to Prowl substantially reduced the crop stand and height. Nortron at 2.0 lb AI/A alone gave acceptable control of all weeds at the early rating date and declined as the season progressed. Prowl at 0.50 lb AI/A plus Ramrod, Nortron, or Dual at the lower rates slightly improved weed control compared to each of the herbicides applied alone.

Potential use of Prowl, Ramrod, Dual, Frontier, Nortron, and Balan in onion weed control programs may offer options to target specific weed problems with single or combinations of herbicides when integrated with timely cultivations and postemergence herbicides. All of the herbicides were not completely safe on onions in this test. Several herbicide treatments provided sufficient weed control to enable the onions to establish a stand. Future studies would incorporate the use of timely postemergence herbicide applications and cultivations to ensure season-long weed control.

Table2

This is a part of publication az1101: "1998 Vegetable Report," College of Agriculture, The University of Arizona, Tucson, Arizona, 85721.
This document located at http://ag.arizona.edu/pubs/crops/az1101/az1101_13.html
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