Most soybean acres in the lower Mississippi River Valley or Delta are not irrigated. Nonirrigated soybean production is a high-risk enterprise in this region because of summer heat and drought.

According to Mississippi and Arkansas enterprise budgets, specified costs for growing glyphosate-tolerant, nonirrigated soybeans in the Delta are $150 to $160 per acre. With an estimated rent cost of $50 to $60 per acre, it can easily cost more than $200 per acre to produce a crop that is subject to yield reductions caused by drought in nonirrigated systems.

At a soybean price of $6 to $6.50 per bushel, yields will have to exceed 30 bushels per acre to cover costs in most years.

Producers who do not irrigate can do little to alleviate the effects of drought, but they can use available information to make production decisions that lead to the greatest probability of success.

One decision that can make a big difference in soybean yields is variety selection. Within that process lies the decision of what maturity group a variety should be chosen from to provide the greatest chance for success from nonirrigated soybean production.

Data from nonirrigated soybean variety trials conducted in Missouri, Tennessee, Arkansas, Mississippi, and Louisiana were summarized to assess the effect of maturity group on soybean 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 (no experimentals) were averaged to obtain the yields shown in the table.

In all cases, the trend was for yields of MG 3 varieties to be at or below yields of MG 4 varieties. Planting dates ranged from mid-April to late May; therefore, it is not known if this trend would have been different with earlier planting.

Twelve comparisons of yields of early versus late MG 4 varieties are available. In six of the cases, the trend was for late MG 4 varieties to produce a greater average yield, while average yield of early MG 4 varieties was greater in only one case. Yields of varieties from the two subgroups were essentially equal in five of the comparisons.

Average yields of MG 4 and MG 5 varieties can be compared in nine cases. In four of the comparisons, average yields from varieties in the two groups were essentially equal. Yields from MG 4 varieties tended to be greater in two cases, and yields from MG 5 varieties tended to be greater in three cases.

Nine comparisons of early MG 5s to late MG 5s can be made. In five of the nine cases, average yield of early MG 5 varieties tended to be greater than average yield of late MG 5 varieties, whereas the reverse trend occurred in two of the nine cases.

This assessment of recent variety trial results indicates that soybean producers throughout the lower Mississippi River Valley should be planting MG 4 varieties to achieve maximum yield with the least time in the field when using a nonirrigated system. If MG 5 varieties are chosen for nonirrigated production, they should be selected from the early MG 5 subgroup.

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 timeframe covered by the two years of trials cited here. Thus, these yield comparisons should be relevant for the majority of soybean producers in the region.