Weed Management in Small Grains Harvested for Grain1
Successful weed control in small grains involves using good management practices in all phases of production. In Florida, winter weeds compete with small grains for moisture, nutrients, and light, with the greatest amount of competition occurring during the first six to eight weeks after planting. Weeds also cause harvest problems the following spring when the small grain is mature.
Crop Competition
Crop competition is one of the most important considerations for weed control, but it is often overlooked in practice. A good small grain stand that emerges rapidly and shades the soil surface is helpful in reducing weed competition.
Utilizing good management practices is necessary to produce maximum small grain yields. Use of these practices also aids in weed control. The plant that emerges first and grows most rapidly is usually the plant that will have the competitive advantage; therefore, everything should be done to ensure that the small grain, not the weed, has this competitive advantage. For optimum management practices, follow UF/IFAS Extension recommendations.
Know Your Weeds
Know your weeds and choose the herbicide that is effective for your specific weed problem (Table 2). In Florida, small grains are infested with several species of winter weeds, most of which can be controlled by the phenoxy herbicides, namely 2,4-D or dicamba. However, phenoxy herbicides may not control some species that inhabit Florida small grain fields. Proper identification is crucial for control of wild radish (Raphanus raphanistrum), which is often confused with wild mustard (Brassica kaber). Wild radish is ubiquitous throughout Florida and is not easily controlled with 2,4-D or dicamba after it is established. Wild mustard occurs sparsely within the Florida small grain growing region and can easily be controlled with applications of 2,4-D. Most producers refer to both species as "wild mustard," therefore compounding the identification problem. If wild radish is misidentified, the degree of weed control given by 2,4-D or dicamba applications may be inadequate. 2,4-D does have activity on wild radish, but the restriction of 2,4-D applications tied to small grain size usually allows the wild radish to get too large to be controlled. Wild radish can be controlled by 2,4-D up to 6 inches or the 3-leaf stage. The confusion of wild mustard and wild radish is the most notable instance where a misidentification can cause substandard weed control in small grains. More information on wild radish identification can be found in EDIS publication SS-AGR-236, Wild Radish—Biology and Control (https://edis.ifas.ufl.edu/wg215).
Chemical Control
Proper herbicide and rate selection is extremely important for weed control in small grains (Table 1). Postemergence herbicides combined with the previously discussed management practices are important for ensuring that the small grain crop has the initial competitive advantage over weeds. Herbicide applications should be made to small weeds unless specifically stated by the label or recommendations.
The most widely used herbicide in small grains is 2,4-D. Small grains vary in their tolerance to 2,4-D, depending on the growth stage when the herbicide is applied, and the particular crop planted (Table 1). Generally, wheat varieties are the most tolerant. Barley is intermediate, and oats are least tolerant to 2,4-D. Rye is intermediate between wheat and barley. As a general rule, the least injury to the grain crop with the use of 2,4-D can be expected when it is applied from the 3–4 tiller to full tiller stage. Applications made after jointing will result in grain head injury and possible reduction in seed fill.
Most herbicides labeled for weed control in small grains belong to or are similar to the organo-auxin herbicide group and have potential for off-target injury to sensitive crops. Care should be taken to avoid spray drift to sensitive crops. Furthermore, Florida has restrictions and prohibitions on organo-auxin herbicides used within the state. Refer to EDIS fact sheet SS-AGR-12, Florida's Organo-Auxin Herbicide Rule—2018 (https://edis.ifas.ufl.edu/wg051), for information regarding this rule.
The herbicides suggested are those that have performed well in UF/IFAS research. Read and follow all label instructions and precautions. Accurate sprayer calibration is extremely important because rates too low may not provide adequate weed control and rates too high may injure the crop. Herbicides, like any pesticide, should be handled with care. Store herbicides behind locked doors in the original containers with the labels intact, and keep them separated from seed, fertilizers, and other pesticides.
