Steven H. Moore and Donald J. Boquet
The success of early season production coupled with the increasing late-season occurrence of Cercospora leaf blight, Asian rust and stink bugs has led Louisiana soybean producers to consider even earlier maturing varieties to maintain profitable soybean production. The purpose of this research was to determine how well-adapted the very early maturing Group III soybeans are in Louisiana.
Flowering in soybeans is most often influenced by the length of nighttime. Early maturing soybean varieties generally require a shorter nighttime period to trigger flowering than later maturing ones. For this reason, early maturing soybeans planted at recommended dates in Louisiana will usually flower and subsequently fill their seeds earlier than later maturing ones
Field tests were conducted in 2005 and 2006 with Group III through Group VI soybeans at the Dean Lee Research Station in the Red River Valley of central Louisiana and the Macon Ridge Research Station in northeast Louisiana to determine yield and agronomic performance. Sixteen Group III varieties in 2005 and 19 Group III varieties in 2006 were planted at relatively early (March) and late (April/May) dates. In addition, the Group III variety Dyna-Gro DG3392 was planted weekly from March 31 through May 12, 2006, at Dean Lee to gain a better idea of when Group III varieties should be planted for best results.
The yield of Group III varieties was significantly affected by planting date. When planted in late March, Group III soybeans yielded an average of 28 bushels per acre, which was lower than other maturity groups (Table 1). When planted after mid-April, Group III soybeans yielded an average of 43 bushels per acre, which was less than the Group IV soybeans, which averaged 49 bushels per acre, but was about the same as Group V varieties, which averaged 42 bushels per acre. With yields similar to Group V, Group III soybeans would likely be more profitable because of lower costs for insect and disease control because of a shorter growing season.
Yield of late-planted Group III soybean varieties averaged about 10 bushels more per acre than Group VI soybeans, which averaged 33 bushels per acre. The difference would have been even greater if the Group VI test at Macon Ridge was not discarded in 2006 because of extreme disease that caused very low yield and seed quality. The relatively large difference in yield between late-planted Group III and Group VI soybeans in 2006 at the Dean Lee Research Station was attributed to late-season Cercospora leaf blight infection and underscores the advantage of producing early-season soybeans.
The Group III varieties averaged 6 inches taller at the later planting dates than at the early planting dates. Plant height of Group III varieties, especially those that were planted early, was shorter than Group IV, Group V and Group VI varieties. Late-planted Group III varieties, however, averaged only 2 inches to 3 inches shorter than the later maturity groups. Plant height in Group III varieties significantly correlated with yield in six of the eight field tests, indicating that most of the yield increase attributed to later planting was the result of increased plant size. Lodging (the propensity of plants to fall over) was fairly moderate in Group III varieties, probably due to their growth habit and relative shortness.
Group III and Group IV soybean varieties usually have indeterminate stem termination, and later-maturing varieties have determinate stem termination. Indeterminate soybeans continue to grow and put on new leaves most of the summer. Determinate soybeans, on the other hand, flower in a short period and do not put on a lot of new growth after pods begin to fill. Early-season indeterminate varieties in the Midsouth often retain green leaves, so harvest aids are frequently used to help dry the crop. Group III soybeans frequently display this behavior, and only a few varieties dried in a normal manner at Dean Lee in 2005 (Figure 1). At Macon Ridge, green leaf retention was noted in the March plantings but not in the April plantings.
The difference in maturity date between early-planted and late-planted Group III varieties was 11 days in both years at Macon Ridge, even though planting dates differed by 20 days to 30 days. It is likely that the colder temperatures associated with March plantings slowed early plant growth and development. Because the time from planting to harvest maturity was reduced for the later plantings, producers should be aware that the earlier harvest they seek by planting at the earlier date may not be realized entirely.
One concern with Group III varieties is the seed quality when maturation occurs during high temperatures in July and August. Seed quality of early-planted Group III soybeans was lower than other maturity groups (Table 2); however, late-planted Group III soybeans had the highest seed quality of all maturity groups in both years. Seed quality was very low for Group V and Group VI varieties in 2006. The results showed that seed quality was lower when Group III soybeans were planted in March, but when planting was delayed until after mid-April, the seed quality of Group III soybeans was not lower than other maturity groups.
In the two-location study with two planting dates about a month apart, yield and seed quality were higher in late-planted Group III soybeans than in early-planted Group III soybeans (Table 1). In the planting date experiment at Dean Lee with a single Group III variety and seven weekly plantings from March to June, yields were lowest for the March 31 planting, increased with each planting until May 5, and then decreased for later plantings. The optimum planting date was determined to be April 16. Although these results are only for one variety produced in one environment, these data support the results of the other studies that show the optimum planting date for Group III soybean varieties is mid to late April.
Group III soybeans have potential for profitable production in Louisiana. Yields averaged as high as 47 bushels per acre when Group III soybeans were planted after mid-April. March-planted Group III soybeans produced lower yield than those planted after mid-April. The yield of Group III varieties was highly correlated with plant height. It is likely that plant height and yield of Group III soybeans would increase in closer row spacing. Seed quality was lower for early-planted Group III soybeans; however, the seed quality of Group III soybeans planted after mid-April was as good as or better than later-maturing soybeans. Crop desiccants may be needed to help dry down Group III soybean varieties for harvest. Group III soybeans can produce good yields and may be one solution for escaping late-season disease and insect pressure; however, adapted Group IV varieties have yielded 5 to 10 bushels per acre higher when planted on the same dates as Group III soybeans.
Early season soybean production research programs in the LSU AgCenter were partially funded by the Louisiana Soybean and Grain Research and Promotion Board.
(This article was published in the winter 2008 issue of Louisiana Agriculture.)