By Jeremy Ross, Extension Soybean Agronomist
So far, this has been a year of highs and lows. As of today, I have certified six soybean fields for the Grow for the Green Yield Contest over 100 bu/ac. A new state record soybean yield was set twice within the last two weeks. The first was Martin Henry in Desha Co. with a yield of 113.9 bu/ac, and his record yield was surpassed by James Wray in Poinsett Co. with a yield of 118.8 bu/ac the following week. For every field over 100 bu/ac I have had 20 calls from soybean producers and consultants wanting to know why their soybean yields are considerably lower than the last few years. Many are saying that the soybean crop has looked excellent all year, with tall, healthy looking soybean plants, but when the combines are rolling through the fields the yields are not there. In my professional opinion, I think there was a host of stresses and events that caused the reduction in soybean yields we are seeing.
For the week ending July 24, 2016, the soybean crop was 91% blooming and 70% setting pods, which was 12% and 23% ahead of the 5-year average. The state average high temperature for that week was 95.4 degrees F with the average low temperatures 74.5 degrees F. Even though we have seen temperatures hotter than these in the past, I believe this hot, dry period was the beginning of the problems we experienced this summer. Stress during pod formation (R3 – R4) may affect yield potential by decreasing total pod number, bean number per pod, or seed size. Typically, soybean plants can compensate, at least partially, for temporary stress. During this period, much of the state had not received much rainfall, and many of the soybean fields were being irrigated or had just finished an irrigation going into the wet period during the first of August. Through the third week of August, we experienced higher than normal rainfall, extended cloudy conditions, and relatively hot temperatures. I believe that many of the soybean fields experienced saturated soil conditions for several week due to irrigations prior to the rain events and the extended rainy weather pattern. The ideal ratio for the soil matrix is 50% soil particles, 25% air filled cavities, and 25% water filled cavities. Even though many of these fields did not have standing water during this time, I believe that many of these fields were at 100% saturation for a number of days. Under saturated conditions, little to no oxygen is getting to the soybean roots. Many of these problem fields I have walked the last few weeks have had abnormally low root masses on most of the plants I have examined, and saturated soils can cause this problem. With limited roots, soybean plants will struggle to maximize yield.
Once we moved out of the rainy period in August, plant diseases exploded all over the state. We were seeing just about every soybean disease pictured in the disease compendium. I personal identified frogeye leaf spot, areal web blight, anthracnose, charcoal rot, brown spot, Cerscospora leaf blight, target spot, and some we are still trying to identify. Many of these diseases we are familiar with, and know the potential damage they can cause. The one I’m getting the most calls about is target spot. Many I’ve talked to want to blame all of the yield loss on target spot, but I think that target spot is just one piece of the puzzle on what caused the yield loss we have experienced. We have limited data on varietal differences and chemical controls of target spot. There does seem to be differences in susceptibility to certain soybean varieties, and we hope to have information on susceptible soybean varieties available in the near future. Many of these disease appeared late in the growing season after the growth stage we typically recommend a fungicide application. Many of the plants in these fields lost their leaves prematurely, and this could be a cause for the smaller seed size many are seeing.
I’ve also heard that some producers are blaming the low yields on soybean varieties with a particular herbicide trait. Up to now, I have not seen any particular herbicide trait or variety immune to lower than expected yields. I’ve heard good and bad yields on soybean varieties from all herbicide traits and all companies. The last time we experienced a weather pattern similar to the one the state had this year was 2009. I walked a number of fields in 2009 with issues many of our producers are having now. I would not let what happen this year affect my weed control options for 2017. Producers who have fields with PPO and glyphosate-resistant weed problems could potentially lose more in yield due to weed competition by not staying with an effective herbicide program.
I know many producers want to place the blame on the soybean variety, a missed fungicide application, or another input, and not the impact of the weather events we had during 2016. I think we had the “perfect storm” this year with the growth stage that a majority of the soybean crop was at during July and August, the weather pattern during this time, and the late-season disease development for a reduction in soybean yields. My intentions are to look at more weather data and the crop development during the year to try and confirm the observations I have seen so far this year, and present these findings at our winter meeting. We still have a lot to learn from this year, but like I have told others, don’t do wholesale changes to your production practices due to what happen in 2016. We need to learn from this year, and what happens next year may be totally different.