Soybeans are planted from late March through June in the Mid-South, a span of over three months. Choice of variety based on maturity group within this planting window is an important production and marketing decision.

Long-term (1976-2003) research at Stoneville, Miss., was summarized to provide guidance for making variety selections based on maturity group and planting date. Assessment of long-term yield trends and days-to-maturity from this summarization lead to several conclusions.

Plantings made before May 1. In plantings that were irrigated, Maturity Group 4 varieties produced superior yields (61 bushels per acre) and resulted in greater irrigation efficiency than did Maturity Group 5 varieties (55 bushels per acre). In plantings that were not irrigated, yields from Maturity Group 4 and Maturity Group 5 varieties were similar (38 and 36 bushels per acre, respectively); however, Maturity Group 5 varieties reached maturity 16 to 20 days later than Maturity Group 4 varieties. Thus, they were in the field longer with no perceived benefit from the longer growing season. Therefore, Maturity Group 4 varieties should be selected for plantings made before May 1.

Irrigated May 1-15 plantings. Using Maturity Group 4 varieties resulted in greater yield (54 bushels per acre) and greater irrigation efficiency than did using varieties from Maturity Groups 5, 6, and 7 (49, 49, and 50 bushels per acre, respectively). Thus, Maturity Group 4 varieties should be selected for early May plantings that are to be irrigated.

Nonirrigated May 1-15 plantings. Using varieties from Maturity Groups 4 through 7 resulted in similar yields (25 to 28 bushels per acre). However, the longer days-to-maturity of the later-maturing varieties indicates that early-maturing Maturity Group 4 varieties should be planted in this period.

Irrigated May 16-31 plantings. Using varieties from Maturity Groups 4 through 7 resulted in similar yields (44 to 47 bushels per acre), but irrigation efficiency was greater with Maturity Group 4 varieties. This, plus the longer days-to-maturity of later-maturing varieties, indicates that early-maturing Maturity Group 4 varieties should be planted in this period.

Nonirrigated May 16-31 plantings. Maturity Group 4 varieties (22 bushels per acre) yielded significantly less than varieties from Maturity Groups 5, 6, and 7 (27 to 29 bushels per acre). Since Maturity Group 5 varieties were in the field for a shorter period than Maturity Group 6 and 7 varieties, their use resulted in the best combination of the highest yield and shortest days-to-maturity in nonirrigated late-May plantings.

Irrigated plantings after May 31. Varieties from all maturity groups produced relatively low yields (36, 38, 45, and 43 bushels per acre for Maturity Groups 4 through 7, respectively) even with irrigation. Maturity Group 6 varieties provided the best combination of yield, irrigation efficiency, and days-to-maturity.

Nonirrigated plantings after May 31. Varieties from all maturity groups produced relatively low yields (17, 20, 27, and 25 bushels per acre for Maturity Groups 4 through 7, respectively). Maturity Group 6 varieties provided the best combination of yield and days-to-maturity.

Overall trends. Yields from and irrigation efficiencies of irrigated April and May plantings of Maturity Group 4 varieties are high relative to all other maturity groups. 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 Mid-South. These results promote expanding this concept to include May plantings of Maturity Group 4 varieties that are to be irrigated.

Average yields of varieties from all maturity groups planted after May 1 and not irrigated were below 30 bushels per 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 days-to-maturity 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.

Larry G. Heatherly is a retired USDA-ARS research agronomist and current crop consultant. e-mail larryheatherly@bellsouth.net