“The problem with research of this nature is every year is different. One year we may get a response to yield that is fantastic in Group 3s. Another year we may see the greatest yield response in Group 4s,” says Larry Purcell, professor of soybean physiology at the University of Arkansas.

“One of the ways to address that situation is to look at weather patterns and the probability of drought and when it occurs. If we can match our production system with the prevailing weather patterns on a probability basis, it would benefit the crop.”

Assuming you start out with a high-yielding soybean cultivar, there are four critical inputs for any production system. The four inputs are:

Light. “Lots of talk goes on about plant population. The overriding concern with plant population is to get a closed canopy to intercept light,” says Purcell, who spoke at the Tri-State Soybean Forum in Tallulah, La.

Time. How much time is needed to produce a crop? How long can you expect to have adequate rainfall to grow a crop?

Water. What is the water availability? Can you expect to have good rainfall during the course of production?

Nutrients. “While I’m not going to talk much about nutrients, to fill out the list properly a nutrient and soil fertility program is essential.”

The first question to address is how much light is needed to produce a soybean crop? The answer is not nearly as much as was once thought.

“When you start looking at what a soybean crop needs to produce good yields, there are some surprises. By the time we accumulate about 800 units of light, there isn’t a further increase in plant weight or yield. A typical season will provide some 2,000 light units.”

The next question is how much time is needed to accumulate the 800 units? Using data sets from Stoneville, Miss., the 83-year means shows that 800 units are available 75 days from the end of March.

How much time is available in a growing season? Much more than needed to produce a soybean crop. Purcell points to temperature data (again from Stoneville) beginning in January and running through December.

“What’s the probability of the minimum temperature being less than freezing? The numbers show that by the time the end of March rolls around, there’s less than a 5 percent chance that there will be a freezing temperature. The numbers further show that there are around 212 frost-free days in which to grow a crop.”

So, from a light standpoint, there’s more than twice the amount available during a growing season to produce maximum yields. From a time standpoint, about 85 to 90 days from the beginning of the season are needed to canopy and fully intercept the needed 800 units of light.

How much water is needed to produce a soybean crop depends on several factors. Soil water storage capacity, soil depth and soil texture all must be considered. Rainfall patterns and evapotranspiration (how rapidly a crop uses water) and how efficiently a crop uses water are also important.

Purcell points to a 7-day running sum of rainfall chart. Using Stoneville data from the start of January through December’s end, Purcell explains the figures are calculated by taking rainfall from a specific date and adding in the amount from the previous 3 days and following 3 days.

“What this shows is from the beginning of January through day 150 – which is the first of June – there is about 1 inch of rain per week at Stoneville, which is typical of much of the Mid-South. From June 1, rainfall declines until mid-August when there is very little rain. Then, later in the fall intermittent showers begin.”

If you look at a 7-day running sum and evapotranspiration (the amount of water used when the crop is fully developed) it shows there is plenty of rainfall to meet the crop’s needs until June 1. From late June through August, evapotranspiration is much greater than the rainfall. The difference between what’s needed and actual rainfall is known as the crop/water deficit. For those using an irrigation program, it’s the crop/water deficit the program is calculating.

“Usually, if you use a crop irrigation scheduler and you’re on a heavy silt clay, you probably water when the water deficit is at 50 millimeters (2 inches). The probability of a 1-inch water deficit is 40 percent during the first week of April. For a 2-inch deficit, there’s a 20 percent probability at the start of June. This probability increases until we reach about 40 percent around July 4.

“So there’s a window of opportunity between when we can plant the crop and when there’s a high probability of having to irrigate.”

Something that also affects crops is the efficiency of water use – or how much plant weight can be produced per unit of water available.

“An 83-year means from Stoneville shows that you can produce a lot of soybean plant material during April when efficiency is at 4 pounds gained per 1,000 pounds of water. That ability – which allows a 60-bushel per acre yield for about 8 inches of water -- declines rapidly, though. By early June, the crop is only gaining 2.5 pounds of material per 1,000 pounds of water (to produce a 60-bushel crop at such levels requires 13 inches of water during the season).”

Shifting attention to non-irrigated yields at the Extension experiment station in Kaiser, Ark., Purcell says some interesting things have been seen.

“We’ve looked at several maturity groups from 00’s through 4’s for the past 4 years. In 2001, we found Group 0’s and 00’s didn’t yield well -- something around 25 bushels per acre. This is primarily due to light. These groups never reach the 800 units of light needed for high yields.”

Meanwhile, the Group 1’s, 2’s, 3’s and 4’s all yielded around 40 to 45 bushels per acre. These groups were able to avoid drought and did well.

“In 1999, the year was hotter and drier. We weren’t able to plant as early as we wanted. We got in around May 10. The Group 3’s and 4’s suffered greatly compared to 1’s and 2’s. Group 1’s and 2’s topped the test at between 30 and 37 bushels per acre.”

Bottom line: can soybean production be matched to rainfall to avoid drought?

“I believe we can come very close to that most years. To canopy and intercept the needed 800 units properly requires 85 to 90 days after planting. The probability of drought in most of the Mid-South is low during the first 70 to 75 days of the growing season. That means the numbers don’t quite match up, but it’s close.”

What are the earliest maturing soybean cultivars that can be planted in the Mid-South?

“From what I’ve seen, I think the answer may shift from the north Delta south. With high populations, narrow rows and irrigation, we can expect to have 60 to 65 bushel yields with Group 1 soybeans. When we go earlier than that, the plants seem to experience height problems and don’t have the time to gather enough light.”

e-mail: dbennett@primediabusiness.com