Soybeans are planted from late March through June in the midsouthern US, a span of over three months. Choice of variety based on maturity group (MG) within this planting window is an important production and marketing decision.
Long-term (1976-2003) research at Stoneville, MS was summarized to provide guidance for making variety selections based on MG and planting date. Assessment of long-term yield trends and days-to-maturity (DTM) from this summarization lead to several conclusions.
Plantings made before May 1. In plantings that were irrigated, MG IV varieties produced superior yields (61 bushels/acre) and resulted in greater irrigation efficiency than did MG V varieties (55 bushels/acre). In plantings that were not irrigated, yields from MG IV and MG V varieties were similar (38 and 36 bushels/acre, respectively); however, MG V varieties reached maturity 16 to 20 days later than MG IV varieties. Thus, they were in the field longer with no perceived benefit from the longer growing season. Therefore, MG IV varieties should be selected for plantings made before May 1.
Irrigated May 1-15 plantings. Using MG IV varieties resulted in greater yield (54 bushels/acre) and greater irrigation efficiency than did using varieties from MGs V, VI, and VII (49, 49, and 50 bushels/acre, respectively). Thus, MG IV varieties should be selected for early May plantings that are to be irrigated.
Nonirrigated May 1-15 plantings. Using varieties from MGs IV through VII resulted in similar yields (25 to 28 bushels/acre). However, the longer DTM of the later-maturing varieties indicates that early-maturing MG IV varieties should be planted in this period.
Irrigated May 16-31 plantings. Using varieties from MGs IV through VII resulted in similar yields (44 to 47 bushels/acre), but irrigation efficiency was greater with MG IV varieties. This, plus the longer DTM of later-maturing varieties, indicates that early-maturing MG IV varieties should be planted in this period.
Nonirrigated May 16-31 plantings. MG IV varieties (22 bushels/acre) yielded significantly less than varieties from MGs V, VI, and VII (27 to 29 bushels/acre). Since MG V varieties were in the field for a shorter period than MG VI and VII varieties, their use resulted in the best combination of the highest yield and shortest DTM in nonirrigated late-May plantings.
Irrigated plantings after May 31. Varieties from all MGs produced relatively low yields (36, 38, 45, and 43 bushels/acre for MGs IV through VII, respectively) even with irrigation. MG VI varieties provided the best combination of yield, irrigation efficiency, and DTM.
Nonirrigated plantings after May 31. Varieties from all MGs produced relatively low yields (17, 20, 27, and 25 bushels/acre for MGs IV through VII, respectively). MG VI varieties provided the best combination of yield and DTM.
Overall trends. Yields from and irrigation efficiencies of irrigated April and May plantings of MG IV varieties are high relative to all other MGs. This supports the premise that early planting of early-maturing varieties should be used for soybean production to achieve maximum yields and production efficiency in the midsouthern USA. These results promote expanding this concept to include May plantings of MG IV varieties that are to be irrigated.
Average yields of varieties from all MGs planted after May 1 and not irrigated were below 30 bushels/acre. Thus, planting soybeans after May 1 and not irrigating offers little chance for profit.
Planting varieties that are later than necessary for maximum yield increases DTM and the concurrent risk of detrimental late-season effects from insects, pathogens, and drought regardless of planting date. These increased risks may not be reflected in yield, but certainly will be reflected in the increased costs associated with their prevention and/or control.
The above assessments were made from averaging long-term data. Thus, there will be years when they may not hold true. However, they provide an accurate guide for producers who are seeking consistency in soybean production.