As canola seed costs are increasing, growers are reassessing seeding rates. Seeding rate needs to be high enough to get a good establishment, as this will set up the crop for its potential yield. It is also good to have insurance against slugs, mites, disease or other seeding issues, but there is no point spending more on seed than needed.
To calculate seeding rate, first decide the target density, or how many plants per square metre you want. The rainfall and the type of seed (that is, hybrid or open pollinated) are important in deciding the target density. The target density recommendations for hybrid seed are lower than open pollinated varieties, largely due to the higher seed cost of the hybrids. Also estimate the field establishment, which is the proportion of viable seeds you plant that are likely to become part of the crop. The moisture conditions at seeding provide a guide to field establishment. Lastly, find out the seed size and germination percentage.
Target plant density
To work out the seeding rate, decide what result is wanted. What is the target plant density, or how thick should the canola crop be? Rainfall is a guide to the target density. Table 1 shows the suggested target crop density for Western Australian farmers.
The cost of seed is also an important consideration, the lower density targets of the hybrid canola is primarily due to the higher seed cost, compared with open pollinated (OP) canola. The target density should not be below 20 plants/m2.
|Rainfall zone||Hybrid||Open pollinated|
Beware of cutting the seeding rate too fine with expensive hybrid seed. It is good to have a safety margin, in case of mites, depth variation, fertiliser toxicity or other unforeseen establishment problems.
Use a higher plant density to combat weeds
It may pay to increase the target density, to ensure good crop competition with weeds.
Make the most of canola and use it as a true break crop by using integrated weed management (IWM) strategies. A high plant density is one such tool. Weed seed set accelerates where canola densities are below 20 plants per square metre. Where there is a high weed burden, target high crop densities to boost crop competition with weeds and help to combat weed seed set.
Although there is no economic yield advantage in having a crop that is too thick, there are generally no yield penalties, especially for growers in the medium and high rainfall zones. Results from 18 Department of Agriculture and Food, Western Australia (DAFWA) density trials over three years and all rainfall zones showed only one trial where yields were severely reduced as density increased. This was in 2013 at Mullewa, where dry conditions and continual aphid attack droughted out canola at crop densities above 20plants/m2. In the other experiments, canola densities above 60plants/m2 and up to 110plants/m2 produced similar yields to lower densities. High density crops may be more prone to disease or lodging.
Establishment of canola is highly variable. Field establishment (or paddock establishment) is the proportion of viable seeds that emerge and grow, compared with the number sown. It is common for only half of the canola seeds sown to emerge and contribute to the final crop yield.
Seeding conditions are the major factor affecting field establishment. Establishment is at its best in warm, wet conditions. In cold conditions, emergence is slower and establishment is reduced. Establishment is also reduced in dry conditions. If sowing into marginal moisture, increase the seeding rate, to compensate for the seeds that will not emerge.
Accounting for differences in field establishment can be the most important factor in calculating seeding rate. The seeding rate of medium sized hybrid seed is 1.9kg/ha under excellent conditions, but 2.6kg/ha when seeding dry and over 5kg/ha if the dry seeding is followed by a tough start to the season.
The precision of the seeder affects the establishment, establishment is reduced if some seeds are sown too deep. Establishment can be reduced further, by fertiliser toxicity, high stubble loads, insect attack or diseases, such as pythium or fusarium root rot.
Hybrid varieties tend to have better establishment than open pollinated (OP) varieties. A guide to the expected field establishment is listed in Table 2.
|Seeding conditions||OP varieties||Hybrid varieties|
|Excellent; warm and wet||65||80|
|Reasonable; just enough moisture||50||65|
|Dry sown; good opening rains within 10 days||45||60|
|Poor conditions (non wetting/hard crusting soil/marginal break)||10-35||20-45|
Seed weight or seeds per kilogram
The important thing is to find out the size of the seed being planted this year, so you can calculate your most appropriate seeding rate. If you are using purchased seed, the number of seeds per kilogram of your seed lot will be on the seed bag, and/or available from the seed supplier. If you are using retained open pollinated seed, refer to your yellow ruler 'Count your canola seed weight' or Estimating the size of retained canola seed.
The bigger the seed, the less seeds there are per kilogram, and the higher the seeding rate will need to be. Using seed with a different seed weight can cause a marked change in your seeding rate. The seeding rate changes from 1.7kh/ha to 2.9kg/ha, where seed is changed from small-medium hybrid seed (285 000seeds/kg) to large-very large hybrid seed (170 000seeds/kg), using 65% field establishment (as would be expected under reasonable seeding conditions) and a target of 30plants/m2.
There are more seeds for the dollar with small seed, but big seed is more robust for good crop establishment. Refer to Table 3 to see the canola seed weight compared with the number of seeds per kilogram. The canola seed weight, measured in milligrams, is the same as the thousand seed weight (TSW), measured in grams.
Variation between and within varieties is a normal part of our canola industry. The seed size of canola is determined by genetics and the environment. Hybrids generally have bigger seeds. However, the environment can have an overriding effect on seed size. Different seed lots of any variety may have quite different seed sizes, if they are produced from different source crops. Hybrid canola seed is generally in the range of medium to large sized seed.
Open pollinated canola seed is usually in the range of very small to medium size. It is best to use seed bigger than 3.3mg, that is with less than 300 000seeds/kg. There is a good relationship between canola seed diameter and seed weight. Grading seed over a 1.8mm sieve should increase the seed weight to 4mg. Grading over a 2mm screen should increase the seed weight to 5mg.
|Seed size||Seeds/kg||Seed weight (mg)|
|Very small||350 000||2.9|
|Very large||150 000||6.7|
The germination percent is the proportion of viable seed in the seed lot. Each seed lot sold has a germination test. This information is available from the seed supplier. Commercially available seed generally has a high germination, above 90%.
For retained seed it is recommended to get the seed tested by a laboratory, such as DDLS - Seed testing and certification services. Otherwise, do a germination test at home.