Late Soybean Planting Poses Unique Weed, Disease Situations

LEXINGTON, KY.
   Soybean growers are full-speed ahead with planting, however, the wet spring has put many of them behind schedule.
   According to the U.S. Department of Agriculture Weekly Weather Crop Bulletin, 48 percent of Kentucky soybeans were planted as of June 7. This is well behind the five-year average of 71 percent.
   Soybeans planted after June 15 are considered late and could cause yield loss. Producers currently planting need a final stand count of 100,000 plants per acre. This number should increase to 140,000 plants per acre for soybeans planted after June 15 and 160,000 plants per acre for double-crop soybeans to help offset yield losses, said Chad Lee, grain crops extension specialist with the University of Kentucky College of Agriculture.
   Even with late-planting looming, it is always better to wait for a couple days of good planting weather than try to mud-in soybeans under wet conditions, he added.
   As producers go to plant their crops, many are noticing the delayed planting has resulted in taller, more developed weeds in fields. To combat this problem, producers need to have a good weed management program in place, said J.D. Green, UK extension weed scientist.
   “A good burn down program is needed to kill existing weeds at planting time and allow soybeans to germinate and get off to a good start,” he said.
   Soybean growers may want to consider using higher rates of glysophate and add 2,4-D to combat weeds at planting time. However, producers should apply 2,4-D with caution where sensitive crops, such as tobacco, grapes or ornamentals, are growing nearby.
   Depending on past history and each producer’s situation, producers may find it useful to add a soil residual herbicide at planting for early-season weed control. The soil residual herbicide kills weeds as they germinate.
   During the growing season, producers need to remove weeds that appear after crop emergence in a timely manner because too many weeds competing with soybeans could result in yield losses.
   The good news for growers still planting is lateplanted soybeans have a lower disease risk than early-planted soybeans in nearly all diseases with the exception of soybean rust.
   “Late planting in soybeans really doesn’t affect the disease situation much. In fact, it probably actually helps it,” said Don Hershman, UK extension plant pathologist. “Usually the only disease where there could be an increased risk is with soybean rust.”
   He added even though there’s an increased risk, it doesn’t mean soybean rust will affect crops this year. Typically, the state has hot, dry weather after the plants have flowered and before pod set, which normally occurs during July and August. However, if those months are wetter than normal, the soybean rust risk could increase in late-planted and double-cropped soybeans.
   Another major factor in soybean rust development is the rate of disease development in the South. Growers can keep track of the soybean rust situation throughout the season by visiting the USDA’s soybean rust Web site at
http://sbr.ipmpipe.org/cgi-bin/sbr/public.cgi. Δ