October 4, 2000 | By: Laura Skillman

The goal of every Kentucky wheat producer is to reach the maximum yield potential from each variety they grow. Selecting and achieving the optimum seeding rate during planting is the first step toward this goal, which requires that grain drills be calibrated with each seed variety/lot that's selected.

A recent field study has shown that individual seed metering/delivery units on drills can vary by more than 10 percent above and below the target-seeding rate, which affects seed costs proportionately, said Sam McNeill, University of Kentucky Extension agricultural engineer.

For this reason, it's best to collect seed from 3 to 5 drop tubes across each 10 to 15 foot section of the drill during calibration to obtain an accurate value of the average seeding rate.

Most drill operator's manuals provide seeding rate tables that are useful for "coarse tuning" your drill but these have been found to vary by 10 percent or more from measured values in calibration trials for most soft red winter wheat varieties. With some diligence drills can be calibrated to within 5 percent of a target seeding rate.

Operators who calibrate their drills each year and who keep records of their drill settings for a range of seed sizes from year to year can reduce the time required to calibrate their equipment provided that seed of similar size is used.

Seeding rates are typically increased as the planting season progresses. No-till operators also match seeding rates to the amount and condition of residue that's encountered at planting time. The degree of residue decomposition, soil and residue moisture, and post-harvest residue treatment (mowed or unmowed) all affect drill performance, stand establishment, final stands, and ultimately yield.

For these reasons, a spreadsheet has been developed to help farmers calibrate their drills, keep track of seed costs, and keep records of their wheat enterprise. Originally developed by Mike Ellis, a Shelby County no-till farmer and crop manager, to facilitate drill calibration, the spreadsheet has been expanded to include seed costs, different row spacings and other useful information.

The spreadsheet is designed to calculate the weight of seed that should be delivered by the drill in a 200-foot strip based on the target plant population, row spacing, and seed tag information (number of seeds per pound, seed germination and purity.) By also entering the numbers of acres for each variety and seed cost per bag, the total number of bags needed and seed cost per acre are calculated.

By simply changing the target population on the spreadsheet, the total amount of seed needed and its cost for a given operation is quickly calculated – a useful feature that helps farmers select profitable target populations.

Seed costs for the same target population can range between $15 and $45 per acre this fall depending on seed size, quality and price. Since wheat seed is sold by the pound and seeding rates are based on a specified number of seeds per unit area, smaller seed of equal quality is the better buy provided the same yield potential exists between the varieties being compared.

To illustrate this point, one can look at the impact of different seeding rates on the total seed cost with four different varieties on a 500-acre operation. In one example, a difference of 25 plants per square yard changes the total seed cost by $825. Moreover, the difference in total seed costs between timely planting (325 plants per square yard) and very late planting (400 plants per square yard) can approach $2,500 or $5 per acre. This difference provides motivation to control seed costs by calibrating grain drills this fall.

Specific details of grain drill calibration procedures and this spreadsheet are provided in a new extension publication (AEN-81). Stop by your county Extension office to obtain a free copy.


Sam McNeill, (270) 365-7541