Soybean Maturity and Fall Freeze Dates

Dr. Don Hershman released a Soybean Rust Spray Advisory for the western half of Kentucky (I-65 and west) this morning. If soybeans are at R4 (full pod) to R5 (beginning seed) then they are at risk of losing yield from soybean rust. Soybeans at R4 or R5 are also at risk to lose yield from another factor - freeze.

The median first freeze dates (32 degrees Farhenheit) for most of western Kentucky are near October 22 (about 30 days from this posting). Of course, October 22 is an average, meaning that the first freeze date can occur before or after the average. In one out of 10 years, the earliest freeze date is around October 6 (about 15 days from this posting). On the other side of the spectrum, the earliest frost freeze date can occur as late as around November 6 (about 45 days from this posting).

Generally, soybeans in Kentucky require about 30 days for seed fill (R6 growth stage). So, soybeans currently at R5 would require about 35 days to reach physiological maturity. The cool, cloudy weather might delay maturity a couple of days.

So, if you are considering spraying a fungicide to prevent yield losses from soybean rust, also consider your chances for a freeze event.