If you want the ideal, uniform, early-established canola stand of five to eight plants per square foot, don’t rely on a scale.
“When it comes to setting a seeding rate to achieve that stand, the common five pounds per acre is not precise enough,” says Autumn Barnes, agronomy specialist and stand establishment lead for the Canola Council of Canada.
Seed size, predicted emergence percentage and other risk factors may mean five pounds will be too high or too low for best economic results for a given field. “The best economic rate requires some extra thought and calculation,” Barnes says. “And the final decision can’t be made until you know the thousand-seed weight of purchased seed lots.” Step one for this “extra thought and calculation” starts with a target.
Step 1: Set the target
Where does the five-to-eight-plants target come from? Alberta Agriculture’s longtime and well-respected (and soon-to-retire) canola specialist Murray Hartman ran a meta-analysis of various recent studies of hybrid canola in Western Canada. He found that overall, hybrid canola achieved 95 per cent of its yield potential with six to eight plants per square foot, 90 per cent with three to four plants per square foot and 85 per cent at two to three plants per square foot. When stands have fewer than two or three plants per square foot, yield potential can drop dramatically.
A target of five to eight plants per square foot emerging after seeding allows for the loss of a couple of plants to frost, insects or other establishment threats while maintaining yield potential and competitive ability with weeds. This target density range is also wide enough to allow for some uncertainty due to emergence percentage and seed size variations within a seed lot.
“A seeding rate that targets five to eight plants per square foot is like yield insurance,” Barnes says.
The CCC has an online tool to calculate a target stand based on risk parameters. Go to canolacalculator.ca and click on the Target Plant Density Calculator. With sliding scales for five major risk factors (stand uniformity, frost risk, weed control, insects and growing season length), the calculator provides a recommended target of between four (best case across the board) and nine (worst case across the board) plants per square foot.
“A useful result requires some educated forecasting based on past experience and an honest assessment of the five major post-emergence risk factors on your farm,” Barnes says.
Step 2: Calculate the seeding rate
Use the Seeding Rate Calculator at canolacalculator.ca to set a seeding rate for each seed lot — and for each field if emergence is expected to vary a lot between fields. You need three numbers for this calculation: seed size (in grams per thousand seeds), target plant density from step one, and an estimate of emergence percentage.
The typical emergence rate for canola in Western Canada is 50 to 70 per cent. Consistent seeding depth into warm, moist soil will often produce higher emergence rates. Inconsistent seeding depth into cool, dry soils will produce lower emergence rates. Damage from seed-placed fertilizer, seedling diseases, frost, insects and weed competition can also reduce emergence and seedling survival.
“Of course you can’t precisely know your emergence ahead of seeding time, but farmers can develop a good understanding of emergence on their farms over the years by doing plant counts and comparing that to the actual number of seeds planted per square foot,” Barnes says.
At seeding time, the moisture and temperature conditions of the seedbed can be assessed to allow for last-minute fine-tuning of the expected emergence and, therefore, the seeding rate. Low expected emergence at any seeding rate or plant density target will be hard on a grower’s bottom line. Identifying and correcting issues that decrease emergence percentage will pay dividends.
A scenario with emergence of 60 per cent, seed size of five grams per 1,000 and a target of five to eight plants per square will calculate to a seeding rate of 4.0 to 6.4 pounds per acre. At $12 per pound for seed, this rate will require a seed investment of $48 to $76.80 per acre.
Farmers could cut their costs with a lower seeding rate, but if that lower rate only achieves three plants per square foot at harvest, the drop in yield and revenue would exceed the seed cost savings. And that doesn’t include all the other risk factors that can reduce profits with a thin stand. These include:
- Non-uniformity. With fewer plants overall, the risk of dropping below two plants per square foot in patches (such as hilltops, for example) becomes much higher. Yield loss in areas below two plants per square foot can be 20 to 40 per cent when compared to areas with five to eight plants per square foot, based on Hartman’s analysis.
- Weed management. With fewer plants per square foot, it takes a few days longer for a crop canopy to fill in. This can mean the difference between one in-crop herbicide application per season and two.
- Insect risk. Consider flea beetles alone. With the same number of flea beetles spread across fewer plants in a field, there will be more beetles (and feeding) per individual plant. This damage will increase the need for a foliar spray as thresholds will be reached sooner than if the same number of beetles were feeding on more plants. Aggressive flea beetle feeding could also set back crop maturity or introduce disease. A foliar insecticide to control flea beetles will also kill beneficial insects in the field, potentially increasing the need for more insecticide applications in the future.
- Flowering and maturity delays. A lower seeding rate means fewer, bigger plants per square foot. That means more branches. Plants with more branching extend the flowering window, which increases the at-risk period for sclerotinia. It can also delay swath timing or straight combining while waiting for seeds on side branches to reach maturity.A Saskatchewan study called, “Response of canola to low plant populations and evaluation of reseeding options,” by Anne Kirk et al., compared various plant stands at field trials across the province from 2010 to 2012. The study confirmed that thin stands flower longer and take longer to mature. When site years were combined, the study found that a reduction in plant density from 70 to 21 plants per square metre (from seven to two per square foot) resulted in a six-day increase in flowering period and a three-day increase in days to maturity. Green seed risk increases with delayed maturity, and since the highest proportion of green seeds is found on later branches, these stands are predisposed to more green seed and potentially lower grades. For these reasons, short-season zones and fields seeded very late should use the higher end of the suggested plant density ranges.
- Stubble density and anchoring swaths. Finally, with just a few stems per square foot, swaths may not anchor as securely as if they were held down by many uniform stalks. This could be a big problem with high-wind events after swathing.
“When the cost of an input like seed increases relative to crop revenue, economic optimal seed rates will be lower,” Hartman says. “That is why target seeding rates are so different compared to 10 or 20 years ago.”
Hartman’s meta-analysis estimates that with current prices and fairly typical seed sizes and emergence rates, the average economic optimal level was in the range of four to six plants per square foot. In farm fields, additional considerations such as non-uniformity, reduced maturity and green seed risk will increase the desired target density.
“Because there are more downside risks with falling below the target density than going above, is it reasonable to advocate for a target range of five to seven plants per square foot?” Hartman says.
To get more precise, Hartman says seed size and emergence percentage will determine the most economically optimal seeding rate within that range. For example, a situation with small seed (four grams per thousand seeds) and high emergence (80 per cent) under current prices would have an economic density at the top of the target-stand range. That’s because seed costs in this situation are lower, and therefore, the economic target stand will be higher. In contrast, large seed (6.5 grams per thousand seeds) and low emergence (40 per cent) would have an economic target at the bottom end of that plant-density range.