Based upon the number of field visits and phone calls that I have had this year, we are experiencing somewhat poorer overall weed control in glyphosate-resistant fields compared to the universally outstanding weed control experienced with those systems a few years ago.

There have been many reports that higher glyphosate rates were required to effectively control problem weeds like waterhemp this year. This phenomenon is commonly referred to as "rate creep" and is usually mentioned in glyphosate-resistant weed conversations.

Although many weed scientists believe that weed tolerance/resistance-derived "rate creep" is probably occurring or will occur in the near future, I believe the vast majority of glyphosate efficacy problems this year have a much simpler and basic explanation.

I believe the observations discussed above are the result of several interrelated factors. These are the same factors that occur with all weed control programs. Herbicide use rate, the weed species present, weed size, spray volume, herbicide application timing and environmental conditions have all played roles in the few disappointing weed control fields we experienced with glyphosate-resistant crops this year. To address these issues, let's ask a few questions.

Why would glyphosate-resistant systems work better, with lower use rates, in 1997 compared to 2003? I believe we did a much better job of timing during those initial years. Most non-glyphosate products used before glyphosate-resistant crop technology was developed have a very narrow optimum application window. Producers were accustomed to the need to apply herbicides at optimum application timing. Weed control in early years of the glyphosate-resistant cropping systems was very good, mostly because of the effectiveness of glyphosate as a non-selective, broad-spectrum herbicide. However, the fact that the glyphosate was applied at proper weed control timings was also a factor in its widespread effectiveness and popularity. Relatively low rates of glyphosate work extremely well when application timing is optimum. These low rates worked so well that we became somewhat apathetic toward timing issues.

In the early years of glyphosate-resistant systems, we also had a two-pass philosophy. The overall efficacy of glyphosate caused many producers to try to stretch the first application until after more weeds germinated, thus creating an opportunity for needing only a single-pass application program. This delay in application timing has led to a dependency on glyphosate to control more and larger weeds. Even with glyphosate, as we stretch the optimum weed control window, higher rates are required to control the larger weeds present. Also, with more weeds and a heavier soybean canopy, the smaller weeds may not be physically contacted by the glyphosate herbicide. These delayed application issues are confirmed by manufacturer recommendations. Just recently, they have raised product rate and spray volume recommendations for delayed glyphosate application situations.

Work done by Aaron Hoefer at the University of Missouri from 1998 to 2000, and reported by Reid Smeda and Brent Sellers earlier this year, demonstrates the importance of application timing in glyphosate-resistant cropping systems. This work verified that the most effective weed control using glyphosate occurs when weeds are small.

The bottom line is to apply glyphosate early at labeled rates and then plan for a second application if necessary. This was the recommendation in the 1990s when the glyphosate-resistant systems were initiated and is still the optimum recommendation today.

The University of Missouri work showed that for early planted, 30-inch row soybeans, the two to four-leaf soybean growth stage is generally the best time for the first glyphosate application. For later-planted, drilled soybeans, the optimum application timing window moves up slightly to the 1 to 3-inch soybean growth stage. Of course, these timings can slide earlier or later depending upon weed species and weed growth stages. If you cannot meet the proper application timings discussed above, then higher application rates or multiple applications will be required.

Another issue this year has been that a few producers are claiming to experience better weed control with generic formulations of glyphosate compared to their higher priced branded counterparts. Why would this occur when we know that the quality and consistency of branded glyphosates is excellent? I believe the lower pricing structure for gen-erics has led to higher actual use rates compared to the branded products. With later applications, these lower branded use rates may not be effective.

If "rate creep" is a resistance issue, it'll continue to become a larger problem. However, until it becomes more prevalent, you can get the best weed control with glyphosate by matching use rates to weed populations and weed sizes present at application time.