Variable Rate Seeding for Soybeans
In U.S. soybean production, variable-rate seeding [VRS] is being used on an increasing number of acres because of the increasing cost of planting seed. Ideally, VRS is applied to distinct zones in a field that have been identified based on their distinct soil properties [e.g. CEC, organic matter, soil pH and potassium level, soil texture, terrain attributes such as elevation] and yield history. The intent is to plant different areas of a field with different seeding rates that are based on the perceived economic benefit that will be derived from planting fewer seed in certain areas of a field. The assumption is that fewer seed can be planted on the “better” field areas [higher CEC, medium-textured soils with greater organic matter and higher CEC that should support more vigorous plant growth] vs. using a perceived best seeding rate on the entire field. It is likely that two rates–i.e. “normal” and “lower”–in an individual field will be the most manageable and economical.
Variable-rate seeding of soybean should be conducted only after first identifying distinct management zones [within a field] that should be based on the factors listed above. Otherwise, the effect of lowering the seeding rate in some field portions may not accomplish the desired effect–i.e., there should be a valid reason based on soil factors and yield history for planting fewer seed in identified field zones.
An article titled “Comparison of variable-rate prescriptions and optimum seeding rate in soybean“ appears in Crop, Forage, & Turfgrass Mgmt. [2021;7:e20130]. The conduct of the research reported in this article and subsequent results follow.
• On-farm studies were conducted at four Ohio locations in 2017, and three Ohio locations and three Michigan locations in 2018.
• The objectives of the research reported in this article were to 1) determine the agronomic optimum seeding rate [AOSR] for soybean in predetermined management zones of a field, and 2) compare the AOSR to each producer’s VRS practice. Each cooperating producer selected the factors [e.g. yield history of a field, soil map unit, soil organic matter, soil pH] that were used to define the management zones within an individual field.
• Four seeding rate treatments were applied at each location each year. They were uniform rates of 100, 140, and 180 thousand seeds/acre and a variable seeding rate of 80 to 180 thousand seeds/acre in the Ohio studies. In the Michigan studies, uniform rates of 70-95, 110-145, and 150-195 thousand seeds/acre [dependent on location] and a variable seeding rate of 85 to 140 thousand seeds/acre were used. The variable seeding rate at all locations was based on properties of pre-determined management zones within a field at each site.
• Plant population [stand counts conducted at V2-V3 and R8 stages] and seed yield data were recorded for each seeding rate treatment in all management zones of a field.
• Management zone seldom affected plant population and never influenced soybean seed yield at any of the sites.
• In 13 of the 28 management zones across the 10 site-years of the study, there was no significant response of yield to seeding rate treatment. This was likely because soybean plants can compensate for low populations by increasing the number of branches per plant.
• The authors calculated an AOSR for each management zone that had a significant yield response to seeding rate, and found that in most cases [53%] the farmer-selected seeding rate resulted in underseeding in management zones with a calculated AOSR. This underseeding was attributed to low stand establishment that resulted from adverse conditions that occurred after planting.
• The authors recommend that producers take a stand count after VSR planting that is based on predetermined field management zones to ensure that the seeding rate they chose for each zone was in fact correct for those zones, and if it was not, adjust the seeding rate up or down accordingly.
• The takeaway from this research is 1) producers should identify under-performing areas of each soybean field, and determine if a higher seeding rate will improve performance in those areas, and 2) determine if a higher seeding rate in under-performing field zones can be offset by a lower seeding rate that may be used in normal and/or high performing zones so that seed costs can be held constant or even lowered.
An article titled “Crop physiological considerations for combining variable-density planting to optimize seed costs and weed suppression“ was published in 2022 in the journal Weed Sci. [doi: 10.1017/wsc.2022.62]. The conduct of and results from the portion of this study that dealt with soybean follow.
• The study was conducted under the premise that high plant densities of a crop [in this study, corn, cotton, and soybean] will contribute to weed suppression.
• The high cost of seed will likely cause reluctance among producers to adopt and use this practice across a whole field that has sporadic areas of high weed density.
• A field study was conducted in 2019 and 2020 at Rocky Mount, NC, and in 2020 at Goldsboro, NC. Soil texture at both sites was a loamy sand.
• In each year, six treatments were used. They were: 1) normal density [control]; 2) 75% of normal density; 3) 50% of normal density; 4) 25% of normal density; 5) a sequential arrangement of alternating 25% and 75% densities; and 6) a sequential arrangement of alternating 75% and 25% densities.
• C25947LL, a MG 5 soybean variety, was planted in early May each year.
• To offset the high cost of seed when using greater crop plant densities to suppress weeds, the authors developed a model to optimize seeding rate so that higher seeding rates are used only in field areas where targeted weeds are present, and lower seeding rates are used in field areas where there will be little or no weed interference.
• The results from this study were used to develop a model that can be used to optimize seeding rate in field areas devoted to high and low seeding rates without increasing the cost of seed.
• The authors concluded that using their developed model for seeding rate optimization would allow the use of variable rate seeding on a large scale–i.e., in field-scale plantings–to benefit weed suppression while minimizing seed costs.
• The optimization model allows growers to adopt variable rate planting to increase seeding rate in areas with a dense population of weeds, and decrease seeding rate in areas that are nearly or completely weed-free.
• Soybean’s ability to compensate for variable population density made the variable seeding rate a non-factor in the crop’s sensitivity to yield differences among the seeding rate schemes used in this study.
• Further research is needed to determine which, if any, problematic weed species in soybean are significantly susceptible to suppression by this approach that uses VRS to aid in weed control.
An article titled “The art and science of variable rate seeding” by Hawkins and Singh provides the following points that should be considered before applying VRS technology to a field.
• When using yield history of field areas to optimize VRS, it is important to use multiple years of data. Also, 1) accurate yield data are critical when using it to optimize VRS in a field, and 2) relatively stable yield responses over time are ideal.
• For soybeans, seeding rates are usually decreased in high productivity field areas, and increased in field areas with a history of low productivity.
• When using VRS, it is imperative that seeding rate prescriptions are validated over time to ensure that they are in fact accurate for the areas where they are used.
• Use check strips in fields where VRS is used to compare the prescribed seeding rates to the seeding rate that is used as a standard practice. This should lead to seeding rate adjustments that may be needed to maximize returns to the VRS practice.
An article titled “Variable Seeding Rate for Soybean” by Laura Lindsey provides the following points that support the use of VRS, or that should be considered before using VRS technology.
• Most new planters are capable of VRS.
• For VRS to be successful, relatively uniform management zones within a field must be identified so that a valid “prescription” seeding rate for each zone can be determined.
• Creating field management zones based on a soil map is likely inaccurate for VRS purposes. Rather, creating management zones based on soil properties that are listed above is likely more accurate.
• The variables that are used to identify uniform management zones within a field will be specific for each individual field.
An article titled “Eight Tips to Follow for Variable-Rate Seeding in Soybeans” by Laurie Bedford appears in Successful Farming magazine, and offers useful tips for using VRS. Also, soybean producers are encouraged to access the “Soybean Seeding Rate and Plant Population” White Paper that is posted on this website.
Composed by Larry G. Heatherly, Apr. 2023, larryh91746@gmail.com