With the recent drought affecting most of Louisiana, there are some points about non-optimal soybean planting dates that need to be addressed. Here in Louisiana, the optimal planting period for soybean production (MG 5s and 6s) is around May 10 and goes until June 15. After June 15, the planting period is termed “late or non-optimal.” For MG 4s, the optimal period to start planting is around April 15 and ends about the first week of June.

According to Jim Board, a research agronomist with the LSU AgCenter, “this conventional planting period is largely governed by the day-length pattern during the spring and summer. Soybean is a short-day plant, meaning lengths of developmental periods are shortened as day length declines. Based on early studies, it was demonstrated that soybeans planted outside the May 1 to June 15 window did not have enough days to R5 to achieve enough dry matter to attain optimal yield. Other things can affect yield, but this is the main factor.

Introduction of the early planting production system by Dr. Larry Heatherly (retired agronomist with the USDA/ARS Soybean Production Research Unit at Stoneville, Miss.) was based on early planting of MG 3 or 4 varieties (varieties that were earlier than those traditionally grown) to avoid mid-summer drought (mid July through the end of August) that afflicted much of the Mid-South. The purpose of this early system is to advance planting date and maturity group so that the most drought-prone developmental period for soybean (R3-R5) occurs before the mid-summer drought begins.

Although this system has shown advantages for areas that typically have mid-summer drought, it would not necessarily be an advantage for farmers who use irrigation and or live in areas not routinely plagued by drought, such as central and south Louisiana”.

To date, we have not received any appreciable rainfall, but some is forecast. It might be early June before we can get the more than 200,000 intended acres planted. The USDA has predicted Louisiana soybean production to be around 850,000 acres this year and, through an informal survey and visual observations, I estimate that we have a little over 650,000 acres planted. Most of the 200,000 acres yet to be planted are in southwest and central Louisiana. The wheat-soybean double-crop acres are included in that number. If we could plant today, this number might increase.

So what do we need to think about if we plant in June? First, do not panic and do not change much about production practices. We can still make a decent crop when we plant in early to mid-June. It is when planting gets into late June and early July that day length will really start to shorten and reduce the number of days to R5, which reduces yields. The first thing that must be discussed at these dates is maturity groups.

I am not trying to persuade producers as to which groups to plant, but to provide some data for you make the best decision for your individual production practices. In Louisiana, MG 5 and 6 beans still traditionally out-yield earlier maturity groups, but the gap is closing very rapidly under early or optimal planting dates. Please take a second or two to process that statement. Even though MG 4s yield exceptionally well in many “regions” of the state - at non-optimal dates, a later MG will potentially outyield an earlier MG, according to research conducted by Steve Moore, Jim Board, Don Boquet (all LSU AgCenter research agronomists) and me when I was a graduate student a decade ago.

Why is this? Once again, it comes down to biomass production and length of the vegetative growth stage. Increased biomass generally means increased yields later in the season. Specifically, soybean needs to achieve a biomass of about 600 grams per square meter by R5 to be in the range for optimal yield.

What other cultural practices increase your success with mid-June-planted beans? First, narrow the row spacing and increase plant populations. Narrow row spacing is critical at non-optimal dates for rapid canopy closure and yield maximization. Increasing the plant population by about 10 percent to 15 percent generally helps also. Some other points to note: these beans will not be ready to harvest in August, and disease and insect pressure can be higher than optimal-planted crops.

Now, let’s discuss some data. I have taken the CVT (commercial variety trial) data from the last five years (2000-2004) and averaged it across maturity groups and trial locations. When averaged across all locations for the past five years, yields were 39.6, 43.3 and 48.8 bushels per acre for MG 4s, 5s and 6s, respectively. There is a positive trend in yield as you go to MG 4 to 5 and 5s to 6s. In actuality, over the past five years the MG 5s have outyielded the MG 6 by 3.7 bushels per acre and the MG 6s have outyielded the MG 5s by 5.5 bushels per acre. When you compare the MG 5s to the MG 6s, there is a 9.2 bushels per acre difference in yield.

Some of the criticism of this data set is that it is CVT data generated with small plots. That is true, but the data is still relative and valid. All planting dates for this data set were in the recommended date of planting window mentioned earlier. This trend was also verified in one year of data last year through the crop demonstration program. In the 2005 demos, yields from 97 varieties were 34 bushels, 43 bushels and 48 bushels per acre for MG 4s, 5s and 6s, respectively. These demos were planted on over 160 acres over a wide range of cultural practices and planting dates. We are doing our best to make sure we can get our 2005 MG 6 demos in, but we are having difficulty in getting the recommended varieties we need.

In summary, there is variability among maturity groups in Louisiana when they are planted at optimal planting dates. More research is needed, and we will pursue that over the next two years. When soybeans are planted after mid-June, some factors regarding MG and cultural practices need to be considered. However, with all environmental conditions being equal, there is an advantage in planting a later MG as opposed to an early one at non-optimal dates. From this data set, it does appear as if there is still a slight yield advantage with the MG 5s and 6s when compared to MG 6s when planted at optimal dates. The unfortunate news is that MG 6 availability is becoming less each year because of a number of factors; demand is number one. In Louisiana, about 300,000 acres do exceptionally well with the later varieties, and this number of acres may not be large enough for the all seed companies to keep pursuing varieties for this market.

Bottom line – I like MG 4s in the northeast, south central, sugarcane area and portions of northwest Louisiana. They are here to stay, and yields are increasing every year. However, for central and southwest Louisiana, the MG 4 success stories are rare. This is why MG 5s and 6s should be planted if we can plant within the next week or two to maximize yields.

Dr. David Lanclos is as Extension agronomist with the LSU AgCenter, Dean Lee Research Station, Alexandria, La.

e-mail: dlanclos@agcenter.lsu.edu