Harvest Aid Weed Management in Wheat

June 2019

Heavy rains in some areas have resulted in wheat fields that are weedier than normal and led to questions about the use of herbicides for harvest aid weed management. A harvest aid application will help reduce the amount of green, weed material the combine has to process. Green material at harvest can lead to higher grain moisture and cause moisture/plant material build-up on critical combine components like concaves, chaffers and sieves. For wheat, 2,4-D, dicamba, glyphosate, metsulfuron and Aim® EC (as well as combinations of these products) are labeled for use as harvest aids (Table 1). Sharpen® also is labeled; however, it is not included in the table on page 2 as maximum residue limits (MRLs) have not be set for all export markets. There are commercial products not labeled for harvest aid use, so it is important to read individual product labels. Paraquat is one active ingredient that is not labeled for use as a harvest aid in wheat. Paraquat residues are easily detectable and have resulted in condemned wheat. For products that are labelled, follow directions carefully regarding herbicide carriers and additives. Using diesel fuel as the carrier for harvest aid herbicides will result in loads of wheat being rejected at the elevator due to contamination and odor.
All pre-harvest applications should be made when wheat is in the hard dough stage (30 percent or less grain moisture). The pre-harvest interval for most harvest aid herbicides is seven to 14 days. Weeds may be large and fairly mature at the time of a harvest aid application, therefore it may take seven days or longer for weeds to dry down. Because of this, it is important to use the full product and carrier rates to achieve maximum weed control. Depending on the weed species, harvest aid herbicides may stop weed growth, but will not kill weeds. Be sure to check the product label for appropriate use rates and rotation restrictions. The use of 2,4-D, dicamba and metsulfuron may impact a following double crop.
If winter annual grassy weeds are the problem, it may be necessary to delay harvest until weed seed heads are mature enough so they can be threshed by the combine and separated from the wheat by the combine’s cleaning fan. Bromus species (downy brome, true cheat and rescuegrass) and wild oats do not usually delay harvest, as they mature before or near the same time as wheat. However, ryegrass can require delaying wheat harvest, by which time lodging and summer weeds can cause serious harvest issues.

Table 1. Herbicides labeled for use as a harvest aid in wheat.     
HerbicideMode of actionWeed notesMaximum ratePreharvest intervalNotes
2,4-DSynthetic auxinNo grass control, kochia and wildbuck are tolerant1 pint/A (5.5 lb ae/gal)
2/3 pint/A (6 lb ae/gal)
14 daysBe aware of plant-back restrictions with the use of 2,4-D if planning to double crop.
dicambaSynthetic auxinNo grass control8 fl oz/A (4 lb ae/gal)7 daysApply when wheat is in the hard dough stage or when the green color is gone from the nodes/joints of the stem. A germination test should be performed on preharvest-treated seed to ensure satisfactory germination. Be aware of plant-back restrictions with the use of dicamba if planning to double crop.
glyphosateEPSP synthase inhibitor Resistant horseweed, kochia and pigweed1 qt/A (3 lb ae/gal)
1.5 pints/A (4 lb ae/gal)
22 fl oz/A (4.5 lb ae/gal)
7 daysDo not apply to wheat grown for seed, as reduction in germination or vigor may occur. There are no rotation restrictions with glyphosate.
metsulfuronALS inhibitorResistant horseweed, kochia and pigweed0.1 oz/A10 daysTo improve weed management, metsulfuron should be applied in combination with glyphosate, 2,4-D, or dicamba. The addition of a surfactant is required.
Aim EC (carfentrazone)PPO inhibitorNo grass control2 fl oz/A7 daysApplications should be made in spray volumes sufficient to provide complete coverage of foliage. The addition of a surfactant is required.

Horseweed/marestail (Conyza Canadensis L.) in Kay County during the 2017-18 winter wheat season.


Misha Manuchehri
Small Grains Weed Science Extension Specialist

Heath Sanders
SW Area Extension Agronomist

Josh Bushong
NW Area Extension Agronomist

DASNR Extension Research CASNR
OCES  Contact
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Oklahoma State University
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