Field Notes: Corn and soybean planting - Are we on schedule?

Liz Stahl, Extension Educator - Crops

After rains put a halt to planting this season, significant progress has been made over the past week where conditions allowed. Drs. Jeff Coulter, Extension corn agronomist, and Seth Naeve, Extension soybean agronomist, discussed what farmers should be looking out for where crops were planted early as well as where seed is still in the bag on the May 15 Strategic Farming: Field Notes session. They were joined by moderator Dave Nicolai, Extension Educator –Crops.

Corn updates

Early planted corn has emerged

Around mid-April, farmers were faced with the decision to plant or not to plant as soil conditions were good as well as the weather. After this time, however, there was a two-to-three-day window where daily air temperatures dipped below freezing. Although stands have been a little more spotty, the earliest planted corn has been emerging around the same time as corn planted around April 22. Upon closer inspection, some of the seeds from pale and less vigorous plants has been mushy, especially in poorly-drained areas of fields.

The seed is an important source of energy for seedlings through the V1 stage of corn. After this time, the seminal root system takes over, which should help any seedlings that are looking a little tough to recover. Germination and emergence is based on the amount of growing degree days accumulated after planting. Any corn planted now should emerge in at least 9 to 10 days due to warmer conditions after planting.

Got corn to plant yet?

If you have corn to plant yet, long-term planting date studies indicate that corn planted on May 15 can be expected to yield 95 to 98 % of corn planted in late April under idea soil conditions. Corn planted on May 20 and May 25 can be expected to yield 92 to 95 % and 87 to 92 % of corn planted in late April under ideal soil conditions, respectively.

Note that if planting is delayed until May 22 to the 28, a hybrid 5 to 7 relative maturity units earlier than what is considered full-season for the area would be recommended. Until this time, it is recommended to stay the course with your original seed choices. If corn planting is delayed until May 29 to June 4, a hybrid 8 to 15 relative maturity units earlier than what is considered full-season for the area would be recommended. See Considerations for late-planted corn in Minnesota for more details.

While planting date is an important piece in laying the groundwork to optimize yield, very early planting dates in corn have not been shown to increase yield potential. Soil conditions at planting play a major role in stand establishment and seedling vigor, and ultimately can have a significant impact on yield potential as well.

Soybean updates

Planting date impacts

Recent data confirms historical research results that show that throughout late-April and May, there is a small but consistent response to delayed planting in soybean, averaging only about a 0.1 to 0.2% drop in yield per day as planting is delayed. This means it takes about 10 days to see a 1 to 2% drop in yield. Once we hit the beginning of June, however, we can see a 1.0% to 1.25% drop in yield per day as planting is delayed. 

As in 'real life,' early planting dates in these studies were often planted under less-than-ideal soil conditions. When soil conditions are unusually good in late-April and early-May, yield potentials should be maximized.  Naeve recommends following soil conditions, then looking at working days and how to best manage risk to determine what crop sequence and planting date will work best in your situation.

Managing fields that have emerged

As with corn, soil conditions at planting are a key factor influencing stand establishment and ultimately yield potential in soybean. Naeve reports seeing fields that have been rotary-hoed in his travels throughout the state, indicating there has been some crusting issues in fields. A rotary hoe can be very effective at helping a crop that is struggling to emerge through a soil crust in both corn and soybean. In soybean, however, one must be very careful when seedlings are just starting to emerge (when you can see the hypocotyl arch) so you don’t break off plants below the cotyledons, as this will kill the seedling.

Got soybeans to plant yet?

Soybeans are less sensitive to planting depth compared to corn. If you have soybeans to plant as we move later into the season and rain is in the forecast, it becomes a balance between waiting for good soil conditions (which may not come soon) or “mudding” in the crop. If you can get the soybeans planted and more rain is expected after planting, this can help erase crusting issues. If you mud the crop in and then it doesn’t rain, this can result in some real issues with soil crusting and ultimately poor stands.

Regarding soybean maturity, it is recommended to stay the course with your original soybean maturity until the second week of June. At that point a soybean variety with a relative maturity of 0.5 units shorter than the original choice would be recommended. If planting (or replanting) is delayed until late June, it is recommended to plant a variety 1.0 maturity unit shorter than what you’d normally plant.

Although many farmers like to roll their soybeans to help with harvestability, Naeve recommends looking carefully at the weather forecast and field conditions. Seedlings can be protected from injury and crusting and sealing issues avoided in fields with higher amounts of residue. In fields where there is little corn residue and/or where tillage has been more aggressive, there is significant potential to seal the soil surface with a roller, especially in drown-out spots where there may be no corn residue.

Replant decisions

Considerations for soybean:

Naeve recommends one be very critical in assessing stands before deciding to tear a stand up and replant. Research has shown a final stand of about 100,000 plants per acre or less results in maximized yield. If you discover lower-than-optimal stands early in the season (e.g. around 50,000 to 75,000 plants per acre), it is possible to interseed around 75,000 seeds per acre to boost stands early in the season. Later planted rows will mature a few days later than the early planted rows, but this shouldn’t be an issue if you wait a few days until both rows are ready to be harvested in the fall.

Refer to the U of MN Soybean damage and replanting guide for more details to help in replant decisions.

Considerations for corn:

Coulter notes that for corn, we can have populations down to 23,000 plants/ac and still end up with a respectable yield, in this case around 92% of optimal. Ideally corn seedlings would be evenly spaced, but the reality is that if spacing between plants is a bit uneven, it won’t have much of an effect on yield.

Yield potential for an earlier planted field with reduced stands should be weighed against the yield potential for later planted corn, along with the additional costs and time involved for replanting. Weed control may be a bigger challenge where stands are spotty and/or reduced as well. The Corn grower’s guide for evaluating crop damage and replant options provides more information regarding replant decisions.

Field Notes podcasts available

Check out these links if you’d like to listen to the podcast of this session (https://strategicfarming.transistor.fm/ ) or review additional resources (z.umn.edu/strategic-farming).

Join us Wednesday mornings!

Join us next week when we welcome Drs. Dean Malvick, Extension plant pathologist and Bob Koch, Extension soybean entomologist to discuss "Seedling Diseases and White Grub Management.”

University of Minnesota’s Strategic Farming: Field Notes webinar series, offered Wednesdays morning from 8 to 8:30 am through August 21, provides useful, timely, and relevant research-based information on cropping issues throughout the growing season. For more information and to register, visit z.umn.edu/strategic-farming.

Thanks to the Minnesota Soybean Research and Promotion Council and the Minnesota Corn Research and Promotion Council for their generous support of this program!