Author’s note – I wrote this before this week’s rain, which is usually the way it works with articles about dry weather.
Winter annual populations in many fields seem to be low to fairly nonexistent at this time due to the dry end of summer conditions we have been experiencing. Growth (or regrowth) of perennial weeds is also less vigorous than in wetter years. This has ramifications for both fall herbicide treatments ahead of corn or soybeans, and decisions about the need for burndown treatments in no-till wheat. The bottom line on fall herbicide treatments for corn or soybeans – don’t be in a rush to apply. There is really no risk of less effective control by delaying treatment, even through mid to late November. Herbicides are more effective on dandelion after a frost in late October or November anyway. It’s important to allow perennials such as Canada thistle to reach a height of at least 8 to 12 inches to obtain maximum herbicide effectiveness and the resultant reduction in population. Delaying application until late in the fall obviously has to be weighed against the possibility that it will turn wet at some point, rendering fields unsuitable for traffic, but fields are dry enough that substantial rain would be required for this to occur. Waiting until early November could also allow for a more informed decision on whether fall treatment is actually necessary. One possibility is that winter weed populations will be low this year, and scouting fields in November prior to application would provide better information on this. Where scouting at that time fails to turn up many winter weeds, the better decision may be to delay application until spring, which makes better use of residual herbicides and can reduce overall weed management costs.
This same thinking can be applied to winter weed management in no-till wheat. A preemergence application of glyphosate can control winter annuals fairly well in a typical year when their emergence is well under way by late September. However, we have also effectively controlled winter annuals and dandelion by omitting the preplant burndown treatment, and instead applying POST herbicides in November, using mixtures such as dicamba plus Express (or the equivalent generic product). The November treatment can be considerably more effective for control of control of dandelion compared with the late September glyphosate application - see related C.O.R.N. article from 2008:
The current lack of winter annual weeds or actively-growing dandelions in many fields lead us to suggest that this may be the year to skip the preplant burndown in wheat, and instead apply POST herbicides later in fall (or in spring if weed populations in November do not merit treatment). This will allow time for rain and subsequent emergence of those weeds that are primed to germinate when soil moisture is sufficient, and for the resumption of active dandelion growth. This decision should obviously be made on a field by field basis based on scouting at the time of wheat planting, and fields with substantial weed populations at planting should be considered candidates for preplant burndown treatment. One of the key decisions here is whether the weed population at the time of planting is substantial enough to suppress wheat establishment before POST herbicides can be applied. Labels for many wheat herbicides specify that they should not be applied in the fall until wheat has 2 to 3 leaves, which does allow substantial time for growth of weeds that have already emerged.
For no-till wheat fields that require a preemergence burndown treatment, glyphosate is often the most cost-effective option. Sharpen is labeled for preemergence use in wheat at rates up to 2 oz/A, and it can be added to glyphosate to improve activity on marestail and other weeds. Gramoxone is also effective on small winter annual weeds. We often get questions about the preplant use of 2,4-D or dicamba in wheat. This subject was covered well in the latest issue of the Penn State Field Crop News, found at this link:
An EF2 tornado hit the OARDC campus on Thursday, Sept. 16th at about 5:30 pm. Several of us were on campus, making a quick run to the basements. Others of us were in our cars and just got lucky! The sky was not a deep gray, and we kept looking out the window to see why the sirens were going on so long, but headed to the basement anyways. As the storm went over the top of the building we could feel our ears pop. After the rain let up – we honestly did not think it was so bad until we opened the doors to the greenhouses. There was glass everywhere in Plant Pathology greenhouses. Fortunately, the framing held up. There were places where metal from other places had pierced the greenhouse. Entomology was out of the way of the tornado’s main path, and their greenhouses held up pretty well. We then headed outside to begin to look around, the path was fairly narrow, but not much was left standing.
Several key freezers are on generators, so they were up and running by Friday, this was then followed by electricity restored on Saturday. We lost some things, but most of it can be replaced. In the greenhouses, several students are going to have to repeat some work that they started, but on the whole, Plant Pathology greenhouses were fairly empty. Students were finishing things up to start classes and making some time for harvest. Horticulture and Crop Science greenhouses were set back with the loss of some crosses and seed. One field, all of the crossing tags were missing. So this will need to be repeated. Amazingly, the field plots surrounding the center are fine. Not a single pod or ear was blown of the plants, and corn and soybean harvest and wheat planting will continue as planned.
The good news, after only 10 days, the campus looks much better, and thankfully, no one has been hurt of sceriously injured. We have been on campus twice to get things so we could continue our work and today marks the return to work. The campus is still closed to the public. Repairs to both Entomology and Plant Pathology greenhouses will begin this week. So by the time we finish with our field data, we can begin to start the greenhouse experiments.
There are several links to articles and reports about the tornado. Dr. Moser’s talk at Farm Science Review is especially moving.
With dry weather and corn stubble sitting in the field, residue can be quickly turned to ashes due to accidental fires. A question frequently asked afterward is how much nitrogen (N) just went up in smoke? N is volatilized and lost when plant material burns. Phosphorus and K remain and return to the ground with ash. However, P and K could be lost if ash is blown away from the field during or after the fire. Fire damage in a field is usually variable in scale. Not all material is completely turned to ash, and rarely is the entire field burned (from one end to another). Understanding what was burned and how much area was affected has an impact on the total amount of N lost. The amount of N contained in corn residue has been well documented from the late 60’s and clearly delineated by John Sawyer at Iowa State (http://www.ipm.iastate.edu/ipm/icm/2000/10-23-2000/dryfallfires.html).
To estimate how much N was lost, the grain yield level for the previous year must be considered. The harvest index (in corn, the ratio of grain weight to total plant dry weight) is another piece of information that must be known (if unknown – assume 0.5). Providing a relatively accurate measure of the area affected is obviously important. Additionally, recognizing the residue remaining and adjusting the material burned can provide a more quantitative measure of the actual damage (if this is unknown - assume 100%).
The table below provides a simple estimate of N lost based on the previous year’s corn yield (assuming a harvest index of 0.5 and yield adjusted to 15.5% moisture). (Remember: the N contained within the corn residue would not have been released and made plant available for next season’s crop.)
|Yield (bu/A)||N lost (lb/A)|
Along with the loss of N, carbon contained in the plant material is lost as well. It would have been incorporated into the soil organic fraction. This too has value. While there is no specific dollar amount tied directly to a loss of organic matter, an Iowa State University article recommends that one dollar per acre should be claimed. Unfortunately, the economic impact associated with the loss of residue cannot be fully realized until later, especially in fields with high erosion potential.
Sawyer, J. 2000. Estimating losses when cornstalk fields are accidentally burnt. Iowa State Integrated Crop Management Newsletter http://www.ipm.iastate.edu/ipm/icm/2000/4-10-2000/burnfield.html
Sawyer, J. 2000. Dry fall leads to field fires. Iowa State Integrated Crop Management Newsletter.http://www.ipm.iastate.edu/ipm/icm/2000/10-23-2000/dryfallfires.html