[Image: clawson checking beans]Louisiana soybean growers have become increasingly interested in early planting, and LSU AgCenter researchers have been evaluating how different cultural practices affect soybean performance.
Three LSU AgCenter scientists have been looking at optimal planting dates for soybeans in maturity groups III, IV, V and VI over the past few years under grants funded by the Louisiana Soybean and Grain Research and Promotion Board.
"Optimal planting date can be influenced by production practices that include both row spacing and variety," said Dr. Ernie Clawson, who conducts research at the Northeast Research Station in St. Joseph. "Row spacing effects may occur because day length or weather can limit soybean growth when planted early – making canopy closure and full utilization of resources unlikely in wide rows."
Clawson says different varieties in the same maturity group can respond differently to planting date or row spacing because of growth potential, critical day length or other factors.
Soybeans flower in response to day length and temperature. The earlier varieties bloom when days are long and nights are short, and the later-maturing varieties bloom during relatively shorter days and longer nights. Days are longer during the summer, so early-maturing soybeans will begin to flower then and are ready for harvest earlier in the year.
Clawson, whose study comparing planting dates and row spacing is concluding with third-year results in 2008, says narrow rows had yield advantages in 2006 but much less in 2007.
"The 2006 data suggest that the earliest optimal planting date was in early April for Group III and Group IV beans in narrow rows and mid-April for the same maturity groups in wide rows," Clawson said.
"For most varieties in each maturity group, late March was the earliest optimal or near-optimal planting date in 2007 – regardless of row spacing," he added, noting that Cercospora may have limited yields of soybeans planted at later dates, especially in the Group V variety.
In both years, the single Group V variety responded well to late-March planting dates in both narrow and wide rows, he said.
Dr. Don Boquet at the Macon Ridge Research Station and Dr. Steve Moore at the Dean Lee Research Station evaluated Group III through Group VI varieties at eight planting dates from March 15 to May 26 in narrow and wide rows.
"In 2007, row spacing had no effect on yield at the mid-March and late-May planting dates, but with early-April through mid-May planting dates, average yield in the 16-inch spacing was 6 bushels per acre higher than in the 40-inch row spacing," Boquet said.
In addition, Boquet said soybeans planted in mid-March yielded 50 to 60 percent less than soybeans planted in late March and early April.
"Farmers should be very cautious about planting soybeans in Northeast Louisiana before late March, regardless of maturity group," he advised.
Boquet said it is becoming clear that planting dates for soybeans have changed dramatically from 10 years ago. The researchers attribute this to changes in genetics in varieties that respond better to early planting.
The LSU AgCenter scientists also said difficult-to-control insect and disease problems have lowered the yield potential and increased the risk for soybeans that reach maturity later than mid-September.
"The yields of Group V varieties at early planting dates in this study are of particular note," Boquet said. "It represents a complete reversal from responses of the past when varieties in this maturity group were genetically disposed to extreme dwarfing under the short day conditions in March and April."
Factors other than yield also need to be considered in selecting varieties to plant, researchers advised.
"At the present, I do not know of a time when planting Group III varieties will result in yield improvement over another maturity group," Moore said.
"The possible significant place for Group III varieties in Louisiana may be if Asian soybean rust ever develops into a significant production issue," Moore added. "Then the earliness afforded by Group III varieties might help escape that disease."
Boquet said seed quality at harvest also was affected by planting date.
"The best quality seed was from the mid-April planting date," he said, adding, "Increased plant height and improved seed quality are additional reasons to plant in April rather than March. –Rick Bogren
(This article was published in the 2008 edition of the Louisiana Soybean & Grain Research & Promotion Board Report.)