The path to variable (optimum) rate application

Variable rate (VR) application – perhaps better termed the “optimum” rate – aligns with the potential of productivity zones within each field, ideally leading to increased profitability and environmental sustainability.

Successful adoption of VR depends on three factors:

  1. Identification of zones for specific rates or treatments
  2. Equipment capable of varying rates
  3. Clear benefit to the farm business

Identification of zones

Hill tops and saline areas often have lower productivity, and rich black soil usually produces higher yield than sandy soil.

Farms can manage obvious zones with low-tech methods that don’t require digital prescriptions. These include:

Other low-tech methods to consider include turning off nitrogen application in saline areas, turning down nitrogen rates on hill tops, and applying sulphur to areas with low organic matter or sandier soil where crop deficiency is more likely to occur.

Prescription maps

Digital prescriptions fed into GPS-guided control systems add a new level of precision. Prescriptions are written based on various data sources, including satellite biomass maps (NDVI and others), variable soil qualities and characteristics, soil nutrient analysis, topography maps that show hills, depressions and side slopes, and yield maps. Maps can be constructed with all or some of these sources.

Machinery software programs often include map making tools to help farmers make their own prescriptions. Machinery companies may offer videos or workshops to help them figure out how. If that is not one’s area of expertise or interest, farms can pay $5 to $10 per acre to companies experienced in the full-package map making service.

One shortcoming with prescription maps, some say, is that they’re based on historic data and can’t account for moisture variability year to year and within each season. For example, low areas are the most productive in dry years and the least productive in wet years. However, when looking at long-term results, areas with higher moisture will have the richer soil and will almost always yield more than hill tops. Like most agronomy decisions, VR maps are about using the information available to make decisions based on probabilities.

Equipment

Seeding tools achieve variable rates with electronic-controlled metering systems adjusted automatically based on the prescription maps. With multiple tanks, these seeding tools can vary the rates of various products at once, including seed, nitrogen and micronutrients. To give a specific example related to enhanced efficiency fertilizer, one tank could have standard urea for hill tops and mid-slopes and nitrification-inhibitor-treated urea for low areas subject to excess moisture. 

Experienced farmers who know their land can have simple high- and low-rate settings for nitrogen metering, switching to low rates on hill tops and saline areas. With maps and modern equipment, farmers can achieve more complex VR objectives automatically, even when they themselves are not operating the seeding tools.

VR capability goes beyond the seeding tool. Fertilizer spreaders have map-driven VR capability. Sprayers provide VR or on-off application using pulse width modulation solenoids at each nozzle, allowing the tool to control rates based on prescription maps. Looking to the near future, prescription-guided drones will apply targeted sprays without having to cover the whole field. 

Some equipment can provide real-time VR based on what it sees in the field. Greenseeker and optical spraying systems are examples. Technology will continue to improve, and variable rate could eventually shift from prescription maps based on historical data to real-time adjustment based on equipment-based sensors for soil moisture, soil type and soil nutrient levels.

Benefit

Farms will decide the moment at which, for them, the potential increase in profitability and lower environmental footprint is worth the investment in VR equipment, knowledge and time. 

VR adoption is low but on the rise. In farmer surveys, the Canola Council of Canada asked respondents to choose which practice “best describes” their fertilizer program for canola. In the 2020 survey, 36 per cent of farmers chose “adjusted on a field by field basis” and 10 per cent “used a variable rate based on zones within each field.” The rest said their fertilizer program was exactly the same for all canola fields. In the 2022 survey, 40 per cent adjusted on a field by field basis and 14 per cent used variable rate, each up four points from 2020. Also notable in 2022, 27 per cent of farms with at least 5,000 acres said variable rate best described their fertilizer program.

The decision to adopt and stick with VR requires some thought. Farms need to understand what they want to obtain from VR. Common objectives are to:

  • increase profitability on existing land
  • reduce input costs
  • maximize yields
  • fix or reduce risk on areas that have low yield potential and negative returns.

In many cases, a comfort level for adoption and long-term success with a VR program depend on a trusted and dedicated service provider.

VR can increase yield and reduce emissions

A recent Agriculture and Agri-Food Canada study (Aaron Glenn et al) showed that VR nitrogen can increase yield and also reduce nitrous oxide emissions. The project, which focused on rolling topography in the Prairie pothole region, concluded that applying 50 per cent more fertilizer than the field average in areas with high yield potential resulted in greater productivity (not a surprise) and also lower nitrous oxide emissions due to improved nitrogen-use efficiency (an added bonus). On the flipside, low-yield zones had higher emissions despite receiving 50 per cent less fertilizer than field average.

In short, the Glenn study showed that farms can achieve higher yields and lower emissions by increasing nitrogen in areas of a field with high yield potential and by reducing nitrogen in areas with low yield potential. The published report added that these results were more pronounced in years with higher moisture.

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