One of the first memories of my extension career is a farmer from northwest Iowa explaining his approach to managing the weed seedbank. He stated that he carried a box of atrazine 90DF in his combine. Whenever he encountered a velvetleaf or cocklebur he would stop, pull the offending weed, place it in a rack he had built on the combine, and then spread atrazine granules around the area. He said he wouldn’t get any soybean in that area for a few years, but there weren’t any cocklebur or velvetleaf either. While I don’t condone that approach to weed management, as I drive around the Iowa countryside it is apparent that changes are needed in how we view long-term weed management.
Figure 1. Late breaking waterhemp in soybean
I suspect most have realized that I’m referring to the prevalence of waterhemp in soybean fields across the state (Figure 1). I’m confident that in most fields the waterhemp has not had a major impact on soybean yields; however, all of those plants are producing seed (OK, only the females). I decided to do a highly non-scientific survey to estimate just how many waterhemp seed are being produced in those fields where waterhemp is poking it’s head a foot or so above the soybean canopy. I randomly pulled two female plants from a soybean field (Fig. 2) and dried them in an oven. I used an equation I developed many years ago that determines the number of seeds per plant based on shoot dry weight. Based on this equation, plants of the size in the picture contributed approximately 21,000 seeds to the seedbank.
Figure 2. Waterhemp that extended 6-12 inches above the soybean canopy.
For the purpose of this example, I will assume our field had one female waterhemp every 15 foot of row (30” rows). This would result in 1162 female waterhemp per acre and more than 24.4 million seeds being added to the seedbank. I then made several assumptions about the fate of those seeds based on previous research. Significant seed numbers are lost due to predation and other factors, so I calculated that only 60% of the seed produced was transferred from the mother plant into the seedbank. If 6% of the seed germinated in 2016, those plants would result in 878,000 waterhemp plants per acre, or 20 waterhemp per square foot. If you are fortunate to have a herbicide that provides 95% control, this would only leave one surviving waterhemp per square foot.
It is important to realize that waterhemp seed is fairly persistent in the soil, thus waterhemp populations would be much higher in this field due to the preexisting seedbank. ISU research in the 1990’s determined 2% emergence from seed placed in the seedbank four years earlier, and that 12% of the waterhemp seed remained viable in the seedbank after the fourth growing season. Thus, growers will be battling waterhemp from this year's seed for the remainder of the decade.
Weed management programs need to be adjusted to provide better full-season control of waterhemp. An industry rep attending a Crop Advantage Series meeting in 2014 commented that farmers had learned that late-emerging waterhemp was not affecting their yields, so were reluctant to spend more money to manage this problem. Hopefully the simple calculations in this paper illustrate the ‘hidden cost’ of late-season waterhemp. Not only will those seeds increase the cost of weed control in the future, some of those seeds will contain the mutation that provides resistance to the next herbicide group. Continuing to accept marginal levels of weed control will not only complicate weed management, but also reduce the time before resistance evolves to additional herbicide groups.
Waterhemp is a formidable opponent, and it isn’t easy designing programs that provide full-season control. It might be easy to say that the late escapes are due to the excessive rain many areas experienced, but late-season waterhemp infested fields has become an annual experience. Tight margins make it difficult to increase weed management expenditures, but we are fighting a losing battle. Implementing herbicide strategies with multiple, effective sites of action is critical, as is including residual herbicides with postemergence applications. We also need to carefully evaluate where alternative control tactics can be fit into the current management system. Now is the time for integrated weed management.