Equipment Needed for Variable Rate Seeding
In order to create a variable rate seeding recommendations three pieces of equipment that are needed. You will need a GPS system that has 6 in pass to pass accuracy or better. You will need an in-cab controller to control the planter drives and a planter with variable-rate drives. These drives can either be hydraulic drive units that control a section of rows or electric drive units with a drive on each row unit.
Why variable rate seeding?
Most fields are going to vary in productivity 50 to 150 bu/ac in corn. Fixed rate seeding is a tradeoff in that you over plant in low yielding area and under-plant in high yielding area. This sacrifices yield on your more productive areas and increase seed cost on lower-yielding areas.
What is the Limiting Factor?
The foundation of creating variable rate seeding recommendations is based on Justice Von Liebig's law of the minimum. The law of the minimum states that growth is dictated not by total resources available, but by the scarcest resource (limiting factor). For variable rate seeding to work, in the higher-yielding areas of the field, plant population is that barrel stave that most limits growth/yield wherein lower-yielding areas of the field some factor other than the population is limiting and farmers are spending too much money on seed.
Fixed Rate Field
This is an example 40 acre field with four equal sized management zones. If the field is planted at a fixed rate of 32,000 seed per acre population, plant population is the limiting factor in Zone 1 and 2 and thus yields reduced from their potential. Where in zones 3 and 4 something other than plant population is the limiting factor and seed cost could be reduced and not limit yield.
Variable Rate Field
We will now variable rate plant this field. The hybrid ‘curve’ we are using is a simple straight line curve where at 150 bu/ac yield potential 24,000 seeds/ac will be planted and at 250 bu/ac yield potential 40,000 seeds/ac will be planted. In our example field in zone one, we increase our population by 5,000 seeds/ace. By increasing the population we decrease the likelihood that population is the limiting factor and yield increase 10 bu/ac. Notice that the size of each ear of corn decrease from 40 longs to 36 longs. In zone two population increase 2,000 seeds/ac and yield increase by 5 bu/ac. Population can’t be raised as high as in zone one because other factors become limiting more quickly. For zone 3 and 4 population is not the most limiting factor and population can be reduced and maintain the same yields. The benefit is the savings in seed cost and not increase productivity.
Variable Rate ROI
From our example field, we can calculate the return on investment (ROI) for switching to variable-rate planting from fixed rate planting. In order to do this, we have to make a couple of assumptions. The price for corn grain is $3.50 a bushel and a bag of seed corn cost $250. The basic formula to calculate ROI is the price for corn grain times yield minus the cost per each seed times the population. For Zone 1 you would take ($3.50 X 240) – ($250/80,000*37,000) = $724.38 gross cost per acre minus seed cost.
Methods to Create Management Zones
There are many different method/strategies to create management zones. We will review some of the most common creation methods. One of the most common and simplest methods to create management zones is the USDA soil survey and assign a population to each soil type in the field. While the USDA soil survey data is a great resource it lacks the resolution needed to create variable rate seeding zones. I am going to group aerial imagery and yield data into the same category as a post mortem diagnostics tool. Both will tell you what happened the previous year but don’t provide much insight to what other potential limiting factors and the why of the management zone and how to help increase overall productivity. Organic matter sensors are becoming more common and are a great tool to help make management zones. While organic matter is a major factor in the productivity of a field it is only part of the equation and must be complemented with other soil layers and management practices. Similar to organic matter sensors, the standard soil fertility testing is a great tool but only part of the equation and must be complemented with other soil layers and management practices. Most soil test only look at pH, P, K and other nutrients in the upper 2-10 inches of the soil. There is a lot more to determining soil zones than current nutrient status. Soil electrical conductivity correlates the electrical resistance of the soil to other soil factors. The primary correlations are to soil texture and organic matter. While this is a good tool it can be fairly expensive and still only gives part of the picture. In Advantage Acre we have chosen to use a management zone creation method called Functional Soil Mapping. Functional Soil Mapping uses elevation data paired with soil data to make a highly accurate soil map that can be used to create management zones.
Functional Soil Mapping
The function of soil mapping technology is a patented technology that was developed by soil scientist at Purdue University. The field mapping technology works on the premise that the primary variability in soil on a field level is caused by changes in soil elevation and how water has shaped and changed the soil over time. To create these maps, we take the digital elevation model (i.e. elevation data) for each field and use the data to calculate other values from the data. From this dataset, we calculate the optimal sampling location in the field and finally sample the field for soil attributes that have the greatest influence on crop productivity. The sampling is first done using the data from the USDA soil survey and can then be updated with data collected from the field.
Productivity Index Calculation
After calculating the functional soil map, multiple layers are combined to create a productivity index. The index combines multiple soil layers that are critical to crop production and weights each layer to create management zones for each field.
Weather Modification of the Productivity Index
The productivity index management zones are then modified using weather trends long-term weather forecast. The modification takes into rainfall, soil texture and where each management zone is located in the field relative to water ponding and runoff.
The productivity index is just a scale of 0 to 10 showing differentiation between different areas in the field. To create variable rate recommendation we need to add a yield goal to the field. This yield goal is entered in the seed plan in Advantage Acre. From this yield goal management zones are created around the variability in the field from the productivity index.
The final step is to match a hybrid response to population. When a user selects a hybrid, each AgReliant hybrid has its own unique curve the will relate population response to yield. Curves for each hybrid were developed from intensive population trials conducted by AgReliant. The example curves show five different hybrids and their response to population. If you select a hybrid with a response to population like the orange line, in most cases variable rate seeding will not be needed. Contrast that to a hybrid like the gray line where population changes dramatically between 100 and 200 bu/ac but levels off over 250 bu/ac yield potential.
Steps to Create Variable Rate Seeding Recommendations in Advantage Acre
To create variable-rate seeding recommendation in Advantage Acre is an eight-step process that can be completed in less than five minutes.