Is it time to reduce seed costs by adopting variable-rate seeding? This practice has been accepted in corn, but instead of lowering the rate you push up the rate in areas with high yield potential. Well, in soybeans you do the opposite—you lower the seed rate in race horse areas and increase the seed area in marginal parts of the field or even push up the rate in marginal soil areas.

The key to why this works this is understanding the soybean plant. At the end of the day yield is all about numbers of pods per acre followed by seeds per pod and seed weight. To get pods you need nodes per plant and plants per acre. Late planting and marginal soils mean fewer nodes per plant. Early planting and deep, fertile soils means more nodes per plant.

For corn, how a hybrid responds to conditions and population changes has a large impact on the population of seed being planted and should be factored in when making a population map. However, with soybeans this is less important because of soybean’s compensentory nature and its ability to occupy a space or a gap and produce more branches and nodes.

Variable-rate seeding is the practice of adjusting the amount of seed planted based on location-specific information such as yield maps, landscape position, soil type, fertility or a variable impacting yield. The key is deciding which of those variables are most important and then being able to develop seed rate maps in the field. Making management zones is both an art and a science. While variable rate requires more management up front to design the zones and plant them, the return on investment will probably exceed expectations.

If you are thinking about starting a variable rate seeding program for soybeans, have a tractor with GPS and computer, a planter with variable rate capability and controller, and a precision ag specialist who can assist you in designing the zones, there are probably 4 or 5 variables to consider when making a planting rate map based on management zones. And keep it simple and limit yourself to either 2 or 3 planting zones: high and low, or high, medium and low.

• Yield maps: Looking at yield maps is the place to start. Collect past yield maps of corn, soybeans or wheat. Use these maps to identify high- and low-yielding areas of the field. Zones should be evident over multiple years of yield data. Aerial images with NDVI and biomass readings can also indicate areas of high and low yield potential.
• Water availability: Water availability is usually the single biggest driver in yield and that is related to annual rainfall, and soil depth, texture and water-holding capacity. The quickest way for you to get at this is by looking at a soil map for the field.
• Landscape position: Looking at the topography layer along with yield and soil maps will indicate areas of high vs. low fertility, shallow vs. deep soils, poorly drained vs. well-drained soils, and droughty soils vs. soils with good water-holding capacity. Use this information.
• Soil tests and conditions: Soil pH, soil tests levels, EC layers, fertilization practices and compaction will impact the yield potential of the soil. When planning a variable rate map, look at these soil layers and see if they can be factored into a planting rate map. Analyzing the soil will tell you if you have an opportunity for a higher return in a certain area.
• Population rate: Last is determining what population rates you want to plant. A good place to start is limit yourself to 2 or 3 zones and 2 or 3 rates. With 2 zones use your average rate and a rate reduced by 15 or 20%. With three zones the medium zone’s rate should be your average rate and then either increase or reduce the rate by 15 or 20% for the other two.

If you adopt variable rate seeding, develop a map for a field and then apply some different populations. You will learn from the experience and get better at creating planting maps and choosing a population range.

At the end of day you may or may not save any money on seed costs. Sometimes you will increase seed rate in one area and lower the rate elsewhere so seed costs may even out across the field, but you may increase yield in the more marginal areas as a result.

Agronomist Daniel Davidson, Ph.D. posts blogs on agronomy-related topics. Feel free to contact him at djdavidson@agwrite.com or ring him at 402-649-5919.