For the second year in a row, we are scrounging to find enough marestail at the OARDC Western Ag Station to conduct the research we had planned on this weed. After years of having plenty of marestail, we have had to look around for off-site fields where there is still a high enough population. Which, since we are scientists after all, or at least make our best attempts, left us thinking about reasons for the lack of marestail, and our overall marestail situation, and seedbanks.
While the short game in weed management is about getting good enough control to prevent weeds from being a yield-limiting factor and interfering with harvest, the long game is about preventing seed production and managing the soil seedbank. One of the characteristics shared by marestail, giant ragweed, and the nasty pigweeds, waterhemp and Palmer amaranth, is a rapid decline in seed viability in the soil within the first year, and an overall decline to 5% or less viable seed within 3 to 4 years. Another characteristic of marestail and pigweed seed is a relative lack of dormancy, which results in the potential for an almost immediate increase in population the year following a year of substantial escapes and seed production. How big that increase is depends upon how many plants go to seed and how many seeds are produced per plant, with the potential of up to about 200,000 seeds per marestail plant and one million per waterhemp or Palmer amaranth plant. The net result of these two characteristics, though, is that these weeds can ramp up population fast following a year of poor control, but populations can also decline rapidly with good control that prevents seed.
Marestail has probably the most variable emergence pattern of any annual weed we study, which is the reason that control requires a comprehensive management program. Trying to plan herbicide use based on a guess about whether the major emergence will be in fall, or spring, or early summer won’t work. A management program should be planned on the assumption that all of these can occur. We assume that environmental conditions over the year determine the patterns of marestail emergence, and that it’s possible a certain set of conditions have just greatly reduced emergence at our research farm. Not sure we buy this completely though – two years in a row? Thinking about the broader picture, one could conclude that we are just obtaining better control of marestail and reducing seed production due to adoption of more effective management strategies and the increased use of LibertyLink and other technologies. This is certainly occurring at our research farm, and our end of season survey of weeds in soybean fields shows about a 50% reduction in marestail over the past 6 years. Marestail seed is moved by wind, and based on the above thoughts, one could conclude that we just have fewer seed floating around in the Ohio airspace. This means less overall potential for sustained high marestail populations or reintroduction of marestail into fields where effective management has driven down the population. A good thing.
Before we collectively pat ourself on the back for this, we should state that there are growers and areas where marestail is still not well controlled, for one reason or another. And the fact that there is plenty of marestaail going to seed in natural areas, parks, right of ways, etc means there is still a source of new infestations. And having more effective technologies available does not mean we can go back to an oversimplified herbicide program that overuses certain foliar herbicides, such as glufosinate, dicamba, and 2,4-D. The fact that almost all marestail populations are resistant to the two herbicide sites of action that were effective in POST applications to begin with, glyphosate and ALS inhibitors, means that we should assume the potential exists to develop resistance to glufosinate, dicamba, and 2,4-D. So the use of residual herbicides and cover crops are still important, along with diversification and rotation of site of action in herbicide programs. Those few marestail plants that seem to be able to survive even the most effective herbicide program can still produce a few hundred thousand seed, and may be the ones that carry the next type of resistance.
But if we are good enough at reading the signs correctly, this may be an example of how practices affect the dynamics of our weed situation, and how that can change over time from both a local and more global perspective. Or maybe that we just had too much time to think about weeds on a Monday morning late in April when it’s too wet to do anything in the field. Next week – but what about the pigweeds?
Addendum. Info about marestail management can be found in the OSU/Purdue marestail fact sheet. There are lots of herbicide issues right now, and it’s worth tracking other university newsletters to keep up with it all. Examples – thiosulfate use and glyphosate efficacy, inversions and dicamba, cover crops, etc. One way to do this is the Purdue Chat n Chew website, which provides an updated list of links to university newsletter articles.