History of Soybean Management

Soybean growth and development is influenced by environmental factors including photoperiod and temperature. The transition from the vegetative stages to the reproductive stages is predominantly affected by photoperiod. Photoperiod is also known as day length or the period of daylight in every twenty-four hours. The reaction in a soybean plant that triggers flowering takes place in the dark so soybean flowering actually relies on the length of darkness during a twenty-four hour period. Photoperiod lengths vary throughout the year. The summer solstice, usually around June 21, marks the longest day (shortest night) of the year in the northern hemisphere. During the growing season day lengths increase (nights decrease) up to this date. Day lengths then decrease (nights increase) after this date. Flowering of soybeans is triggered when nights reach a certain length of time generally occurring when nights start becoming longer. If the dark period is too short or is interrupted by light, the plant will not flower. While many people consider soybeans to be a short-day plant, it is more accurately thought of as a long-night plant.

In between the first day of spring, around March 20, and the first day of fall, around September 23, day lengths increase (nights decrease) as you head north in the northern hemisphere. Throughout the summer, day lengths will be up to at least two hours longer in Canada compared to the southern U.S. Due to this, the lengths of night vary just as much which becomes an issue when triggering soybeans to flower. A soybean adapted to the southern U.S. (shorter days, longer nights) would flower very late in the growing season if grown in Canada (longer days, shorter nights) as nights become long enough to reach the certain length to trigger flowering. A soybean adapted to Canada (longer days, shorter nights) would flower much earlier in the southern U.S. (shorter days, longer nights) as the nights are much longer and would reach the certain length of night much sooner to trigger flowering. You want soybeans to be as productive as possible by producing lots of leaves and nodes before flowering, but also have enough time to undergo all reproductive stages. To ensure the flowering of soybeans to be initiated at a time that is not too early but not too late there are different maturity groups of soybeans for different parts of the country. Each group requires certain amount of day length (night length) that will trigger flowering. Soybean varieties are divided into different maturity groups. These groups are adapted to relatively narrow bands of latitude. The maturity group 000 is best suited for southern Canada while the maturity group 9 is best suited for the southern tip of Florida (Figure 4). Within each group it is common practice to add a decimal to denote variation within a maturity group, a common maturity group in Nebraska is 2.8. As you can see from figure 5 soybean maturities best suited for Nebraska range between the low 2 maturity group to the low 3 maturity group.

Soybeans are a legume so they typically will not require nitrogen fertilizer. As long as the proper rhizobium bacteria are present, they will infect the roots and form nodules, where nitrogen is fixed by the bacteria. However, if the proper rhizobium is not present, no infection, and consequently, no nitrogen production will take place.

To ensure that rhizobium is present, farmers can treat soybeans with an inoculant containing at least 1000 viable bacteria per soybean seed. Innoculants come in both dry and liquid formulations. The use of innoculnats depends on the field history of soybean production. If soybeans have never been grown before, a high rate of inoculant is recommended to ensure that proper bacteria are present. When fields have been out of soybean production for 5 years or more, inoculation would be recommended. However, in a typical corn-soybean rotation, there is typically no need to inoculate the seed as sufficient bacteria will be present to ensure nodulation.

In the early 1900’s when soybeans were first grown, row spacing was determined by the breadth of the animals used to work the fields. This wide spacing had an impact on methods of weed control and the varieties of soybeans developed. Later on, row spacing was often determined by tractor tire size since cultivation for weed control was required.

However, with the broad spectrum of herbicides available and lodging-resistant soybean varieties available today, soybeans can be grown in many different row spacings, typically ranging from as narrow as 7.5 inch up to as wide as 36 inch rows.

Narrowing soybean row spacing can increase yields if light utilization is the yield-limiting factor. The main purpose of narrow rows is to intercept more light. Unlike fertilizers and water which can use the soil as a reservoir, light falling on bare soil cannot be used. Decreasing row spacing results in more rapid canopy closure and increased light interception. Canopy closure at or shortly after flowering will maximize yields in ideal conditions.

However, Nebraska's growing seasons are rarely ideal. Any yield limiting factor like lodging, weed growth, moisture stress, soil compaction, high soil pH, or nutrient deficiency will reduce soybean yield responses in narrow rows.

Other factors in determining row width are equipment considerations, weed control, variety considerations, and erosion considerations. One of the biggest factors in equipment has been the rise in seed costs and the inability of most drills to ensure seed to soil contact and to singulate seed, resulting in a shift toward wider row spacings.

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An important factor in determining soybean seeding depth is the process of germination and emergence. Soybean germination starts when the seed imbibes approximately 50% of the seed weight in water, swelling the seed and starting the process of cell division. This high water requirement makes it important to ensure that seeds are planted into moist soil with good seed to soil contact. Once imbibition has started the first part of the soybean to emerge from the seed is the radicle root, which will quickly have lateral roots and root hairs form. Next, the hypocotyl starts elongating and forms a hook, which pulls the cotyledons out of the ground. During this process of pulling the cotyledons out of the ground the hypocotyl is vulnerable to breaking from soil crusting or field operations. Seeds planted too shallow may be in soil too dry to start the imbibition process or continue to supply enough water for the growing seedling. On the other hand, seeds planted too deep may not have enough energy to pull the cotyledons from the ground.

The yield potential of any soybean production system can be enhanced by planting as early as possible. Early planting allows the soybean plant to collects more solar radiation and transpire more available water than if planted later. This will increase the number of soybean nodes per plan and thus yields.

In Nebraska if soil temperatures are warm enough to avoid chilling injury, plant your soybean fields as close to May 1 as possible to provide optimal yields. University of Nebraska–Lincoln research has shown that for every day you plant after May 1, you lose ½ bu/ac yield potential. Delaying soybean planting until May 15 could result in a 7.5 bu/ac yield decrease.

Tillage is expensive, time consuming, increases soil erosion, and in many cases is not necessary to maximize soybean yield. Yet adoption of no-tillage systems has been slow because the common thought is that soybean growth and yield are reduced in these systems. Cold soils, weed control, increased crop residue, seedbed preparation and more rapid soybean emergence have been cited as reasons to use tillage prior to soybean planting. Each of these points is true; however, each has little to no effect on soybean yield.

Most of the research conducted in the Midwest has shown that soybean in general does not respond to tillage and other management practices should be given more consideration for gaining maximum yield potential. No matter which tillage system is used, ensuring good seed to soil contact at planting is critical to getting a good soybean stand.

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The use of irrigation in Nebraska’s soybean production systems has steadily increased in recent years to the extent that irrigated soybean acreage now accounts for about 45% of state’s total soybean acreage. Technological advances in irrigation equipment can improve on-farm water application efficiencies (with respect to savings of both energy and water). However, producer adoption of these advances can be slow due to the capital expenditures required for irrigation system upgrades. Greater producer adoption of more effective crop irrigation management can also ensure that water is scheduled and applied in a “when-needed, just-in-time fashion”, and would thus also optimize the efficiency of on-farm water and energy use.

Irrigation scheduling methods fall into two general categories relative to the information used for making when and how much water should be applied. These include (1) using daily Crop ET estimates as a proxy for daily crop water use, and (2) using soil water sensors placed at various crop root zone depths to quantify daily soil water content. A decision making tool, SoyWater has been developed which takes into account both of these factors.

For more in-depth information on irrigation management check out Nebraska Extension’s Agricultural Water Management Guide.