Purdue News
Purdue NewsDecember 12, 1997
The problem occurs when farmers rotate their fields between corn and soybeans: If corn herbicides remain active a year later, they can damage beans, and vice versa. To avoid herbicide-carryover damage, producers need to consider three main factors, Jordan said.
"The first is whether the herbicide was applied to the field or to the crop later than normal last season," he said. "With the late start to soybean planting in 1997 in several areas of the state, producers will need to be sure the number of months required before planting corn in 1998 will be met."
The same evaluation should be made with 1997 corn fields that will be planted with soybeans in 1998. "Check the herbicide labels for plant-back restrictions to determine when the 1998 crop should go in," Jordan said. "In 1997, we experienced crop damage to corn and soybeans when those crops were planted only a few weeks earlier than the plant-back restrictions required."
Jordan said soil pH is the second factor to consider when checking the label. This can be tricky, because many herbicides last longer in soil with pH that's higher than the label states. However, Jordan said, "a few herbicides will carryover when the pH is lower than the label states."
Next, Jordan recommended that producers review the amounts and the timing of the past growing season's rains. "There were a few areas in the state that experienced a severe shortage of rain in June through August," he said. "These areas are prime for herbicide carryover problems in 1998 if you plant early. With most herbicides, the potential will be greatly reduced if we have a wet spring and the planting season is delayed."
If weather in the spring of 1998 is normal, fields could suffer from carryover problems if planted too early, or in soils with pH that's too high or too low, based on label requirements.
In areas that were dry for a significant part of the past growing season, the potential for carryover is greater if there was a prolonged period of dry weather after the herbicide was used. The potential is less if there was good rainfall immediately following application -- and drought followed later. "This is because with rains soon after application, less herbicide was adsorbed by the soil, and the breakdown process was accelerated," Jordan said. "However, if adequate rains were not received until several weeks after application, a larger amount of herbicide was adsorbed to the soil, and the breakdown process was delayed.
"In planning for 1998, first determine the number of months needed before a rotational crop can be planted, then consider the soil pH and rainfall pattern before deciding when to start planting. This will help you determine if it is OK to plant a rotational crop, if herbicide-tolerant crops should be used, or if you should not rotate and stay with the same crop that was planted in 1997."
Producers who have experienced carryover problems or are concerned about carryover should consider using a herbicide in 1998 that has a different mode of action than the one they used last season, Jordan said.
The plant-back, pH, and rainfall restrictions are on the herbicide labels. Additional information can be obtained from Purdue's WS-16 publication, which is available through the county Extension offices.
Source: Tom Jordan, (765) 494-4629; e-mail, jordan@btny.purdue.edu
Writer: Amy H. Raley, (765) 494-6682; e-mail, ahr@aes.purdue.edu
Purdue News Service: (765) 494-2096; e-mail, purduenews@purdue.edu
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