When using the Early Soybean Production System, there is always the question of what maturity group (MG) from which to select varieties. The following guide, based on long-term Stoneville, Miss., data, can be used as a tool for making this decision.
There are nine maturity groups (0 through 8) of soybeans grown in the continental United States from northern Minnesota to central Florida. One degree of latitude equals about 69 statute or land miles. The latitude of central Florida is about 28 degrees N, and the latitude of northern Minnesota is about 48 degrees N. Thus, the south-to-north soybean production area in the United States covers about 20 degrees of latitude, or about 2.2 degrees latitude (approximately 150 land miles) per maturity group.
Summer weather patterns at most Mid-South locations are similar. July and August are the hottest and driest months. Many years of research at Stoneville, Miss., have determined that MG IV varieties have the best combination of high irrigated yield (70+ bu/acre) and least time from planting to maturity (135 to 140 days) when planted in the Early Soybean Production System (late March through late April) to avoid summer drought and heat stresses at that location.
Using this as a guide, the accompanying table gives proposed maturity groups from which to select varieties for planting at Mid-South locations both south and north of Stoneville. Latitude of other locations can be found at http://geonames.usgs.gov/pls/gnispublic.
This presentation assumes that the estimated last spring frost date for each location is sufficiently early to allow planting as early as late March. This generally is a safe assumption as far north as Memphis, Tenn. For locations north of Memphis to Sikeston, Mo., earliest planting should occur in early April when using the average date of last spring frost as a guide.
The interpolation is intended only as a guide for choosing the appropriate maturity group for early plantings at a given latitude. It can be used as one of several tools in making variety selections. Compilation of research results regarding days to maturity and yield from varieties of the different maturity groups at locations throughout the Mid-South will give a more definitive guideline. Producers should seek this information from variety trials conducted in their areas.
Larry G. Heatherly is a retired USDA-ARS research agronomist and current crop consultant. e-mail firstname.lastname@example.org