Field Crops Notes (July/August 2011)

Why Are Toxins So Variable In Small Grain Harvest?

This harvest season we face the challenge of increased head scab (or Fusarium head blight) in many of our fields, and higher DON (or vomitoxin) levels as a result. Many growers would like to know the proper procedures if they have a high level of scab in their fields and if it is possible to reduce the amount of toxin in their grain. Unfortunately, there is not an easy answer because there can be a great deal of variability in toxin levels, and here is why:

Fungicides are not 100% effective
Those growers who were able to apply a fungicide at flowering may find they still have scabby fields. This is because the fungicides available do not provide 100% control. Even in the best situation they will provide about 60% reduction in scab. The entire head of wheat does not flower at the same time, and these fungicides are not able to move into newly forming plant tissue so it is impossible to protect all the flowers at once.

Fusarium head blight (scab) is spotty in the field
Edges of fields can have significantly higher levels of fungus and toxin than the centers, or it may be patchy (for instance, higher levels at low lying areas of the field). By carefully harvesting, it is sometimes possible to avoid these areas thereby reducing the contamination of clean grain. Also, increasing the combine fan speed can help to blow out the lighter and diseased kernels along with the chaff, reducing the amount of severely affected kernels in the final product. Likewise, postharvest cleaning also can remove some shriveled kernels.

Scab does not always directly correlate with DON levels
The toxin is a byproduct of fungal metabolism, so depending on how the environmental conditions match up with the fungal growth stage, you could have a lot of infection but a relatively low amount of toxin, or little infection but higher toxin levels. For instance, the fungus grows well in warm, humid daytime conditions while the toxin is produced in cooler temperatures. Even kernels that do not appear infected can have microscopic levels of fungus, and therefore may also have DON.

Fungus and toxin levels vary in the truckload or bin
Often, the smaller, denser grain settles into the middle of the load and it is this material that tends to have higher toxin levels. It is critical that multiple samples are taken from a truck or bin in different locations so that the material tested is representative of the entire load. Drying the grain down quickly can stop the growth of the fungus, and therefore prevent more toxin from being produced, but drying or roasting will not remove or denature the toxin that is already present.

In our region, we are often at risk for head scab depending on what kind of spring we get. So for the future, we need to use a multipronged approach to scab management:

• Use a head scab resistant variety
• Plant following soybeans
• Visit the online assessment tool www.wheatscab.psu.edu to find out your risk level at flowering
• If your wheat is at risk, apply Caramba or Prosaro (tebuconazole products, to a lesser degree, can also be effective in reducing scab and DON levels)
• Do not apply strobilurin fungicides at flowering

Combining all of these strategies can result in nearly 90% control of DON and scab.

        Alyssa Collins, Director, Southeast Agricultural Research and Extension Center

Weed Management in Double-Crop Soybeans

The following article was written last year by Dr. Mark Loux, Extension Weed Specialist at the Ohio State University. Mark has been conducting marestail (horse-weed) control research in soybean for a number of years. With the increase in marestail problems in PA, this information should be very timely as the barley and wheat acres are planted to soybean.

A weed-free start is the most critical aspect of a weed management program for double-crop soybeans. This can be challenging to achieve where glyphosate-resistant marestail are present after wheat harvest. Problems with marestail include the following:

1. Most populations are now glyphosate-resistant and many of these are also ALS-resistant

2. It’s usually not possible to use 2,4–D ester and wait seven days until double-crop soybean planting.

3. Marestail that were tall enough to be cut off by harvesting equipment will be even more difficult to control.

Where the marestail are not resistant to glyphosate, application of glyphosate at 1.5 or more lbs. ae/A should be an economical and effective approach, or a mixture of glyphosate and Sharpen. The addition of FirstRate or a chlorimuron-containing product can also improve control if the population is not ALS-resistant, and provide some residual broadleaf weed control. However, results of a trial conducted in 2010 suggest that a switch away from a glyphosate-based burndown may be the best strategy where the marestail are resistant to glyphosate.

The population in this trial was resistant to glyphosate and ALS inhibitors and had survived early-May application of glyphosate. We applied burndown treatments on June 1 when most of the plants were about 4 to 15 inches tall. The upper part of the taller marestail plants had branched out in response to the early glyphosate treatment, and were generally bushier than marestail left undisturbed through early June.

We applied a variety of treatments, including combinations of glyphosate with 2,4–D and/or Sharpen, and combinations of Ignite or Gramoxone with 2,4–D and/or Sharpen and/or metribuzin. None of the treatments provided more than 83% control of the marestail four weeks after treatment, although several were in the 80–83% range. The better treatments included (all treatments included AMS also):

83% - Glyphosate (1.5 lb) + Sharpen (1 oz) + 2,4-D ester (0.5 lb) + MSO
82% - Ignite (32 oz) + metribuzin (4 oz of 75DF)
81% - Ignite (22 oz) + Sharpen (1 oz) + MSO
77% - Glyphosate (1.5 lb) + 2,4-D ester (0.5 lb)
75% - Ignite (22 oz) + metribuzin (4 oz of 75DF)
75% - Ignite (11 oz) + Sharpen (1 oz) + metribuzin (4 oz of 75DF) + MSO
74% - Ignite (22 oz) + Sharpen (1 oz) + MSO

We considered this to be a worst-case situation with regard to marestail burndown, and it’s likely that these treatments would be considerably more effective in some populations.

We applied these same treatments on June 4 in another field where marestail had been growing undisturbed up until that point. Plants were up to 30 inches tall in this field but were mostly unbranched. We did not have prior information about the herbicide resistance characteristics in this population, but our results indicated a relatively low level of glyphosate resistance and apparently no resistance to ALS inhibitors. At the 14-day evaluation, almost all of the treatments resulted in 100% control. The primary exception was glyphosate applied alone, which resulted in about 60% control.

Regardless of the type of soybean planted (RR vs. LL vs. non-GMO), it’s essential to control marestail and other weeds present at the time of planting. The treatments shown above should control the other broadleaf weeds present after wheat harvest, although Ignite rates of 22 to 32 oz. should be used where grasses are present. Other considerations include cost of the seed and POST herbicides, potential for POST soybean injury, and need for POST control of marestail that emerges late or regrows following a burndown treatment. Soybeans planted at this time of the summer should not be subjected to injurious POST treatments that might result in a cessation of growth if possible, since there is limited time for regrowth to reach maximum yield potential.

Assuming use of an effective burndown treatment, some of the herbicide/seed type options are:

· Plant any type of soybean and include a residual herbicide with the burndown treatment so that POST herbicides are not needed. A good strategy in Roundup Ready or non-GMO soybeans even where POST treatment is needed, since POST marestail control might be impossible in these systems. Residual herbicides used at this time of the year should be restricted to those that have little or no carryover risk - such as metribuzin, Valor, or low rates of chlorimuron or cloransulam products.

· Plant a LibertyLink soybean and apply Ignite POST as needed. Probably the best option for control of later-emerging marestail or plants that regrow after the burndown.

· Plant a Roundup Ready soybean and apply glyphosate POST. Should work for most weeds, but not a good choice if the POST application needs to control marestail.

Plant a non-GMO soybean and apply conventional POST herbicides (Flexstar, Fusion, Select, etc.) as needed. This system has the most potential for soybean injury, but seed may be cheaper than the other systems. Not a good choice if the POST application needs to control marestail.

Mark Loux, Weed Science, Ohio State University

Brown Stink Bug Damage 

Be on the lookout for brown stink bug damage. It can be found now in fields, usually closer to field buffers. It resembles other damage such as European corn borer. The key to the diagnosis is the halo of yellow tissue around the hole. If you have encountered this type of damage, I would be interested in hearing from you.  Please call me at 610-378-1327 or email mmh10@psu.edu.