University of Arizona a dot Cooperative Extension



Presented at the 51st Annual conference of the California Weed Science Society in Anaheim, CA on January 12, 1999.

Umeda, K. 1999. "Life After Dacthal" - The Continuing Search for Alternatives - Herbicide Weed Control Programs for Dry Bulb Onions . University of Arizona, College of Agriculture and Life Sciences, Cooperative Extension, Tucson, Arizona. URL:

The projected loss of DacthalŽ (DCPA) herbicide will significantly impact many vegetable crop herbicide weed control programs. Dry bulb onion production will be affected with the loss of the effective broad spectrum at-planting time preemergence (PREE) herbicide. Onions emerge and grow extremely slowly after planting and the competitive weeds generally cannot be controlled with postemergence (POST) herbicides until the onions reach a more tolerant size. Hand-hoeing is often a very expensive alternative in place of the POST herbicides. Mechanical cultivation on raised beds is nearly impossible when there are as many as eight or nine seedlines on a forty-inch bed. The search for alternative weed control methods for onions intensified when the cost of Dacthal escalated after production ceased.

During the 1997-98 onion growing season in the low desert growing area of Central Arizona, several small plot field tests were conducted to evaluate potential PREE herbicides for use in onions when applied alone or in combinations and early timing of applications of POST herbicides. Herbicides applied PREE included pendimethalin (Prowl), bensulide (Prefar), propachlor (Ramrod), ethofumesate (Nortron), metolachlor (Dual), dimethenamid (Frontier), benefin (Balan), and thiazopyr (Visor), and lactofen (Cobra).
Treatments including Visor and Cobra severely reduced the onion crop stand when applied PREE. Some treatments that appeared to be relatively safe included Prowl, Prefar, Ramrod, Nortron, Dual, Frontier, and Balan. Prowl alone or combinations with Ramrod, Dual, or Nortron gave good weed control of London rocket, sowthistle, and yellow sweetclover. Prefar combined with Ramrod or Dual was more effective than either of the herbicides applied alone. Nortron combined with Dual or Frontier improved when compared to the herbicides applied alone.

The type of irrigation affected herbicide performance and crop safety. Prowl applied PREE and furrow irrigated did not cause significant onion injury. Under sprinkler irrigation, Prowl treated onions were significantly reduced and the stand was visibly thinned. Dual and Frontier demonstrated the reverse effect by injuring the onions when furrow irrigated and negligible injury was observed under sprinkler irrigation.

Bromoxynil (Buctril) and oxyfluorfen (Goal) provide good POST weed control in onions when weeds are small but the onions must be at the 2- to 3-leaf stage of growth. Crop injury can be associated with the application of the POST herbicides. Prowl and trifluralin are labeled for POST use as a layby application and offer good control of late season emerging weeds. Combinations of Prowl with Buctril or Goal eliminate many existing weeds and provide good control for the remainder of the season. Crop injury was more severe when onions were treated by Buctril plus Prowl at earlier application timings especially when the weather was inclement. Onions must be growing under ideal climatic conditions to ensure the development of the leaf cuticle so that crop injury is minimized.

Rates below the Buctril and Goal labeled rates were evaluated when applied at an earlier timing to determine efficacy and crop safety. Buctril and Goal at 0.063 to 0.125 lb AI/A were applied to small onions at the flag leaf to first true leaf stage of growth. Crop safety was excellent for all rates and weed control was acceptable and exhibited a rate response with improving weed control with increased rates.

Suggestions for onion weed control in 1998-99 include continuing Dacthal use as long it is economically feasible, evaluate Buctril and Goal at early timings for POST applications, and evaluate combinations of Buctril or Goal with Prowl at layby. Use caution when applying POST herbicides and be sure that onions have a well-developed cuticle for protection. Always follow label instructions when using any herbicide.

The continuing search in 1999 and beyond include continuing evaluation of Ramrod, Dual, Frontier, and Nortron in combinations, evaluating Prowl combined with Prefar at rates below normal use rates, evaluating Prowl applied preplant incorporated and PREE, and evaluating new emerging herbicides soil-applied and POST.

(*Product names mentioned are registered trademarks.)


Full Disclaimers

Issued in furtherance of Cooperative Extension work, acts of May 8 and June 30, 1914, in cooperation with the U.S. Department of Agriculture, James A. Christenson, Director Cooperative Extension, College of Agriculture and Life Sciences, The University of Arizona.

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Information provided by:
Kai Umeda, Area Extension Agent, Vegetable Crops
University of Arizona, Tucson, Arizona.
Material written January 1999.

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