By Palle Pedersen, Department of Agronomy
Southeast Iowa has again experienced heavy rainfall and after talking to several agronomists yesterday and today it seems that there are still a few acres left to be planted in this area. With the heavy rain passing through Iowa over night and with more rain in the forecast for tonight it could be another 4-5 days before we can be back in the field planting. Because of that the questions have arise on what to do when planting soybean in late June or later and what to expect.
Late planting of soybean is going to have a significant impact on our yield. The good news is that the response to planting date and the yield loss from planting in late June is less for farmers in southern Iowa compared to farmers in central and northern Iowa. A large study conducted by my predecessor, Dr. Keith Whigham, documented that yield loss from planting in mid-June was much higher for northern and central Iowa than for southern Iowa. In southern Iowa you can expect 82 percent of your yield potential by planting in mid June but only around 60 percent can be expected for farmers in central and northern Iowa.
Changing to an earlier maturing variety is not necessary unless the planting and/or replanting date is very late. It is recommended to plant the "original" full season variety until June 20 in northern and central Iowa and early July in southern Iowa. If planting occurs after these dates it is recommend shorten the maturity group by 0.5 to 1.0. The seeding rate is the same in late April as it is in June. However, weed management should be a top priority at late planting simply because of the lack of canopy and competitiveness. It is highly recommended, if possible, not to plant soybeans now with your corn planter (30 inch rows or greater). Using a split-row planter or a drill should help you to increase light interception and biomass accumulation to maximize your yield when planting late.
Finally, if you are debating whether you need to replant, the magic number is 73,000 healthy uniform distributed plants per acre. Economically, a soybean stand of 73,000 or more healthy, uniformly spaced plants per acre in early June or later is probably worth keeping. And again, weed competition is detrimental at these low populations so be sure to control weeds when they are small.
More information on soybean planting and soybean management decisions can found at www.soybeanmanagement.info
Palle Pedersen is an assistant professor of agronomy with research and extension responsibilities in soybean production.