Seed lot variability can mean inadequate seeding rates
November 30, 1999 By Donna Fleury
Seed lot variability is an important consideration for calculating seeding rates and achieving optimum yield potential. Seed lots of canola vary from variety to variety and even within a variety, making thousand seed weights (TSW) an important factor in setting seeding rates. The goal is to select a seeding rate that will target an optimal plant population in the range of seven to 14 plants per square foot, which allows the opportunity for that variety to achieve its full yield potential. To achieve that target, growers need to pay attention to seed lot TSW variability and ensure they have calculated the right pound-per-acre seeding rate for each seed lot. “We are seeing a trend to lower seeding rates as many growers try to be as efficient as possible with their seed costs,” explains Derwyn Hammond, senior agronomy specialist for Manitoba with the Canola Council of Canada. “Unfortunately, in many cases they are falling at or below recommended seeding rates. If spring conditions are challenging and seed mortality turns out to be higher than expected, growers can end up with a poor plant stand and reduced yields.”
Canola thousand seed weight (TSW) can range in size from less than three grams per 1000 seeds to six grams or more. This seed size variation can have a dramatic effect on seeding rate, and growers need to adjust their seeding rate accordingly to seed the same number of seeds per acre. “In the past, most seed sizes were smaller and growers using an average seeding rate of six pounds per acre typically ended up with good plants stands,” explains Hammond. “However, today many of the newer hybrid varieties in particular, tend to have a larger seed size. Larger seed may help these varieties achieve their higher yield potential, but in the seed bag it also means fewer seeds per pound. To get the maximum yield potential out of your seed investment, you want to make sure you have calculated the right seeding rate.”
The seeding rate formula used to calculate an appropriate pound per acre seeding rate for each seed lot is:
“The biggest challenge in this equation is estimating what that seed survival is going to be,” emphasizes Hammond. Seed survival varies from field to field and is impacted by environmental conditions and soil moisture and temperature. “Paying attention to the right seeding rate is partly about risk management,” adds Hammond. “If you typically target a minimum of eight to 10 plants per square foot for example, that puts you near the middle of the optimum plant range and provides a bit of a buffer in case flea beetles, frost or other factors result in higher than expected seedling mortality. However, the combination of lower seeding rates and large seed size can increase the risk of your plant population falling below the threshold for getting the maximum yield potential out of the genetics.”
Therefore, growers should ensure they have targeted high enough plant populations and estimated seeding survival as accurately as possible to calculate a seeding rate that will allow them to optimize yields
When seed lots vary in TSW, then growers should calculate the seeding rate for each seed lot separately. “In a recent example, one seed lot weighed 3.5 grams and the second weighed five grams per 1000, so you need to calculate the right seeding rate for each individual seed lot,” explains Hammond.
Using a 50 percent seed survival, the 3.5 gram seed lot would need a seeding rate of 4.7 pounds per acre to achieve the minimum plant stand of seven plants per square foot. With the larger seed size of 5.0 grams per 1000 seeds and a 50 percent survival rate, a seeding rate of 6.7 pounds per acre is needed just to achieve the minimum plant stand of seven plants per square foot. However, a recent survey suggests many growers are using a 5.0 lb/acre seeding rate or less on average, which means they would need a minimal survival rate of about 70 percent to achieve that minimum plant stand. For more information, go to: canola-council.merchantsecure.com/canola_resources/product34.aspx.
Calibrate the drill as well
Before calculating and fine-tuning seeding rates, make sure equipment is properly calibrated first to deliver the targeted pounds of seed per acre. “It’s important to do the math; however if you don’t do a good job of calibrating the drill then there really isn’t any point trying to fine-tune the seeding rate using TSW,” says Hammond. “Most drills now have a standard calibration procedure to help determine exactly what your drill is delivering. However, it may be a good idea to test the calibration in the field every once in a while to see how it compares to the actual application rate.”
Collecting seed from several drill runs over a measured distance in the field at the speed planned for seeding is a good idea to confirm settings and rate, as equipment bounce or age and wear can lead to over- or under-application, which can be costly both in terms of seed investment and yield potential. Recalibrate drills when switching between canola varieties or seed lots with substantially different TSW.
Scouting and record-keeping is the final piece that often gets missed. “Without actual plant counts, you can’t determine whether your seeding rate was successful or not,” says Hammond. “If growers are really looking at trimming those seeding rates to be as efficient as possible for their seed investment, then that’s where this extra attention to detail will really pay off.”
Growers seeding at rates that are too low will find their risks have increased and any challenges at seeding and emergence may increase plant stand mortality, leading to significant losses to yield potential. On the other hand, growers who find they are consistently above their targeted plant counts for maximizing yield may have the opportunity to cut back a little on their seeding rate without compromising yield. “Keeping a record over time will help growers understand how successful they were at converting that seed into plants and help target seeding rates to various conditions.”