Ag Answers:Consider Herbicide Carryover Risks When Planning for 1998

Several corn and soybean herbicides can carry over and cause damage in next year's crops, so producers need to consider three main factors to determine their fields' carryover potential, according to Purdue Extension weed specialist Tom Jordan.

"The first is whether the herbicide was applied to the field or to the crop later than normal," he says. "With the late start to soybean planting 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 consideration should be made with corn fields going to soybeans. "Check the herbicide labels for plant-back restrictions to determine when the 1998 crop should go in," Jordan says. "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 says 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 says, "a few [herbicides] will carryover when the pH is lower than the label states."

Next, Jordan recommends 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 says. "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."

Even if weather in the spring of 1998 is normal, fields could suffer from carryover problems if planted to early or in soils with the 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 than if there was good rainfall immediately followed 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 says. "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 okay 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 they used last season, Jordan says.

The plant-back, pH, and rainfall restrictions can be found on the herbicide labels. Additional information can be obtained from Purdues WS-16 publication which is available through the county Extension offices.