Planning onion weed control for 2018
Effective weed control in onions results in higher yields.
Dry bulb onion planting season is here. For successful weed management and maximum yield, have a plan for season-long weed control. Research has demonstrated that a few weeds in an onion field, especially during the first few weeks, may reduce yield. With a number of herbicides labeled for onion, you should be able to maintain control of most weeds throughout the growing season.
Weed control research in 2017 on muck soil demonstrated a 30-40 percent increase in yield in treatments where plots were treated with pendimethalin (Prowl H2O, Satellite Hydrocap) plus bromoxynil (Buctril, Moxy) 11 days after seeding. The same treatments applied three days after seeding had more weeds initially and during the early weeks of the season. The weeds were killed at the one-leaf stage in all plots, but the initial weed competition resulted in lower yields in the plots treated soon after seeding.
The delayed pre-emergence application allowed many weeds to emerge, which were then killed with the bromoxynil. The primary weeds in the research trial were ladysthumb and redroot pigweed. Subsequent applications of pendimethalin and post-emergence herbicides plus hand-weeding maintained all of the plots relatively weed-free during the rest of the season.
Apply the delayed emergence pendimethalin plus bromoxynil just before onions emerge. Emerged onion plants may be killed by the bromoxynil. Additional weeds that may germinate early include common chickweed, common lambsquarters, marsh yellowcress, shepherdspurse and Virginia pepperweed. Common purslane is not killed by bromoxynil, but can be controlled easily with GoalTender. GoalTender often misses ladysthumb and common lambsquarters that exceed two leaves, so early kill with bromoxynil is important for the season-long plan. Delayed emergence application of Prowl H2O also gives the inter-planted barley more time to become established.
The next emergence application of pendimethalin should be made at the onion two-leaf stage). The delayed emergence treatment maintains emergence herbicide activity for one to two more weeks during the critical early growth stages of onion. At the full one-leaf stage (second leaf showing), oxyfluorfen (GoalTender) may be applied at up to 4 fluid ounces (0.125 pound active ingredient) over the top of the onions for broadleaf weed control. Goal 2XL should not be used at the one-leaf stage because it can injure the onions severely. This application is also critical to maintain control of difficult weeds such as common lambsquarters, ladysthumb, marsh yellowcress and Virginia pepperweed.
In a cool, damp spring, common chickweed may germinate over several weeks and remain active throughout the season. Oxyfluorfen does not control common chickweed, so Nortron may be added to the pre-emergence tank mix for control. Chickweed normally dies out in a warm, dry year. If it persists, it can be controlled with a post-emergence application of Starane or Chateau. If the barley has not been killed yet, add a post-emergence graminicide, such as Fusilade, Poast or SelectMax, at the one-to-two-leaf stage.
At the onion two-leaf stage, reapply the pendimethalin or s-metolachlor (Dual Magnum) or dimethenamid-P (Outlook). At the three-leaf stage, apply flumioxazin (Chateau) at 1-2 fluid ounces per acre (0.032-0.064 pound active ingredient) to improve ladysthumb and pigweed control, and help suppress spotted spurge. The Chateau label allows tank-mixing only with Prowl H2O. Apply subsequent treatments of oxyfluorfen, pendimethalin, s-metolachlor, flumioxazin and the graminicides as labeled. The objective is to maintain the field weed-free throughout the season.
Weed control in onions on mineral soil is more difficult because of greater potential for crop injury from the herbicides. Most of the pre-emergence herbicides labeled for onion are labeled for use at the onion two-leaf stage on mineral soil. As we have seen from the discussion above, the first few weeks of onion growth are very sensitive to weed competition. The Satellite Hydrocap (pendimethalin) label allows application pre-emergence to onions on mineral soil at 1.5-3 pints (0.71-1.42 pounds active ingredient per acre). The label warns not to apply before the loop stage if heavy rains or excess irrigation are expected. If on sandy soil, use the lowest rate of Satellite Hydrocap and work up to a rate that is safe on the soil. After the two-leaf stage, onions on mineral and muck soil generally may be treated the same with post-emergence herbicides. Chateau may cause stunting if applied above 1 ounce per acre on mineral soil.
With a wide choice of herbicides and timings, you can maintain onion fields relatively weed-free. Hand-weeding may be required if weeds escape herbicide application, but any weeding activity in onion fields may displace onions and reduce yields. Wet weather may interfere with application plans, so always anticipate problems and work ahead a few days when possible. The practices described above should result in substantially higher yields with only moderately more inputs.
Dr. Zandstra’s work is funded in part by MSU’s AgBioResearch.
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