Producers in the lower midsouth (Memphis to Vicksburg) are striving for earlier planting (late March/early April). The question is what maturity group (MG) should be planted in these ultra-early plantings. Years of research results in the lower midsouth have shown that indeterminate MG IV varieties work best in early soybean production system plantings, or those usually made in April and early May. Will planting earlier than this dictate using varieties from MGs that are earlier than MG IVs?
There are no definitive answers to this question. However, certain facts are known. First, varieties from MGs 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 MG III rather than MG IV varieties should be selected for March and early April plantings at locations within this region.
Research indicates that MG III varieties may be planted to achieve certain goals. First, MG III varieties planted in late March/early April will be ready for harvest in early August, whereas MG IV varieties (particularly those that are irrigated) planted during this time (before April 10) will be ready for harvest in late August. Second, MG IIIs 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 their maximum irrigated yields of 55 to 60 bushels/acre. This makes them an attractive alternative to MG IVs in areas with limited irrigation water.
There are downsides to growing MG III rather than MG IV varieties in the aforementioned timeframe and region. First and foremost, MG III varieties will not yield as well as MG IV varieties, presumably because of their shorter time from planting to maturity (124 to 128 days vs. 138 to 142 days). Preliminary research results from 2003 and 2004 at Stoneville, MS indicate they will yield 5 to 10 bushels/acre less in nonirrigated plantings and 10 to 15 bushels/acre less in irrigated plantings. Second, MG III varieties will not be as tall as MG IV 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 MG III variety that was irrigated yields 55 bushels/acre and is sold for $7/bushel (assumes $1/bushel premium for early delivery), it will gross $385/acre. If a MG IV variety that was irrigated yields 65 bushels/acre and is sold for $6/bushel (no early-delivery premium), it will gross $390/acre. Obviously, a larger difference in yields between MG III and MG IV varieties and/or a smaller early-delivery premium than used for the above calculations favors MG IV varieties in ultra-early plantings.
The above discussion pertains to plantings made from late March to about April 10, or what may be classified as ultra-early plantings in the lower midsouth. There is another aspect of the MG III/MG IV discussion that is worthy of attention.
Results from preliminary research indicate that MG III varieties may be a good fit for plantings made in the first half of May in the lower midsouth. They were almost as tall, were in the field 10 to 12 fewer days, and required 2 to 3 inches less irrigation water than MG IVs planted at the same time. Most importantly, MG III varieties yielded as well as MG IV varieties in both nonirrigated and irrigated plantings.. MG IIIs 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 MG IV varieties, which may result in slightly lower input costs.