C.O.R.N. Newsletter: 2014-11

Weed growth has been somewhat slow this year due to the cold weather in late winter, but the current warm and wet weather should change this.  Those fields not treated with burndown herbicides yet may present some challenges by the time fields are dry enough for traffic again, especially if they were not treated last fall.  Wet weather and delayed planting can have several effects on no-till burndown programs, especially in soybeans.   First, the weeds get bigger and what is a relatively tame burndown situation in April can become pretty hairy by the first two weeks of May.  Second, in the rush to plant when it finally dries out, it can be difficult to keep 2,4-D in the burndown mix and wait yet another 7 days to plant soybeans.  A review of the soybean burndown options for this situation follows.

1.  In all mixtures containing glyphosate, increase rate to the equivalent of 1.5 to 2.2 lb ae/A.  This will help with many weeds, but obviously not with glyphosate-resistant marestail or ragweeds.

2.  One option is to just omit the 2,4-D ester and rely on glyphosate for the burndown.  This is probably the worst choice on this list, but it can work in some fields.  This is not an option in fields with marestail, or glyphosate-resistant ragweeds.  Use a glyphosate rate of 1.5 to 2.2 lbs ae/A.  We recommend applying with a residual herbicide that contains chlorimuron (Canopy/Cloak, Valor XLT, Envive, Authority XL) to improve control of dandelions and many summer and winter annual broadleaf weeds (but not ALS-resistant marestail or ragweeds).  The mixture of glyphosate and chlorimuron or cloransulam is likely to be variable for control of even ALS-sensitive marestail by the time we can get into fields.

3.  Keep 2,4-D ester in the mix with glyphosate and wait another 7 days to plant.  Plant the corn acres first and come back to soybeans to allow time for this.  Have the burndown treatment custom-applied if labor or time is short.

4.  Apply a combination of glyphosate, Sharpen (or other saflufenacil product), and MSO.  Major drawback to this is that saflufenacil cannot be used in combination with residual products that contain flumioxazin (Valor) or sulfentrazone (Authority/Spartan) less than 14 days before planting.  Best options for residual herbicide in this mixture, especially where marestail are present – metribuzin or Canopy/Cloak DF + metribuzin, making sure that the metribuzin rate is at least 0.38 lbs ai/A.  A spray volume of at least 15 gpa should be used and it is essential to use MSO as the adjuvant.

5.  Use Liberty (36 oz/A) for burndown.  This should be applied with a metribuzin-containing herbicide.  One benefit of this compared with option 4 is that Liberty can be applied with any residual herbicide.   However, Liberty can struggle with burndown of some weedy situations, and is best suited for fields that were treated last fall.  A spray volume of at least 15 gpa should be used.  Avoid use of nozzles that produce primarily large droplets.

6.  Substitute tillage for burndown herbicides.  Make sure that the tillage is deep and thorough enough to completely uproot weeds.  Weeds that regrow after being “beat up” by tillage are often impossible to control for the rest of the season.  There can be a benefit to treating fields with glyphosate prior to tillage, where weeds are dense and large, to ensure a weed-free seedbed.

7.  Gramoxone usually works well in late spring only when combined with both 2,4-D ester and metribuzin, especially when weeds have much size.  Use the high Gramoxone rate and crop oil concentrate, and a spray volume of 15 to 20 gpa, depending upon nozzle type.

8.  Some things that probably will not work.  We have not had much success with combinations of glyphosate and 2,4-DB, although high rates of 2,4-DB can help control certain weeds. Valor and Authority do not have enough foliar activity to help control emerged weeds, although they do cause contact herbicide symptomology on weed leaves (and can actually cause antagonism in mixtures with systemic herbicides).  Metribuzin does help control emerged weeds, although this is expressed best in mixtures with Liberty, Sharpen, or Gramoxone.

9.  The cost of several of these options can be about twice the cost of a typical glyphosate/2,4-D mixture.  This is not really the situation to balk at spending a few extra dollars.  Failing to effectively control weeds at the time of no-till soybean planting creates problems for the rest of the growing season.

10.  Late-spring burndown tends to be less of an issue in corn, due to the contribution of atrazine and mesotrione (in Lumax/Lexar) to burndown, and the ability to use dicamba.  Labels for 2,4-D products vary with regard to preemergence use in corn.  Some recommend avoiding application between 7 days before and 3 to 5 days after planting, while others are less restrictive. 

One of the questions that comes up once a grower decides to kill marestail with tillage instead of burndown herbicides is – does he still need residual herbicides?  The thinking here apparently is that: A) tillage takes care of marestail for this crop year; B) marestail is the only weed for which residual herbicides are necessary; and C) POST applications of glyphosate are adequate to control the rest of the weeds that are going to emerge after planting.  Only “A” is correct.  A brief list of reasons why we use residual herbicides may be helpful.

Using a program consisting of only glyphosate is the reason we have glyphosate-resistant populations of marestail.  We also have glyphosate-resistant giant and common ragweed, waterhemp, and Palmer amaranth, and apparently also redroot pigweed based on our latest greenhouse screens.  Early-season control from residual herbicides reduces the number of weeds that will be treated with glyphosate POST, which reduces the risk of additional glyphosate-resistant populations.

Giant ragweed and other large-seeded broadleaf weeds are most easily managed with a combination of residual and POST herbicides.  The residual herbicides can reduce the population and slow the growth of remaining plants, so that the field is more likely to need just one POST application.  Omitting residual herbicides pretty much ensures that two POST applications will be needed if the field has more than a few giant ragweed.

Remember lambsquarters - easily controlled with residual herbicides and tolerant to many POST herbicides?  Back in 2005 or so, before the use of residual herbicides rebounded, we received lots of questions about problems with POST control of lambsquarters with glyphosate.  We decline to answer those questions now because they indicate a failure to use a simple solution.

Remember the two other major reasons we use residual herbicides, to start with some early-season weed control so that: 1) not every field has to be sprayed POST at the same time; and 2) crop yield is not reduced by weeds if the POST is sprayed too late.

To be completely candid and not tactful at all, all of us who provide guidance to growers on weed control issues – dealers, CCA’s, consultants, extension – don’t know what else to do to communicate the importance of residual herbicides.  They are a first line of defense against poor weed control, herbicide resistance, and yield loss due to weeds.  Omit them from your weed management program at your own risk.

Modified Relay Intercropping (MRI) is the planting of soybeans into standing wheat whereas double crop soybeans are planted after wheat is harvested. Vyn et al, found that relay intercropping of soybeans yielded better than double cropping of soybeans north of I - 70 in Indiana (http://www.agry.purdue.edu/staffbio/AY316.pdf).  In 15 years of replicated trials in North Central Ohio on the MRI system, yields have averaged 76 bu/acre for wheat and 30 bu/acre for soybeans (70 bu/wheat and 54 bushel soybeans in 2013). Wheat yields in favorable growing seasons have exceeded 90 bushels per acre and while soybeans have yielded over 60 bushels per acre in small plot replicated trials.

However, to successfully MRI soybeans into wheat, wheat row spacing modification to allow soybean planting equipment to pass without running down wheat plants..  Wheat Row spacing for MRI has ranged from 10 to 20 inches. As wheat row spacing widens, wheat yields may decline http://corn.osu.edu/newsletters/2011/2011-27/wheat-variety-yield-in-15-inch-rows .

There have been various row configurations used to allow soybean planting equipment.  For example, some wheat producers will use a twin row planter or slide row units together to a 6 inch wheat row spacing and leave a 14 inch planting strip for soybeans.  Some producers may wish to utilize corn or soybean planter to sow wheat and this normally is a 15 inch row configuration.  For 2013 data on 15 inch row wheat go to:  http://oardc.osu.edu/wheattrials/default.asp?year=2013  Row spacing data would suggest that wheat is an adaptable plant that will yield well over various row spacing’s up to 15 inches.

To accommodate soybean planting into wheat and allow for better wheat management via fertilizer, herbicide and/or fungicide applications, a tram line is essential. Generally, the tram line will be set up for the  MRI  tractor tires. Planter equipment tires are moved if necessary to follow tractor tires.

In summary, for the best interseeding results, set up your equipment in the fall and practice MRI prior to the actual planting of wheat.  To attempt to interseed wheat  in the spring without having set up the equipment to drive through the wheat may cause significant damage and reduce wheat yield. The act of  interseeding may also reduce wheat yield.  Interseeded wheat yields were reduced about 3 bushels per acre compared to non-interseeded wheat in 3 years of replicated trials in north central Ohio.

For an overview of some of the 2013 MRI plot results go to: http://croppp.cfaes.ohio-state.edu/