By Dr. Bob Scott, Extension Weed Scientist, Christopher Rouse and Dr. Nilda R. Burgos
Who are we?
We are the University of Arkansas Weed Physiology Research group and the Region IV IR-4 Center, led by Dr. Nilda R. Burgos. One of the objectives of our program is to identify viable herbicides for use by Arkansas specialty crops producers and evaluate their utility in our state’s production systems. Our group, along with the Vegetable Research Station, the Fruit Research Station, and the Southwest Research and Extension Center, perform research on various crops grown by Arkansas producers including southern pea, lima bean, cucurbits, small fruits, herbs, edamame, and others. We strive to find new and diverse weed control programs that will help specialty crop producers maximize production while minimizing the impact of weeds on their crops.
What can we learn from 2015?
This season has been beleaguered by extreme weather conditions, particularly intermittent excessive rains. Continuous rainfall has had a major impact on herbicide applications and herbicide efficacy. One consequence has been delays in planting following field preparation because of rain. The rain events promote new flushes of weed emergence, which then necessitate another burndown application of nonselective herbicide (Roundup, or others) prior to planting. When this happens, we recommend mixing it with a soil-active herbicide, or planting into a stale seedbed and applying the burndown herbicide tankmixed with a preemergence (PRE) herbicide immediately following planting. Glyphosate (Roundup) has no soil activity, but if the crop is starting to emerge DO NOT apply glyphosate. Another option is to apply a product such as Spartan Charge preemergence. This product contains both a soil-active herbicide and a postemergence (POST) herbicide that will control newly emerged weeds and also provide residual activity. Another problem we have seen this year is the short-lived activity of PRE herbicides, such as Dual Magnum and Pursuit. We lost weed control much earlier than expected. This was due to excessive rainfall and standing water on the fields, which hastened the breakdown of herbicides and increased loses due to leaching and surface runoff. All we can do is manage the continually emerging weeds. Timing is key for herbicide application; the earlier the better. It is good to include a residual herbicide mixed with the POST herbicide for extended weed control in the season. Consult the label for tank mixture recommendations and maximum use rates for a product within the season. This season, like most, has been another year of fighting Palmer amaranth in most of our production fields in Arkansas. The hot, humid, and mostly wet production season has allowed them to thrive.
Options for Palmer amaranth
For pea producers, Dual Magnum (PRE) is still a good herbicide for Palmer amaranth; however, excessive rains had reduced its residual activity this season. Spartan and Spartan Charge provide excellent long-term residual activity for Palmer amaranth, but too much rain can also reduce its efficacy. Fields should be scouted for pigweeds. As soon as they emerge, an application of Reflex (1 pt/A) should be made. Reflex will burn the southernpea, and there is significant varietal difference in how much burn is observed. The crop will grow new leaves and will recover quickly. Timely application is important to achieve maximum control and reduced injury. Reflex applications should be made when Palmer amaranth are LESS than 2 inches and when the crop has reached 2 to 3 trifoliate, but prior to bud formation. This is the best postemergence Palmer amaranth herbicide currently available to pea growers in Arkansas. Pursuit (PRE and POST) used to be a good option for pigweeds, however widespread resistance to the ALS-inhibiting chemistries has led to most pigweeds in Arkansas being resistant.
Spartan (sulfentrazone): Spartan is recently registered under a 24(C) label for use on dry cowpea in Arkansas. Spartan is a preemergence herbicide that can be applied from one week prior to and up to planting to provide residual control of Palmer amaranth, some annual grasses, morningglories, and nutsedge. The use rates for Arkansas are between 3 and 8 fl oz per acre depending on soil type. Consult the label for your specific area and soil texture. In general, it is not the best option for grass control but it can be mixed with other herbicides for improved activity. Dual Magnum is a good partner for increased spectrum of control; a premix of Dual Magnum and Spartan is sold as BroadAxe. This mixture may cause some stunting if applied PRE, but will not affect yield. Crop safety is better when applied 1 week preplant. There are restrictions on the counties in which this herbicide can be applied. Consult the label for your area.
Spartan Charge (sulfentrazone + carfentrazone): Spartan charge is a premix of sulfentrazone (PRE activity) and carfentrazone (POST activity). This has a 24(C) label for use on dry cowpea in Arkansas. This is similar in activity to Spartan, but will control small emerged weeds in the field at planting. It can be applied from one week before planting through planting. The use rates for Arkansas are between 3.8 and 7.6 fl oz per acre; application rates vary by soil texture. Consult the label for your specific soil type. There are restrictions on the counties in which this herbicide can be applied; consult the label for your area.
What else are we looking into?
We are always looking into new herbicides for use by our growers. Valor SX (flumioxazin) is currently under investigation for crop tolerance and safety. This will provide excellent residual control of Palmer amaranth in pea fields even under less –than-ideal conditions like this year. Another herbicide of interest is Ultra Blazer (acifluorfen) which provides excellent postemergence control of hemp sesbania (coffee bean), Palmer amaranth, and morningglories. This compound is residue testing in cowpea through the IR-4 program for possible 24(C) registration in various states. Cadet (fluthiacet-methyl) is also currently in our field trial for use as a POST herbicide to control some pigweeds and morningglories. Grower support is always needed and input from producers will help us identify the need for new compounds.