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Velvetleaf response to
by Bob Hartzler
December 15, 2000 - With the introduction of Roundup Ready soybeans there has been much speculation over what weed will be the first to create significant problems in this system. The species most frequently mentioned include morningglory, yellow nutsedge, waterhemp and velvetleaf. Velvetleaf is known to have a higher level of tolerance to Roundup than many other common weeds, and thus inconsistent control can be expected. We initiated research in 1998 to develop a better understanding of the impact that velvetleaf surviving Roundup Ultra might have on soybean production.
In 1998 an experiment was conducted near Vinton, whereas in 1999 experiments were conducted near Ames and Kanawha. Fields were planted with Roundup Ready soybeans (10" rows at Vinton, 30" rows at Ames and Kanawha) and no preemergence herbicides were applied. Roundup Ultra was applied at either 1 or 2 pints per acre when velvetleaf reached a height of 3-4 inches, 5 to 6 inches or 10 to 12 inches. Immediately prior to application velvetleaf plants at the appropriate height were identified and tagged so that their fate could be monitored for the remainder of the season. At the end of the growing season tagged plants were harvested to determine biomass production. The number of seed capsules on surviving plants was determined to estimate seed production. Earlier research has determined that the number of seeds per capsules remains relatively constant for velvetleaf plants growing under a wide range of conditions.
Survival of untreated control plants ranged from 90 to 98%. A relatively high percentage of velvetleaf plants survived Roundup Ultra applications (Figure 1). Velvetleaf survival of 1 pt Roundup Ultra ranged from 50% at Vinton to 80% at Kanawha. Increasing the rate to 2 pints reduced survival rates to approximately 12 to 38%. The reduced rate of survival at Vinton may be due to increased competition from the soybeans due to the narrow row spacing at this site.
There was not a consistent response to application timing (data not presented). Velvetleaf height did not influence survival percentage at Kanawha, whereas at Ames and Vinton the lowest survival rate occurred with the early application and the greatest survival was at the middle application timing.
All Roundup Ultra treatments reduced velvetleaf biomass by at least 80% compared to untreated plants (Figure 2). While there were large differences in velvetleaf survival between the two Roundup Ultra rates, the differences in biomass production were small between the two rates (4-9%). The experimental design did not allow us to determine the impact of surviving velvetleaf plants on soybean yields; however, because of the low dry matter accumulated by these plants it is unlikely that soybean yields would be impacted by these plants.
Reduction in capsule production of plants surviving Roundup treatment were similar to reductions in biomass production. Since seeds per capsule remains relatively constant, the number of capsules per plant provides a fairly accurate estimate of seed production by surviving plants. At Kanawha the 2 pint rate reduced capsule production more than the low rate, but there wasn't a significant rate response at the other two locations. There was not a consistent response to application timing.
The relationship between biomass production and capsule production at the three locations is presented in Figure 3. Capsule production for both untreated plants (Figure 3a) and plants surviving Roundup Ultra (Figure 3b) was proportional to dry weight. This means that a plant that accumulates twice as much biomass as another plant will produce twice as many seeds as the other plant. Exposure to Roundup Ultra greatly reduced biomass production of surviving plants; however, the reproductive efficiency of these survivors was not affected (ability to convert biomass into seed).
Within a population of plants there is typically a wide variation in plant size. In the absence of stress there is usually a fairly uniform distribution of plant sizes (an equal number of small and large plants). However, as stress increases an inequity in distribution occurs resulting in the population being dominated by a few large plants and the bulk of the population comprised of many small plants.
The frequency distribution of velvetleaf plants at Ames is provided in Figure 4. The figure provides information on the percentage of the velvetleaf that accumulated various amounts of dry matter. For example, 12% of untreated velvetleaf accumulated between 0-25 g of dry matter (Figure 4a).
Velvetleaf not exposed to Roundup accumulated from 25 g to 275 g of biomass, and there was a fairly uniform distribution of plant sizes (Figure 4a). The size class with the highest frequency of individuals was 100 g and this group composed only 20% of the population. Plants surviving glyphosate ranged in size form 15 g to 165 g (Figure 4b). In contrast to the untreated plants, the distribution of plants surviving Roundup Ultra was highly skewed, with approximately 70% of the plants accumulating 15 g of biomass or less. The significance of this skewed distribution is that a few plants produce the majority of seeds for the entire population. If the survival of the plants that dominate the population is associated with a genetic trait, this genetic attribute could increase rapidly within the velvetleaf population.
In summary, the percent control of velvetleaf was below 'commercially acceptable levels'. However, the growth of velvetleaf plants that survived Roundup Ultra was reduced by at least 80% at all locations and at both rates. The reduced growth of these surviving plants will greatly reduce their ability to compete with soybeans, and under most conditions it is unlikely that they would reduce soybean yields. However, the reproductive fitness of surviving velvetleaf was not affected by exposure to Roundup Ultra. Seed production was proportional to biomass accumulation in both untreated and surviving plants.
Do the results of this research implicate that velvetleaf will be the weed that bursts the Roundup Ready bubble? The ability of surviving plants to produce seed could result in an increase in velvetleaf populations over time and therefore complicate future management plans. However, the reduced competitiveness of plants surviving Roundup (Figure 2) greatly diminishes the cost associated with these weeds. My assessment is that the data indicate that velvetleaf populations can increase under Roundup Ready systems and thus warrant close monitoring to prevent densities from reaching a level that creates significant problems. However, some other species is just as likely as velvetleaf to be the first major problem in Roundup Ready systems, and we still can not predict whether we are one year or two decades from this problem developing.
This article is an adoption of a poster presented at the 2000 North Central Weed Science Society Annual Meeting held in Kansas City on Dec. 11-14. Bruce Battles was responsible for conducting the research and analyzing data.
Prepared by Bob Hartzler, extension weed management specialist, Department of Agronomy, Iowa State University
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