Interpreting weed clues
Weeds present at harvest can tell a story. Learn how to interpret the causes and effects of what you see to improve weed management practices for the next year. If you use a yield monitor, you also may be able to relate crop yield to weed management and adjust your strategy accordingly.
Small weeds. Small grass and broadleaf weeds are likely to have developed after the first month of the growing season, perhaps after a POST treatment or cultivation or after a PRE treatment has become ineffective. These smaller, late developing weeds may produce seed and perpetuate the problem but are unlikely to have impacted yield. These late developing weeds are most likely in areas where the crop canopy developed more slowly, allowing penetration of the light necessary for weed establishment.
Large weeds. Large weeds present at harvest likely are escapes which were not controlled by your primary weed management program. Depending on the number of these weeds, a change may be indicated for your weed management program.
Herbicide resistance. You may be able to see indications of herbicide resistance at harvest although the picture would have been clearer with an earlier examination. Herbicide resistance is first evident as a limited number of escapes in the field. There are many causes of weed escapes other than herbicide resistance. The key is to look for scattered large plants or small patches that were not controlled by your primary program. Dead weeds adjacent to the large ones provide even more evidence that resistance may be present. These fields should be monitored closely the next year.
Weed patches. These tell you that your weed management program is not uniformly effective across the field. There may be several causes, however the effect is the same – these field areas will have higher concentrations of weed seed as compared to the rest of the field. This means the problem next year will be most serious in these patches. If you continue to manage the field as in the past, the patches will persist or become larger. A change in management is needed to prevent “growing” these weed patches.
Perennial weeds. These weeds typically occur in patches and many are less susceptible than annuals to most weed management programs. Perennials usually call for special attention not warranted on the entire field. Identifying problem areas in the fall can make it easier to target them in the spring.
With a little extra effort at harvest you can gather information that will be useful in developing next year’s weed management program.
Alex Martin
Extension Weeds Specialist
Interpreting insect clues
Unlike weeds, insect problems are not always visible from the combine. There are a few cases though when signs of insect damage may be visible at harvest and provide clues to needed management changes.
In corn watch for plants with lodging or broken stalks or tassels. If there are patches in a field with these characteristics, investigate the situation further before harvest. Lodging may be caused by rootworm injury, but also may occur in fields without significant rootworm injury. Digging up some lodged plants, washing off the roots and looking for root injury can help identify the cause. Be aware that root regrowth following rootworm injury may cover up the original injury, so careful examination may be needed. Stalk or tassel breakage may be caused by European corn borer. To verify the cause split some stalks and check for tunneling or the presence of European corn borers. Fields with a lot of lodging or stalk breakage may need to be harvested first to minimize harvest losses from ear droppage.
Rootworm damage suggests that management practices should be modified for 2007. Crop rotation would be one means for reducing the density of rootworms on your farm for the next growing season. Review the control measures you used in 2006 and consider alternative products or practices for 2007.
Damage from European corn borer this year is not a good predictor of injury in 2007, since the moths can fly from field to field; however, review your control strategies for European corn borers. Scouting for European corn borers in susceptible hybrids and treating if economic thresholds are reached can prevent economic losses in the future.
Bob Wright
Extension Entomologist
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