The adoption of herbicide resistant crops (e.g. Roundup Ready and Liberty Link) has increased the interest in preharvest herbicide applications since these crops allow the application of broad-spectrum herbicides late in the season with minimal risk of crop injury. While late-season applications have little impact on crop yields since the majority of competition between the crops and weeds has already occurred, herbicide applications at this time may improve harvest efficiency. Growers also use these treatments in an attempt to reduce weed seed production and the viability of weed seeds. This article will address this last issue.
The effect of late-season 2,4-D applications in corn on cocklebur and velvetleaf seed production was studied in the early 1980's at ISU. The late-season applications were more effective at reducing seed production in cocklebur than velvetleaf. Velvetleaf flowers over a long-period of time and frequently produces significant seed by the time the brown-silk 2,4-D applications are made (At the time of this research, most 2,4-D labels allowed applications at the brown silk stage. Current labels require corn to reach hard-dough stage prior to 2,4-D application). Cocklebur on the other hand initiates flowering later in the season, and thus the brown-silk treatments were more effective at reducing seed production in this species. The research found that the majority of seeds that reached maturity following treatment were viable, even if the seeds were immature at the time of application.
I was unable to find any research looking at the effect of Roundup on weed seed viability, but people have studied the effects of preharvest applications in soybeans on the viability of soybean seed. This article will summarize a paper that reports the effect of preharvest treatments of Roundup, Liberty and Gramoxone on soybean seed. In this study, herbicides were applied at the R5, R6, R7 and R8 stage of development1. The herbicide treatments at the R5 stage reduced soybean yields by 98% (Gramoxone) to 88% (Roundup). The maximum yield loss at the R6 stage was 32% with Liberty, whereas no yield effects occurred with applications made at the R7 or R8 stages of development.
While applications made during early reproductive stages (R5 and R6) resulted in significant reductions in total seed production by soybean, the majority of seed from treated plants that reached maturity were viable. Neither Liberty nor Gramoxone had a significant effect on seed germination, but early applications did influence the % of normal seedlings (Table 1). Although Roundup significantly reduced germination at all application timings except full soybean maturity (R8), the majority of seeds were viable at all application stages. At the R5 stage the furthest developed seed is only 1/8" in length, however, 62% of seeds produced retained their viability when treated at this stage. R5 applications of Roundup resulted in 60% malformed seedlings, however, the abnormal seedlings survived and only a slight reduction in biomass was observed in these plants after one month of growth.
Table 1. Effect of harvest-aid herbicides on soybean 100 seed weight, seed germination, and normal seedlings.
|Herbicide||Application Timing (Soybean stage)||100 seed wt (g)||% germination||% normal seedlings|
|21 oz/A Roundup||R5||13||62||40|
|21 oz/A Roundup||R6||12||82||42|
|21 oz/A Roundup||R7||13||90||86|
|21 oz/A Roundup||R8||13||94||93|
|57 oz/A Liberty||R5||10||93||77|
|57 oz/A Liberty||R6||10||95||62|
|57 oz/A Liberty||R7||13||95||87|
|57 oz/A Liberty||R8||13||96||91|
|2.4 pt/A Gramoxone||R5||9||95||89|
|2.4 pt/A Gramoxone||R6||10||96||88|
|2.4 pt/A Gramoxone||R7||13||97||89|
|2.4 pt/A Gramoxone||R8||13||95||91|
Source: Ratnayake and Shaw. 1992. Weed Technol. 6:339-344.
All herbicides reduced total seed production by soybean when applied at early stages of reproductive development. Roundup applied during early stages of reproductive development was more injurious to seeds than either Liberty or Gramoxone. Roundup is translocated much more readily than either Liberty or Gramoxone, thus it is more likely to accumulate in the reproductive structures. Although Roundup did reduce seed viability, the majority of seed still was viable - even at the R5 stage when no seeds had reached full dry matter accumulation.
The results of these studies were similar to those done with 2,4-D on cocklebur and velvetleaf in that the majority of seeds that were able to mature following application of the herbicide were viable. Growers should be cautious when weighing the benefits of late-season herbicide treatments. While these treatments may improve harvest efficiency in many situations, the benefit in reducing weed seed production will be highly variable. If weeds have entered the seed filling stage when the herbicide is applied, it is likely that the majority of seeds produced by these plants will retain their viability.
1Soybean stages of development
R5 (beginning seed) = Seed is 1/8" long in the pod at one of the four uppermost nodes on the main stem. Reproductive development ranges from flowers just open to pods containing seeds 1/8" long.
R6 (full seed) = Pod containing a green seed that fills the pod cavity at one of the four uppermost nodes on the main stem.
R7 (beginning maturity) =One normal pod on the main stem that has reached its mature pod color.
R8 (full maturity) = 95% of the pods have reached their mature pod color.
Prepared by Bob Hartzler, extension weed management specialist