Managing Weeds in Wheat (A. Stanley Culpepper)

With a warm December, numerous winter weeds are up and growing aggressively.  For wheat that was planted timely, herbicide applications are usually needed around Christmas.

Broadleaf weeds are common in wheat but can be managed effectively if herbicide applications are timely. Numerous herbicide options exist and six of these options are shown below.

Herbicide Option	Wheat Stage of Growth	Weeds
Harmony Extra1 + MCPA2 or Harmony Extra1 + 2,4-D2	Fully tillered	Wild radish, henbit, chickweed, primrose, and most other broadleaf weeds.
Harmony Extra1 followed by MCPA or 2,4-D	After 2 leaf & before flag leaf followed by fully tillered
Harmony Extra1	After 2 leaf & before flag leaf	Henbit, chickweed, most other broadleaf weeds EXCEPT primrose and radish larger than 1”.
2,4-D or MCPA	Fully tillered	Wild radish and primrose.
PowerFlex	Three leaf but before jointing	Wild radish, ryegrass and less than 1.5” henbit or chickweed
Osprey	Emerged but before jointing	Ryegrass, annual bluegrass, and less than 2” henbit or chickweed

¹Harmony Extra, Harmony Extra Total (Sol) and generic products are available, use correct rate.

²MCPA or 2,4-D can be applied at 12-16 oz/A; MCPA is “softer” on wheat.

Annual ryegrass is a serious pest of wheat, especially in middle and North Georgia, primarily due to herbicide resistance.  Growers must rotate herbicide chemistry. Osprey and PowerFlex have the same mode of action thus rotating these herbicides is not an effective approach in rotating herbicide chemistry.  Osprey/PowerFlex should be rotated with Axial; a piece of land should never be treated two years in a row with PowerFlex/Osprey OR Axial.  In addition to rotating herbicides, one must implement cultural practices such as tillage (deep tillage helps considerably) and crop rotation (allowing use of other herbicides such as Roundup) in a ryegrass management program.  There are Georgia growers with ryegrass resistant to Axial, PowerFlex, Osprey, and Hoelon and the production of small grains is no longer economically feasible.  Ryegrass will develop resistance to herbicides more quickly than any other known plant.

All “ryegrass” herbicides should be applied prior to 2 tiller ryegrass!  Mixtures with Harmony will improve broadleaf weed control; however, do not mix these products with 2,4-D or MCPA.

Herbicide Option	Wheat Stage of Growth	Weeds in Addition to Ryegrass
Axial XL	Two leaf up to pre-boot.	None
PowerFlex	Three leaf up to before jointing.	Wild radish < 6”, henbit and chickweed < 1.5”
Osprey	Before jointing.	Annual bluegrass; wild radish < 4”, henbit or chickweed < 2”

Should Growers Be Concerned About Diuron Carryover to Wheat (Culpepper)?

Growers are strongly encouraged to apply diuron + MSMA at cotton layby for the control of Palmer amaranth.   With rapid adoption of diuron use at layby in cotton, many growers are now questioning the potential for diuron injury to a subsequent wheat crop.   The following restriction is made clear on the Direx label when it is applied at cotton layby: “Cotton, corn, and grain sorghum can be planted during the next spring.  Do not replant treated areas to any other crop within 1 year after the last application, as injury to subsequent crops may result.”

Therefore, the Direx label prohibits planting wheat following a layby application of Direx.  So is this restriction based on sound science or is this proposed potential for damage simply not fully understood?  Our initial research suggests that the potential for damage is simply not understood.  During last winter, we applied Direx from 8 oz/A up to 32 oz/A preemergence on wheat and followed immediately with heavy irrigation.   Injury was not detected with 8 oz/A and less than 10% injury was noted with 16 oz/A.  Therefore, our INITIAL research suggest diuron applied at layby in cotton should pose little to no risk to the following wheat crop as long as at least 90 days has passed (Table 1).  Significant (23 to 43%) wheat injury was noted with 24 to 32 oz/A Direx applied PRE to wheat.  It is important to note that this study was conducted on a soil with less than 1% organic matter which increases the potential for injury as compared to soils with greater than 1% organic matter.

Table 1.  Wheat tolerance to Direx applied PRE.
Rate of Direx applied PRE to Wheat	Maximum Wheat Injury
Direx  8 oz/A	0% d
Direx 16 oz/A	8% c
Direx 24 oz/A	23% b
Direx 32 oz/A	43% a
Experiment conducted in TyTy, GA during the 2010 and 2011 season.  Values with the same letter are not different at P = 0.05.

It is important to remember to always follow all herbicide label restrictions.  Our goal is to work with MANA to address rotational restrictions for Direx to wheat.

Post-Harvest Control of Benghal Dayflower (Tropical Spiderwort) (Prostko)

Although much of our attention and effort has been focused on Palmer amaranth control, other weeds are still problematic in Georgia.  While driving around recently, I noticed many harvested corn fields that were covered with Benghal dayflower a.k.a. tropical spiderwort (Figure 1).  Like Palmer amaranth, it is very important to control these fall populations of Benghal dayflower since plants can go from seed to seed in about 40 days.  Here are a few reminders:

1) Tillage is just as effective as herbicides in controlling post-harvest populations of Benghal dayflower.
2) Herbicides that can be used for the control of post-harvest populations include 2,4-D, paraquat, and Aim.  Please refer to page 56 of 2012 UGA Pest Control Handbook for more information.
3) Plants larger than 6" tall will need split applications of herbicides (Figure 2).

Post-Harvest Control of Palmer Amaranth (Prostko)

I have been getting numerous telephone calls lately regarding the post-harvest (i.e. after field corn) control of Palmer amaranth.  It is extremely important to control this fall population of Palmer plants before they produce more seed which can be added to the soil seed-bank.  A year's worth of good control could go down the drain if these populations are allowed to produce seed! In case you were unaware, I have very specific recommendations for this particular situation on page 62 of the 2012 UGA Pest Control Handbook.  It is critical that growers pay close attention to the size of the Palmer.  If paraquat is applied to weeds larger than 6", control will be poor and this lack of control may even help contribute to the future development of paraquat resistance.  I often suspect that some folks think that I make this stuff up.  However, if you or your growers do not believe me, please refer to Figure 1.  Increasing the paraquat rate and tank-mixtures with 2,4-D or dicamba would help but not enough for me to say that it is a great idea to spray pigweed plants greater than 6" tall.

Herbicide Options for Double-Cropped Soybeans

With the current high price of soybeans, many GA growers are planting soybeans after field corn.  Thus, I have received many questions about weed control options for this situation.  Please consider the following:

1) My recommendations for weed control at this point in time are very different from early/normal planted soybeans. 
2) My primary concern is crop rotation.  If wheat is to be planted this fall or field corn is to be planted next spring, herbicide options may be limited depending upon the application date.
3) Table 1 lists the current labeled crop rotation restrictions for common soybean herbicides.
4) In order to prevent any major crop rotation problems, I would suggest a PRE application of Dual Magnum (or generic) followed by an EPOST application of glyphosate + Cobra or Ultra Blazer (RR soybeans).
5) Valor would be an excellent choice in fields where a POST application of Cobra, Prefix, Reflex, and Ultra Blazer may not be needed.  If we are going to protect Valor, growers must avoid PRE applications of Valor followed by POST applications of Cobra/Prefix/Reflex/Ultra Blazer. PPO abuse for sure!
6) If field corn will be planted next spring (~8 months), avoid the use of Canopy, Envive, Prefix, Pursuit, and Reflex.

Reflex Carryover to Field Corn in Georgia (Prostko)

Last few days, I have gotten several questions about potential Reflex carryover to field corn in Georgia.  Here are a few of my thoughts:

1) The labeled rotation restriction for field corn following Reflex is 10 months.
2) Average field half-life for Reflex is 100 days.  It is probably much shorter in our soils.
3)  I have only seen Reflex carryover injury to field corn on the farm 1 time in Georgia.  This occurred in Early Co. in a field that received 3 applications of Reflex in cotton the prior year.
4) Most of the weird symptoms that are occurring in Georgia corn fields at this time are not likely caused by Reflex (i.e. assuming excessive rates or delayed applications were not made). 
5) My colleagues in Tennessee and Arkansas have been reporting some problems with Reflex carryover to field corn but you must remember that these states have heavier soils with higher organic matter contents than Georgia.  
5) The following are some pictures of known Reflex injury to field corn. 

4

Leadoff Herbicide (Prostko)

The UGA Weed Science Group has been getting many questions from County Agents regarding DuPont’s new burndown herbicide Leadoff.  Here is what we know and do not know:

1)   Leadoff 33.4WSG is a mixture of Matrix/Resolve (rimsulfuron – 16.7%) + Harmony (thifensulfuron -16.7%).

2)   The typical use rate for Leadoff is 1.5 oz/A at a cost somewhere between $7-8/A

3)   Rotation restrictions for common GA row crops are as follows:  soybean/cotton = 30 days; corn = anytime; peanut = 45 days.

4)   Leadoff is being aggressively promoted by DuPont sales reps as a residual alternative for the control or Palmer amaranth. 

5)   There has been no research conducted by the UGA Weed Science Group to support or refute these claims. 

6)   Rimsulfuron does provide some soil residual activity on certain weeds.  However, I can find no published data for Palmer amaranth.

7)   The current Leadoff label suggests that it will only provide partial control or suppression of Palmer amaranth.   http://www.cdms.net/LDat/ld2AA003.pdf

8)   I am not sure why would any grower would want only partial control or suppression of Palmer amaranth with Leadoff when other residual herbicides are available that have a proven record of control (i.e. Atrazine, Valor, Dual Magnum, Direx).

9)  A few field trials will be conducted by the UGA Weed Science Group this year to figure out what kind of residual control Leadoff actually provides. 

What's That Weed (Prostko)

Recently while walking through a fallow field near my home in Tift Co., I came across this weed.  At first glance, you might say it is shepherd’s purse (Capsella bursa-pastoris).  However, the common name for this plant is pinnate tansy mustard (Descurainia pinnata).  Pinnate tansy mustard is a spring/winter annual member of the Brassicaceae or Mustard family of plants.  As far as I can tell from the literature, it should not be any more difficult to control than wild radish or other common members of this family.

Capreno Herbicide (Prostko)

Recently, I have been getting a few questions about Bayer's new corn herbicide, Capreno.  I have conducted numerous field trials with Capreno over the last several years.  Consequently, Capreno is now listed in the 2012 UGA Pest Control Handbook (page 52).  A few general thoughts about Capreno:  1) I would always recommend tank-mixing atrazine with Capreno; 2) Capreno + Atrazine should be good on most of our common weed species including pigweed and Texas millet; 3) I have no clue what Capreno will do on tropical spiderwort/Bengahl dayflower?  The following are a few slides with additional information about Capreno: