Mother Nature certainly has not done us any favors this growing season. After a 21 day spell, we have seen rain almost every day since. Due to all of the rain, saturated soils and poor conditions of crops, many producers are considering replanting soybean fields (if they have not already).
The first question on most minds is “How much yield should I expect from late-planted soybeans?”. According to UK Extension Grains Specialist Dr. Carrie Knott, based upon a 6-year (2006 to 2011) study that examined two relative maturity groups (late group II and mid group IV) the most recent estimate of Kentucky’s soybean yield loss per day is: 0.50% yield loss per day for mid group IV soybean beginning May 9 and 0.42% yield loss per day for late group II soybean beginning April 20.
To calculate expected soybean yield, based upon planting date:
1. Calculate the number of days after the optimal planting date
a. Optimal Planting Date
i. May 9 for mid group IV
ii. April 20 for late group II
2. Calculate expected yield loss
a. Multiply the number of days past the optimal planting date by yield loss per day
b. Yield Loss per Day
i. 0.50% for mid group IV
ii. 0.42% for late group II
3. Multiply average soybean yield by expected yield loss
Additional Considerations
When choosing a cultivar for July or August plantings consider one that is 0.5 to 1.0 relative maturity less than a full season cultivar. This may help reduce the risk of frost damage in the fall, due to an early frost event. Full-season soybean cultivars range from 3.5 to 5.0 relative maturities in Kentucky.
Be aware that reducing the relative maturity of the soybean cultivar by more than about 1.0 relative maturity may increase the risk that the pods will be near or at the soil level. This is because the internode distances are typically reduced as planting date is delayed, which can have a much more dramatic effect on shorter relative maturities.
The calculations above for expected soybean yield assume “normal” weather conditions occur after planting. That means adequate precipitation occurs and no early frost. If either we get an early frost, drought-like conditions, or other stress events occur, soybean yield may be markedly lower than estimated above.
On a different note, Southern Rust has been confirmed in Webster County. It was found on one leaf in a variety plot by one of UK Extension Plant Pathologist Dr. Carl Bradley’s interns. This is about 1 week earlier than when it was detected in KY last year. Most hybrids are susceptible to southern rust. Late-planted corn will be the most at risk. If corn is beyond R3, then probably no need to consider spraying it with a fungicide. Southern rust may still develop in fields that are beyond R3, and it may look really bad, but any yield reduction at that point in time would be small. When scouting for Southern rust look for pustules appearing primarily on the upper leaf surface that are orange to light brown, round, and densely packed on the leaf surface.
For more information on late planting of soybeans or if you suspect your corn may have Southern Rust, contact the McLean County Cooperative Extension Office at (270)273-3690. Educational programs of the University of Kentucky Cooperative Extension Service serve all people regardless of race, color, age, sex, religion, disability or national origin.
Upcoming Events
July 29th Extension Blood Drive, 11am-4pm
July 30th Farm to Fork Dinner, 6pm at Myer Creek Park
August 4th Forestry Program, Noon
August 9th McLean Co. Beef Field Day, 9am at Cody Robertson’s Farm
August 10th Hay Testing Blitz
August 11th UK Hemp Field Day
August 11th CPH Sale
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