Wet fields
made spring weed control difficult to impossible in many winter wheat fields,
and prolonged wet conditions have encouraged prolific weed growth from large
competitive broadleaf weeds like giant ragweed and lambsquarters. As we approach harvest in southern WI (week
of August 21st) growers simply have limited herbicide options for preharvest
weed management:
1. 2,4-D products.
There is a 7 day pre-harvest interval with this product. The downside of 2,4-D
is you are NOT allowed to feed
treated straw to animals. That restriction alone probably leaves just one
option….
2. Glyphosate products. There is also a 7 day pre-harvest interval with this product, and it
can NOT be applied until the grain
is at the hard dough stage (30% moisture or less). Grain treated with glyphosate at this growth
stage should not be used for seed as germination can be significantly lowered.
Some benefits to applying
preharvest glyphosate may include desiccation of green weedy plants to enable
an easier combine harvest and quicken the ability to bale straw following the
grain harvest. However, drawbacks
include a narrow window of application timing ahead of harvest, wheel tracks
(if ground applied) will reduce grain yields, and moreover many of the weeds
like giant ragweed and lambsquarters will be large and difficult to control. Thus, consider a preharvest glyphosate application
as a last resort because partial control of large weeds will greatly increase
selection pressure for glyphosate resistance.
We already have glyphosate-resistant giant ragweed in southern WI ,
and other broadleaf weeds continue to be a concern.
Shawn Conley and Vince Davis
