Herbicide performance and alternative weed management

Most soil-applied herbicides provide acceptable weed control if there is sufficient moisture to allow the products to perform. However, weed control expectations by growers are generally higher than that which herbicides provide. Current tendencies are to automatically look to another herbicide to provide supplemental weed control when initial treatments fail to meet expectations. Although this strategy may be appropriate in many cases, economically it may be better to consider mechanical weed management practices. These strategies must be used in a timely fashion for maximum efficacy, not unlike supplemental herbicide application. Rotary hoeing and cultivation are effective techniques for weed management and have significant economic benefits when compared with the cost of herbicides. However, it is important to consider that mechanical strategies also require management skills for optimum weed management and these skills may be different than those used with herbicide treatments.

What factors impact the use of mechanical weed management strategies? Primary tillage is the first consideration and probably the most important factor. Many producers do not accept that mechanical weed management is important in no-tillage production systems. In fact, mechanical strategies are extremely effective and highly desirable strategies in these production systems.

One of the major changes that occurs when tillage is eliminated is a relatively rapid change in weed populations. Large-seeded, deeper-emerging weeds such as common cocklebur and velvetleaf become less prominent and small-seeded, shallow-emerging weeds increase in reduced tillage systems. These shallow-emerging weeds respond readily to mechanical management tactics. Also, when plant residue levels increase on the soil surface, variability in soil temperature and moisture increases resulting in extended germination periods for weeds. Integration of mechanical and chemical control tactics is the most efficient method of managing the prolonged emergence patterns of weeds typically found in reduced tillage systems.

Consider using the rotary hoe as an early-season weed control strategy.

Obviously, cultivation is not a viable option with narrow-row soybeans. However, rotary hoeing is effective and a desirable component of a weed management system in narrow-row soybeans. It is important to consider that certain older rotary hoes will not operate effectively in high-residue systems, and new models that have been specifically designed to work in high-residue environments must be used.

The most critical decision for mechanical weed management is timing. In fields with soil-applied herbicides, anticipating how the herbicide will perform will improve the timeliness of mechanical strategies. Rainfall is the main factor influencing early-season performance of herbicides. There are no consistent guidelines for the amount of rainfall needed to make a herbicide effective; however, precipitation of less than one-half inch generally is insufficient to initially activate herbicides. Soil type, herbicide chemical characteristics, tillage system, and cropping system influence the relationship between the herbicide performance and necessary rainfall.

Unfortunately, weed seeds may germinate even if rainfall amount is less than needed to facilitate herbicide activity. Factors to consider when predicting herbicide performance are when the last tillage event occurred relative to herbicide application, amount of rainfall following application, and soil moisture status at planting.

If the last tillage was several days prior to herbicide application, there is a greater likelihood that weed seedlings will emerge through the herbicide treatment, and supplemental mechanical strategies should be employed quickly. If the soils are extremely dry, the amount of rainfall needed to improve herbicide activity is much greater than if soils are relatively moist. Finally, if a herbicide has little solubility in water, more rainfall is needed for herbicide activity.

Generally speaking, the closer the sequence of final tillage treatment, crop planting, and herbicide application, the better the success of the herbicide treatment and the longer the delay for a supplemental mechanical tactic. One inch of rainfall within a few days of the last tillage and herbicide application on dry soil may be sufficient to provide herbicide activity.

The best way to resolve these difficult management predictions and decisions as to the appropriate timing of mechanical strategies is to carefully scout fields. Observe when weed seeds begin to germinate and their relative population. Also determine the relative depth of weed seed germination, as it will influence the timing delay and effectiveness of mechanical strategies. Generally, if weeds have emerged, a rotary hoe treatment will not be as effective and a secondary strategy should be considered. Also recognize that crops can tolerate considerable injury from a rotary hoe without a significant response to stand or eventual yield.

Cultivation also should be done in a timely fashion on smaller weeds and at shallow depths. Lay-by cultivation may diminish the need for late herbicide applications. Importantly, the negative response of crops to later herbicide applications is more important than the weed control that might result from these treatments. Cultivation is a better and cheaper option.

This article originally appeared on pages 82-83 of the IC-480(10) -- May 18, 1998 issue.

Updated 05/17/1998 - 1:00pm