We have high expectations of herbicide programs, and we have a lot of good herbicides to choose from. Something that can get overlooked as herbicide programs are planned, however, is the effect of early planting on the duration of weed control that is required.
We plant earlier on average than we did 25 years ago, and then we have years like this one, where we plant even earlier because – we can – or we’re coming off a wet year – or we need to get out of the house – or whatever. Within the time frame of about mid-April through mid-May, crops planted earlier do not necessarily develop more rapidly, so the time until crop canopy may not vary much with planting date. Herbicide programs are intended primarily to control weeds until the crop canopy has developed sufficiently to shade out later-emerging weeds. So the net result of early planting can be an extension of the duration of control that needs to be provided by herbicides.
The further conclusion here is that herbicide programs have to be better now than they were 20 or more years ago to account for early planting, and they have generally evolved to accomplish this through the availability of more effective herbicide combinations and the use of a planned PRE + POST approach.
Some things to consider relative to this subject as the season progresses:
#1. Weeds that can emerge over a broad period of time (or – in weed scientist geek terms – weeds that “exhibit plasticity in germination”) cause the most problems as planting moves earlier. Summer annual weeds in this category that we commonly deal with in Ohio include giant ragweed, foxtails, shattercane, burcucumber, waterhemp, and also pigweed and morningglory in some years. These weeds are often most likely to emerge late when an herbicide program that has been applied early in the season loses activity.
#2. Corn needs to be kept free of weeds until it’s about 20 inches tall through the use of a comprehensive full-rate PRE herbicide program, or the use of a combination of PRE and POST herbicides. Early planting puts considerable stress on the capability of a total PRE herbicide approach to last long enough, so be sure to scout fields to determine whether a follow up POST program is required.
#3. In the planned PRE + POST approach for corn, use of a more comprehensive, higher rate PRE herbicide can improve overall effectiveness by controlling weeds more effectively up until the time of the POST, or providing some residual control even after the POST. Our research shows that the minimum PRE herbicide/rate in this system should be something like 75% of the full use rate of an atrazine premix or similar product. Rates should be increased and/or a more comprehensive mix of PRE herbicides used for especially early planting or weedier fields. When the time between planting and crop canopy closure stretches out, there can be considerable value to adding herbicides with residual activity to the POST application.
Examples – the addition of atrazine or Callisto to POST application of glyphosate, Ignite or other non-residual herbicides. Capreno is a product with that controls emerged weeds and provides residual grass control. The use of residual POST herbicides is especially important when the corn is relatively small at the time of the POST application, or less than about 12 to 14 inches tall. This approach can supplement the residual weed control about the time the herbicides applied at planting are losing effectiveness.
#4. One of our areas of emphasis always is the appropriate management of giant ragweed in soybeans, and early planting makes this more of a concern. Giant ragweed can emerge well into June, making it difficult to manage with one POST application. The reality is that moderate to high infestations of giant ragweed, and even fields with patches at this level, are most consistently controlled with two POST applications. It is possible to obtain adequate control with one POST application, where the PRE herbicides have reduced the size and density of the population considerably. Where the population has evolved any loss in response to glyphosate, a two-POST approach is more likely to provide control. We conducted a study in four grower fields last year that showed the value of the two-application POST approach in Roundup Ready soybeans. A brief report on this research is available on the OSU weed science website here.
This research verified that giant ragweed populations are more effectively reduced when POST herbicides are applied twice, and the first application occurs when plants are no more than about 6 to 10 inches tall. The approach that many growers use – making a single POST application to larger plants to avoid making a second POST application – results in reduced control and increased survival and seed production. Early planting makes the two-application POST system even more appropriate, because in a stretched out season, it’s impossible to time one POST application appropriately to ensure that plants are not too large, and the application is late enough to control the plants that are still emerging in June.