Ernest L. Clawson and Donald J. Boquet
In recent years, reports of high yield potential and the advantages of an early harvest have created interest in early planting of soybeans in Louisiana. Little research information is available on the responses of Maturity Group (MG) V soybeans to early planting dates. In addition, the acreage devoted to early-maturing soybeans such as MG IV varieties has increased in recent years. The currently recommended planting dates for MG IV soybeans in Louisiana range from April 15 to May 15, but additional research is needed to confirm this recommendation. Although new to this state, MG III soybeans have been planted by some producers because of their potential for even earlier maturity.
The growth and yield responses of soybeans to planting date depend upon the environment, variety and production practices. If planted too early, soybeans may have poor emergence or limited growth because of cool temperatures. The short days of early spring also may affect the success of earlyplanted soybeans. When soybeans are exposed to days shorter than a critical length, they progress rapidly toward maturity. If this occurs before the plant reaches an adequate size, the soybeans are stunted and low-yielding. The day length that causes this response differs by variety. If planted at a time when low temperatures and short days may cause poor growth, narrow row spacings are often beneficial. They allow the crop to use available space even if individual plants are small. This helps to control weeds and may also increase yield. Testing Planting Date Responses
LSU AgCenter researchers conducted studies on planting dates for soybeans in 2005 and 2006 at the Northeast Research Station at St. Joseph, La
. In 2005, separate, nonirrigated tests were conducted for narrow rows (20-inch average row spacing, planted flat) and wide rows (40-inch row spacing on raised beds). The studies included six MG IV and two MG III varieties. The earliest planting date was April 15 for narrow rows and March 31 for wide rows. Planting dates were established approximately 10 days apart, depending on field conditions.
In 2006, the two row spacings were established on wide seedbeds (80-inch centers) in a single irrigated study. One MG V, four MG IV and three MG III varieties were included. Planting dates ranged from March 15 to June 2. Yield and the timing of maturity were measured for each plot in all studies. Preliminary Results
In the narrow-row study in 2005, yields were generally high when planted April 15, April 29 and May 16. All varieties yielded poorly when planted on June 3. Some MG IV varieties produced their highest yields when planted on April 15, while others produced the greatest yield when planted on April 29. The highest yield for each of the MG III varieties was obtained when planted on April 29. MG IV varieties were higher yielding than MG III varieties in most cases. This was especially true when planting was on April 15.
In the conventional-row study in 2005, all varieties, regardless of maturity group, produced the greatest yield when planted on April 21. All varieties produced low yields when planted in June. Maturity Group III varieties tended to yield less than MG IV varieties, especially at the earlier planting dates. Low yields of MG III varieties at the earliest planting dates may have been caused to some degree by delayed harvest, which led to preharvest seed losses through shattering — pods opening and spilling their seeds before harvest.
In 2006, narrow rows generally yielded more than wide rows for all maturity groups in March and early April plantings (Figures 1, 2 and 3). For MG III and MG IV varieties, the earliest planting date for near-optimal yield was April 7 in narrow rows and April 18 in wide rows. The MG V variety yielded highest when planted between March 27 and April 18. This may indicate a major change in MG V adaptation because, in earlier research in the 1980s and 1990s, optimal planting dates for MG V varieties were in May. MG III varieties were generally lower-yielding than those of other maturity groups at all planting dates.
To obtain one week of earlier maturity, MG III and IV varieties had to be planted approximately 12 days earlier in 2005 and approximately one week earlier in 2006. For the MG V variety in 2006, one week’s earlier maturity required 15 days’ earlier planting. At a given planting date, MG III varieties were earlier-maturing than MG IV varieties. In 2006, each was earlier-maturing than the MG V variety, which did not mature before September 8 at any planting date. Implications for Soybean Producers
The early results of this study have a number of implications for Louisiana soybean producers. First, MG III varieties did not yield as well as MG IV varieties and probably should not be planted unless early maturity is a production objective. Producers may experience yield reductions from planting MG III or IV soybeans before mid-April in wide row spacings. Although more years of data are needed, the results from 2006 suggest that these maturity groups may be planted for high yield in early April if narrow row spacings are used. The differing responses between years suggest that irrigation extends or widens the optimal planting window for MG III and IV soybeans in years where rainfall is limited. Because the MG V data represent only one variety in a single year, firm conclusions cannot yet be made about planting dates for MG V soybeans. The study will be continued at the Northeast Research Station in 2007.
Ernest L. Clawson, Assistant Professor, Northeast Research Station, Saint Joseph, La., and Donald J. Boquet, Professor, Macon Ridge Research Station, Winnsboro, La.(This article was published in the winter 2007 issue of Louisiana Agriculture.)