C.O.R.N. Newsletter: 2016-21

The month after wheat harvest provides an opportunity to control marestail and prevent further increase in the soil seedbank, but coming up with the right strategy has not necessarily been easy.  Keep in mind that the primary goal of marestail control here is preventing seed production, which doesn't mean that any treatment applied has to provide 100% control of the plants themselves.  We have conducted several studies targeting tough marestail situations, representing plants that have been previously treated with herbicide or mowed, or survived tillage, and also one study in a wheat stubble situation.  The reason all of these studies are pertinent is that there can be a couple different types of marestail plants in wheat stubble.  The generally easier plants to control are those that small and have lurked within the wheat, below the height of the cutter bar.  These plants grow taller in response to light once the wheat is harvested, but are free of prior damage that would make them more difficult to control.  The other plant type is the one that was tall enough to be cut off by the cutter bar, and these also regrow following harvest, but the prior stress makes them much more difficult to control.  And of course there can be emergence of new plants after harvest, depending upon moisture and the degree of soil cover by residue.  A few things we have learned relative to this situation:

1.  The goal of preventing or greatly reducing seed production can be accomplished without herbicides.  Mowing is effective and can be timed for the early flower stage, before plants have produced seed, and late enough that substantial regrowth and potential for much additional seed is low.  Downside – more time consuming than herbicide application.  And of course tillage can work here too.

2.  If both mowing and herbicides are going to be used, we would suggest using herbicides first and follow with mowing later as needed.  Trying to control plants that have regrown after mowing will be difficult.

3.  With regard to herbicide treatments, earlier is better.  Some regrowth of damaged plants is fine, but allowing plants to get larger almost always reduces control and may require a more comprehensive treatment.  In a 2014 wheat stubble study, we applied herbicides on either July 25 or August 7.  These were mostly plants that had not been damaged by the wheat harvest.  The combination of glyphosate plus 2,4-D applied on July 25 provided 85% control and resulted in only few plants being able to produce seed.  All other treatments (Sharpen, Liberty dicamba, etc) provided 100% control.  When application was delayed until August 7, control with glyphosate plus 2,4-D or dicamba dropped to 70 to 74% and 38 to 43% of the surviving plants produced seed.  Control with dicamba plus 2,4-D also was reduced to 78%, but plants did not produce seed.  The other treatments still provided upwards of 85% control and prevented seed production.  We suspect that the difference in control between application dates would have been accentuated for marestail plants that had been damaged by the cutter bar and regrown.

4.  Some growers try to apply late enough in summer to control both marestail and volunteer wheat.  This is likely to make control of marestail difficult.  The post-harvest treatment is also not likely to serve the purpose of controlling marestail plants that emerge in late summer into fall.  A mid to late fall application will still be most effective for those.

5.  In the other studies where we were trying to control marestail plants that had undergone a previous stress (such as cutter bar damage), it was difficult to obtain greater than 80% control with any treatment, and we actually rarely killed more than 10% of the plants.  We did not follow surviving plants through to maturity to determine effect on seed production.  The more effective treatments tended to be combinations of more than one herbicide that had activity on marestail (excluding glyphosate), such as glufosinate plus 2,4-D or Sharpen.  Overall, the variability of control increases and predictability of effectiveness decreases for plants that have regrown following damage or prior treatment, and allowing them to get taller does not help. 

6.  Every year we get a few calls into late August about control of large marestail in wheat stubble and at that point we really don’t have a recommendation to offer.  They will often have gone to seed by then anyway.  Mowing is certainly still an option then, but we would encourage application of herbicides fairly soon to avoid this problem.

We will post tables of results for the studies mentioned above in the marestail section of the OSU weed management website – u.osu.edu/osuweeds.

The frequency of Palmer amaranth infestations in Ohio seems to be holding relatively steady again into this year with the exception of one county, as far as we know.  In two areas where Palmer amaranth was most prevalent - far southern Scioto County and an area along the Madison-Fayette County line north of Jeffersonville – Palmer has not increased its footprint.  Palmer has been found in several new fields in Mahoning County, however, and the discovery occurred too late to implement effective POST herbicide treatments.  Waterhemp infestations, or at least the discovery of them, have been increasing with no real geographic pattern to their locations.  It can be found almost anywhere in the state but the highest concentration is in west central Ohio.  Essentially all of the Palmer amaranth in the state is resistant to glyphosate and ALS inhibitors.  Resistance characteristics of waterhemp populations are more variable.  They are all resistant to ALS inhibitors, but resistance to glyphosate is more likely to occur in the western half of the state.  We have also characterized some populations in western Ohio as having some level of resistance to PPO inhibitors.

Neither one of these weeds is a picnic to deal with, and the trend across the country is for them to develop resistance to any new herbicide sites of action that are used in POST treatments.  So preventing new infestations of them should be of high priority for Ohio growers.  When not adequately controlled, Palmer amaranth in Ohio can take over a field faster than any other annual weed we deal with.  Taking the time to remove any Palmer and waterhemp plants from fields now will go a long way toward maintaining the profitability of farm operations.  There is information on Palmer amaranth and waterhemp identification on most university websites, including ours –  u.osu.edu/osuweeds/ (go to “weeds” and then “Palmer amaranth”).  The dead giveaway for Palmer amaranth as we move into late summer is the long seedhead, and those on female seed-bearing plants are extremely rough to the touch.  We recommend the following as we progress from now through crop harvest:

- Take some time now into late summer to scout fields, even if it’s from the road or field edge with a pair of binoculars.  This would be a good time to have a friend with a drone that provides real-time video, or your own personal satellite.  Scouting from the road is applicable mostly to soybean fields, since corn will often hide weed infestations.

- Walk into the field to check out any weeds that could be Palmer amaranth or are otherwise mysterious.  If you need help with identification, send photos to us or pull plants and take them to someone who can identify them. 

- Where the presence of Palmer amaranth is confirmed, check to see whether plants have mature seed (the plants with the rough seedheads), by shaking/crushing parts of the seedhead into your hand or other surface that will provide contrast.  Mature seed will be small and very dark.

- Plants without mature seed should be cut off just below the soil surface, and ideally removed from the field and burned or composted.  Plants with mature seed should be cut off and bagged and removed from the field, or removed via any other method that prevents seed dispersal through the field.

- If the Palmer amaranth population is too dense to remove from the field, some decisions need to be made about whether or how to harvest.  Harvesting through patches or infested fields will result in further spread throughout the field and also contamination of the combine with Palmer amaranth seed that can then be dispersed in other fields.  So consider: 1) not harvesting areas of the field infested with Palmer amaranth, and 2) harvesting the infested field(s) after all other fields have been harvested, and cleaning the combine thoroughly before further use.  This also applies to any Palmer amaranth infestations that are discovered while harvesting.

- Scout field borders and adjacent roadsides, and also CREP/wildlife area seedings, which can be infested due to contaminated seed produced in states where Palmer amaranth is endemic and not considered noxious.  Reminder - ODA will test any seed used for these purposes at no charge.

- Feel free to contact OSU weed science for help with identification or management of Palmer amaranth.  Mark Loux – 614-292-9081, loux.1@osu.edu.

Editor's Note:  This is a synopsis of the article last week from Peter Thomison, with the pictures attached.  I've included a bit of the information to "explain" the images, but if you want "The Whole Story," please refer to last week's CORN Newsletter (2016-20).

On a typical midsummer day, peak pollen shed occurs in the morning between 9:00 and 11:00 a.m. followed by a second round of pollen shed late in the afternoon.  Pollen grains are borne in anthers (Fig. 1), each of which contains a large number of pollen grains. The anthers open and the pollen grains pour out in early to mid morning after dew has dried off the tassels. Pollen is light and is often carried considerable distances by the wind. However, most of it settles within 20 to 50 feet.  

Figure 1:  Pollen shed begins in the middle of the central spike of the tassel.

Pollen shed is not a continuous process. It stops when the tassel is too wet or too dry and begins again when temperature conditions are favorable. Pollen stands little chance of being washed off the silks during a rainstorm as little to none is shed when the tassel is wet.  Silks are covered with fine, sticky hairs, which serve to catch and anchor pollen grains (Fig. 2).

Figure 2.