Early-maturing soybean varieties often yield less than adapted varieties By Bill Wiebold

Because Asian soybean rust was found in the southern US in late 2004, Missouri farmers may be thinking about planting soybean varieties that mature earlier than varieties adapted to their region. Soybean is highly sensitive to photoperiod, so soybean varieties are adapted to narrow bands that run nearly east and west across the country. The soybean maturity groups associated with Missouri range from MG III in the north to MG V in the south.

The reason that farmers may plant early-maturing varieties is that they hope that their soybean are plants further along in the seed-filling process before rust moves from the southern USA to their fields. This strategy makes sense because the effects of most stresses on soybean yield depend on the stage of development at which the stress occurs. Soybean plants affected by rust or other stresses in mid-seed filling (R6) suffer less yield loss than soybean plants affected by rust at beginning pod development (R4). In general, two varieties that differ by a whole maturity group will differ for the date on which they mature (R7) by about 10 days. Unfortunately, it is hard to predict variety response to in-season growing conditions and this difference might be fewer or more than 10 days.

Stages earlier than R7 are of more interest to farmers, and we have little data upon which to base predictions. In central Missouri, we planted varieties ranging in maturity from 2.8 to 4.6 (mid-II to mid-IV). This experiment was conducted for only two years in one location so information is limited. We found that two varieties that differed by a whole maturity group differed for beginning flowering (R1) date by about five days and for beginning seed-filling (R5) date by about seven days. So, planting varieties from earlier maturity groups will affect dates at which plants reach important growth stages.

However, yield potential of earlymaturing varieties is probably less than adapted varieties – at least in central and north Missouri. Again, Missouri data are very limited. The earliest variety used in the study discussed above, had special oil characteristics and its yield may not be indicative of other earlymaturing varieties. But, early-maturing varieties (e.g., MG I and II) planted in north or central Missouri will be shorter, branch less, and have smaller leaf areas than adapted varieties. For these reasons their yield potential will be predictably less. Farmers thinking about planting early-maturing varieties will find little data on which to base variety selection. They may use data from variety trials held in states to the north, but yield performance will likely change.

Planting early-maturing varieties in central and north Missouri is not recommended. In SE and SW Missouri we have some data that indicate that MG III varieties may be an appropriate choice for a portion of the soybean acreage. However, yield variation from year to year may be greater for earlymaturing varieties than for adapted varieties, so predicting yield potential will be harder. Early-maturing plants will mature when weather conditions are hot and humid. Seed-borne diseases including phomopsis are likely to occur and grain quality may be reduced. Reduced grain quality may lead to lower grain prices.

Bill Wiebold, Ag Extension-Plant Sciences
(573) 882-0621