Producers in the lower Mid-South (Memphis, Tenn., to Vicksburg, Miss.) are striving for earlier planting (late March/early April). Which maturity group should be planted in these ultra-early plantings?

Years of research results in the lower Mid-South have shown that indeterminate maturity group 4 varieties work best in early soybean production system plantings, or those usually made in April and early May.

Will planting earlier than that dictate using varieties from maturity groups that are earlier than maturity group 4s? There are no definitive answers to this question. However, certain facts are known.

Varieties from maturity groups I and II have too short a growing season to produce optimum yields. They will mature in less than 120 days, which does not allow enough time to fully express yield potential or to overcome the effects of short-term stresses. They will be short-statured (especially on clayey soils) and likely will not form a complete canopy even in narrow rows.

Thus, the question is whether or not maturity group 3, rather than maturity group 4 varieties, should be selected for March and early April plantings at locations within this region.

Research indicates that maturity group 3 varieties may be planted to achieve certain goals.

First, maturity group 3 varieties planted in late March/early April will be ready for harvest in early August, whereas maturity group 4 varieties (particularly those that are irrigated) planted during this time (before April 10) will be ready for harvest in late August.

Second, maturity group 3s will have a shorter growing season and thus less exposure to environmental stresses during the hottest, driest part of the growing season.

Third, they normally will require one less irrigation to achieve maximum irrigated yields of 55 to 60 bushels per acre. That makes them an attractive alternative to maturity group 4s in areas with limited irrigation water.

There are downsides to growing maturity group 3 rather than maturity group 4 varieties in the aforementioned timeframe and region. First and foremost, maturity group 3 varieties will not yield as well as maturity group 4 varieties, presumably because of the shorter time from planting to maturity (124 to 128 days vs. 138 to 142 days).

Preliminary research results from 2003 and 2004 at Stoneville, Miss., indicate they will yield 5 to 10 bushels per acre less in nonirrigated plantings and 10 to 15 bushels per acre less in irrigated plantings.

Second, maturity group 3 varieties will not be as tall as maturity group 4 varieties, and this may pose canopy and harvest problems on clayey soils.

Third, their shorter growing season provides less time for them to recover from short-term stresses that may occur.

What is the proper choice?

If a maturity group 3 variety that was irrigated yields 55 bushels per acre and is sold for $7 per bushel (assumes $1 per bushel premium for early delivery), it will gross $385 per acre. If a maturity group 4 variety that was irrigated yields 65 bushels per acre and is sold for $6 per bushel (no early-delivery premium), it will gross $390 per acre.

Obviously, a larger difference in yields between maturity group 3 and maturity group 4 varieties and/or a smaller early-delivery premium than used for the above calculations favors maturity group 4 varieties in ultra-early plantings.

This article pertains to plantings made from late March to about April 10, or what may be classified as ultra-early plantings in the lower Mid-South. There is another aspect of the maturity group 3/maturity group 4 discussion that is worthy of attention.

Results from preliminary research indicate that maturity group 3 varieties may be a good fit for plantings made in the first half of May in the lower Mid-South. They were almost as tall, were in the field 10 to 12 fewer days, and required 2 to 3 inches less irrigation water than maturity group 4s planted at the same time. Most importantly, maturity group 3 varieties yielded as well as maturity group 4 varieties in both nonirrigated and irrigated plantings.

Maturity group 3s planted during this time were harvested in early September, thus missing August delivery. Therefore, their advantage in these plantings appears to be a shorter growing season to produce the same yield as maturity group 4 varieties, which may result in slightly lower input costs.

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