C.O.R.N. Newsletter: 2016-22

As we mentioned in a previous newsletter article, there is a product newly labeled for spider mite control in soybean and sweet corn (Agri-Mek SC).  We would like to test this product in Ohio soybeans, and are looking for a producer field for this trial.  The soybean field must have an at-threshold spider mite population to treat, and the producer must be willing to leave some untreated check strips.  The product will be free.  If you would like to participate in this trial please contact Kelley Tilmon at tilmon.1@osu.edu or 330-202-3529

We have spent a lot of time educating and warning about Palmer amaranth, but while we were doing this, waterhemp has become a more widespread problem.  Waterhemp is only a little less fun to deal with than Palmer amaranth, and has a tendency to fairly rapidly develop resistance to any new sites of action used repeatedly against it in POST treatments.  Submitted questions and photos to confirm identification of this weed increased substantially this summer over previous years.  Waterhemp infestations can be found around the state, with a concentration in west central Ohio.  We assume all waterhemp populations are resistant to site 2 herbicides (ALS), but in our screening so far, not all populations are resistant to glyphosate.  Populations in western Ohio tend to be glyphosate-resistant at this point, but populations in other parts of Ohio not necessarily so.  Some populations that are glyphosate-resistant appear to have developed at least a low level of resistance to site 14 (PPO) herbicides as well. 

In areas of the corn belt further west where waterhemp has been the major glyphosate-resistant weed problem for some time, the evolution of multiple resistance is common.  Recent surveys by the University of Illinois determined that approximately 60% of Illinois waterhemp populations were resistant to glyphosate and also to site 2 and 14 herbicides.  Due to the tendency for waterhemp to evolve resistance to new sites of action used against it, the U of I recommends a strategy of not using a new site of action (usually PPO) until resistance to glyphosate has been confirmed.  So in a field situation where a grower is unsure about resistance, they suggest applying glyphosate to small waterhemp plants, and then monitoring to determine if the population is responding.  If there is no response, the next step is immediate application with a PPO inhibitor.  The drawback here of course is that glyphosate-resistant waterhemp will continue to grow during this brief period of evaluation following glyphosate application, possibly reducing the effectiveness of the PPO herbicide. 

Waterhemp requires a combination of residual and POST herbicides, with the goal of applying POST herbicides to plants not more than 6 inches tall.  In a number of Ohio fields with waterhemp that we know of, growers were either unaware they had waterhemp and inadvertently let it get far bigger than this, or were apparently still adhering to the “I’ll spray glyphosate whenever I get around to it – what, me worry” approach.  Waterhemp produces a lot of seed and one or two years of this approach will guarantee the presence of waterhemp for years to come. 

It’s largely too late to provide much helpful guidance for this summer’s waterhemp control programs.  What’s essential to accomplish still this year though, for future planning, is to figure out whether a waterhemp population is resistant to glyphosate and/or site 14 herbicides.  Fortunately, there is a painless way to do this, through a service offered by the University of Illinois Plant Clinic.  Link to the U of I newsletter article that provides the needed info - http://bulletin.ipm.illinois.edu/?p=3619.  In a nutshell, you collect five new leaves that have developed following an herbicide application, and send them to U of I via rapid delivery along with one sample submission form per sample.  Cost is $50 per sample, which includes screening for both glyphosate and site 14 herbicides.  We highly recommend making this investment where resistance characteristics of a waterhemp population are unknown.  This information will inform future decisions on herbicides for control of waterhemp, ensuring that appropriate herbicides are used and that the population does not develop resistance to site 14 herbicides as rapidly.  The LibertyLink soybean system is, of course, another option for management of waterhemp, as well as Xtend and Enlist soybean systems whenever they become available.  It should be noted that: 1) use of glufosinate, dicamba, or 2,4-D to help manage this weed does not change the approach, and these herbicides must be applied to small plants to control waterhemp; and 2) the assumption is that inappropriate and continued use of these herbicides would lead to the development of resistance to them.  And in fact there are already some populations of waterhemp farther west with resistance to 2,4-D.

Although it may be a dim memory at this point, we started the 2016 growing season on the wet side.  Some planned spring forage seedings did not happen due to wet conditions and a compressed spring planting season.  Add to this the fact that some alfalfa stands are not holding up as planned because of harvest injury during the wet and rainy conditions of 2015 and now the dry summer conditions of 2016 and there are potentially a lot of acres of alfalfa or another perennial forage that need to be planted as we look ahead to 2017.  August gives us another window of opportunity to establish a perennial forage stand and it fits nicely into rotations after wheat grain harvest. 

Typically the main risk with an August planting is a question of sufficient moisture for seed germination and plant growth and it looks like this year will not be an exception, as the weather outlook for August is for rainfall to be below normal.   Most of our perennial forage grasses and legumes are shallow seeded crops and should be planted no deeper than ½ inch and ideally closer to ¼ inch.  If the seed bed is too dry germination and emergence will be poor.  This year’s weather pattern needs to be considered and planting may depend upon a favorable forecast for rain. It is best to plant right into moist soil or right before a large rain system is forecast. No-till planting is very helpful for preserving valuable moisture this time of year.

There are some advantages to late summer forage planting as compared to a spring planting. One big plus is that planting time and field preparation is not competing with corn and soybean field work. No-till planting following a small grain crop often works well. Late summer planting means forage seedlings are not competing with the flush of annual spring and summer weed emergence/growth. The soil borne root rot and damping off disease organisms that thrive in cool, wet soils are not an issue. However, late summer forage planting has some other risks that must be managed.

Ideally, planting should be completed by mid-August in Northern Ohio and by the end of August in Southern Ohio. These timelines are based on average frost dates and the time needed for forage plants to develop a root system capable of overwintering.   For example, at about 8 to 10 weeks after emergence alfalfa plants pull the growing point below the soil surface, a process is called ‘contractile growth’.   Once contractile growth occurs the alfalfa plant is considered a true perennial.  The alfalfa plant needs to reach this growth stage to overwinter. Clover plants also need to have a crown formed, and grasses should be at least in the tillering stage of development before the onset of winter. 

If the fall is warm and extended, similar to what we have experienced the past few years, it might be possible for successful establishment with later planting dates.  Some alfalfa growers believe that the late summer planting deadline dates can be moved back by several weeks due to climate change.  But who can predict the weather? How lucky do you feel? Late summer and early fall planting dates of forages were tested in Pennsylvania in the mid-1990’s at two locations that historically are a little milder than most of Ohio’s winters. The year after seeding legumes, forage yield declined as planting dates were delayed after early August. For each day planting was delayed after August 1, total forage dry matter yields the next year were reduced by an average of 158, 105, and 76 lbs./acre for alfalfa, red clover, and birdsfoot trefoil.  Grasses were usually less affected by later planting dates. For example, orchardgrass yields only decreased significantly when planting was delayed past late-August and perennial ryegrass yields were actually greater in late-August than in early August. However, for each day planting was delayed after August 30, yields declined 100 lb. /acre for orchardgrass and 153 lb. /acre for perennial ryegrass. Reed canarygrass, a slow establisher, was more sensitive to planting dates. Reed canarygrass yields the year after seeding declined 120 lbs. /acre for each day planting was delayed after August 1.  So the best policy is usually to plant most perennial forages as soon in August as possible, when soils conditions allow and when soil moisture is present.

Sclerotinia crown and stem rot is a concern with no-till seedings of alfalfa in late summer and especially where clover has been present in the past. This is a pathogen that causes white mold on alfalfa seedlings. They become infected during cooler rainy spells in late October and November, the disease develops during the winter, and seedlings literally "melt away" in winter and early spring. It can be devastating where the pathogen is present. No-till is especially risky where clover has been present because the sclerotia germinate from a shallow depth. Early August plantings dramatically improve the alfalfa's ability to resist the infection. Late August seedings are very susceptible, with mid-August plantings being intermediate.

In a no-till situation, minimize competition from existing weeds by applying a burndown application of glyphosate before planting. Post-emergence herbicide options exist for alfalfa. After the alfalfa is up and growing, late summer and fall emerging winter annual broadleaf weeds must be controlled. A mid- to late fall application of Butyrac, Pursuit or Raptor and Buctril are the primary herbicide options. Fall application is much more effective than a spring application for control of these weeds especially if wild radish/wild turnip are in the weed mix. Pursuit and Raptor can control winter annual grasses in the fall but should not be used with a mixed alfalfa/grass planting. Consult the 2016 Ohio and Indiana Weed Control Guide and always read the specific product label for guidelines on timing and rates before applying any product.

If tillage is used to prepare the soil for planting, a firm seedbed is needed to ensure good seed-to-soil contact. Follow the "footprint guide" that soil should be firm enough for a footprint to sink no deeper than one-half inch. A pre-plant herbicide is not needed for a tilled seed bed.  Generally, the risk associated with establishing a tilled seed bed for a late summer planting is the loss of moisture.  Finally, keep in mind that any time forages are planted the following factors must be managed:

• Soil fertility and pH: The recommended soil pH for alfalfa is 6.8. Forage grasses and clovers should have a pH of 6.0 or above. The minimum or critical soil phosphorus level for forage legumes is 25 ppm and the critical soil potassium level is somewhere between 100 and 125 ppm for many of our soils.
• Seed selection: Be sure to use high quality seed of adapted, tested varieties and use fresh inoculum of the proper Rhizobium bacteria. “Common” seed (variety not stated) is usually lower yielding and not as persistent, and from our trials the savings in seed cost is lost within the first year or two by lower forage yields.
• Planter calibration: If coated seed is used, be aware that coatings can account for up to one-third of the weight of the seed. This can affect the number of seeds planted if the planter is set to plant seed on a weight basis. Seed coatings can also dramatically alter how the seed flows through the drill, so be sure to calibrate the drill or planter with the seed being planted.
• Seed placement: The recommended seeding depth for forages is one-quarter to one-half inch deep. It is better to err on the side of planting shallow rather than too deep.
• Do not harvest a new perennial forage stand this fall. The ONLY exception to this rule is perennial and Italian ryegrass plantings, which should be mowed or harvested to a two and a half to three-inch stubble in late November to improve winter survival. All other species, especially legumes, should not be cut.