Source:
Integrated Crop management News
By Palle Pedersen, Department of Agronomy,
Iowa State University
Many farmers were able to get back into the field this week and
start finishing up planting. However, there are still many areas
where fields remain flooded and it will take awhile before we
can get back in and replant. Some fields need to be replanted
and some don’t. It is important to accurately estimate a
surviving stand and then evaluate the economics of replanting.
Consider the yield potential of late-planted soybeans, along
with costs associated with late planting. Final stands of at
least 73,000 uniformly distributed plants per acre will
consistently yield more than 90 percent of optimum plant
population. If you are replanting an area that has been flooded,
it is highly recommend to use a fungicide seed treatment and
Bradyrhizobium inoculants. The level of seedling diseases
probably will be high with our current conditions and the level
of Bradyrhizobium in the soil could have been reduced,
particularly in fields that were flooded for an extended period
of time.
After the flood in 1993, an extensive research project was
conducted by Keith Whigham (former ISU Extension agronomist)
throughout Iowa from 1995 to 1997 to assess management decisions
when planting occurs in late June and early July. Based on that
data, the yield potential from planting in mid-June was
approximately 60 percent of the optimum yield in northern and
central Iowa and 80 percent of the optimum yield in southern
Iowa. When planting was delayed until early July, soybean yield
potential dropped even further and producers would have
approximately 33 percent of the maximum yield in northern Iowa
and 50 percent in central and southern Iowa available.
Another thing that was investigated in that study was when we
should be switching to earlier maturity group varieties. Based
on that study, it was concluded that producers should plant
their original soybean variety unless planting is delayed beyond
late June in northern and central Iowa and beyond early July in
southern Iowa.
We
are frequently concerned about late maturity of full-season
varieties planted in mid-June or later. Planting a full-season
variety in Iowa in late June or early July will, on average,
delay physiological maturity of the soybean crop to mid October.
Soybean yield potential and seed quality may be negatively
affected if frost damages the soybean crop before the plants
reaches physiological maturity (R7).
Therefore, we are recommending that growers in central and
northern Iowa switch to a shorter maturity group and shorten the
maturity group by 0.5 to 1.0. Southern Iowa growers can wait
another 10 days before needing to switch to a shorter maturity
group. No data supports planting soybeans as a grain crop after
mid July in Iowa.
More information about soybean management can be found at
www.soybeanmanagement.info.
Palle Pedersen is an assistant professor of agronomy with
research and extension responsibilities in soybean production.
