The Mid-South states conduct soybean variety trials to provide data to assist producers in making the correct variety selection. Choosing the correct variety for a given set of conditions is probably the most important production decision that a grower will make.

In recent years, varietal choices have increased. One reason for this is a wider range of maturity groups from which to choose. Producers in the Mid-South are now planting varieties from maturity group (MG) 3 through maturity group 6.

Over the range of planting dates used in the Mid-South, this means that days from planting to maturity can range from about 110 to about 165 days. Obviously, this means that the difference in days in the field between early- and late-maturing varieties can be quite large.

The objective when selecting a soybean variety is to choose one with a combination of high yield, resistance to or tolerance of pests, and the shortest possible time from planting to maturity. The latter component is often not considered, but is important since risk is reduced when the crop is in the field for the shortest possible time.

Data from irrigated soybean variety trials conducted in Missouri, Tennessee, Arkansas, and Mississippi were summarized to assess the effect of maturity group on yield. Yields from studies conducted in 2005 and 2006 at the locations shown in the accompanying table were used.

Planting dates were generally from mid-April through mid-May.

For the purpose of comparison, maturity is divided into five levels. The maturity groupings are MG 3, early MG 4 (4.0 to 4.6), late MG 4 (4.7 to 4.9), early MG 5 (5.0 to 5.5), and late MG 5 (5.5 to 5.9). Within each maturity class and test location of each year, yields of the five top-yielding varieties were averaged to obtain the yields shown in the table.

MG 3 varieties did not yield as well as MG 4 varieties. In seven of the eight possible comparisons, MG 4 varieties had greater yields. Planting dates ranged from mid-April to late May; therefore, it is not known if this trend would have occurred with earlier planting.

In 18 of the 21 location/year combinations where a comparison was possible, the trend was for MG 4 varieties to yield as much as or more than MG 5 varieties. This trend occurred within a range of planting dates from mid-April to late May.

The three occasions where this trend did not hold occurred in 2006 at Stuttgart and Rohwer, Ark., and at Clarksdale, Miss., which are only about 50 miles apart in a north-to-south direction.

Within the MG 4 category, varieties in the 4.0 to 4.6 maturity range yielded as much as or more than later MG 4 varieties in 16 of the 20 possible comparisons. The four locations where this trend was reversed occurred at the more northern locations of Keiser, Ark., and Milan, Tenn.

There were 15 possible comparisons of early to late MG 5 varieties. In 14 of those cases, the earlier MG 5 varieties had yields that were equal to or greater than those from late MG 5 varieties.

This assessment of recent variety trial results using available varieties (no experimentals) and the shown planting dates indicates that soybean producers throughout the lower Mississippi River Valley should be planting MG 4 varieties to achieve maximum yield when irrigation is used during the growing season.

Even though MG 5 varieties had yields that were equal to those from MG 4 varieties in many cases, the shorter growing season of the MG 4 varieties is preferred from a management standpoint.

The cited variety trials contain no plantings earlier than mid-April. It is possible that ultra-early planting dates could change the yield relationships among the maturity classes. However, most of the Mid-South's soybeans are planted in the time-frame covered by the two years of trials cited here. Thus, the yield comparisons made here should be relevant for the majority of soybean producers in the region.

Data from only irrigated trials were used to remove drought stress as a factor in this assessment. My next article will address this topic for nonirrigated soybean production.