As we wait for the snow to melt and the fields to dry up, there are some things producers can do to ease the transition into seeding. One important decision that can be made at this time of year is calculating seeding rates. For calculating seeding rates it is important to have information on the quality of the seed, and the size of the seed. Seed size is determined by the weight of one thousand seeds and is expressed in grams. Hence, the term thousand kernel weight (TKW).
Why do you need to know the TKW of your seed lot? Each variety can have an expected range of seed sizes but the growing conditions from where it was harvested can also influence the development and therefore size of the seed. Knowing the seed size of your seed sample is critical to calculating seeding rates. Large seed will require a higher seeding rate than small seed when targeting plant populations or seeding densities.
Targeting a certain plant density is important to ensure there are enough plants in the field to maximize yield potential. Optimum plant densities for crops grown in Saskatchewan are listed in Table 1. To calculate your seeding rate from a target plant density you need to know the TKW, germination, and expected emergence rate. TKW and germination numbers can be obtained from certified seed lab or can be obtained from tests done at home. The advantage to the tests from the certified seed lab is the documentation you will receive that provides proof of the seed quality.
The expected emergence rate varies with crop type, timing of seeding, and risk factors associated with seeding. Risk factors include soil characteristics such as moisture and temperature as well as anything that can affect the speed of germination and the ability of that seeding to emerge. Factors such as depth of seeding, airflow of the seeder, and seed placed fertilizer can negatively impact emergence if care is not taken.
The average emergence rates for larger seeded crops, such as cereals and pulses, are typically 5 to 20 per cent less than the germination rate. With good germination many producers use an average emergence of 80 to 90 per cent for cereals and pulses. Small seeded crops are more sensitive to adverse conditions. Crop such as canola and mustard have emergence rates of only 40 to 60 per cent with an average of 50 per cent. However, there have been cases of emergence in canola dropping to 20 per cent under more stressful conditions. If there are added risks of reduced emergence then your seeding rate should be increased or the risk removed. For example, if you are seeding canola early into cold soils and are seed placing more than the recommended safe rate of fertilizer then you should be increasing your seeding rate.
Now for the calculations. To simplify the calculations target seeding density has been added to the table. The target seeds per square meter are based on good quality seed with over 90 per cent germination and average emergence rates. Again, if seed quality is lower or if you are adding risk then seeding rates should be adjusted upward. If you have seed with high germination, seeding shallow into warm moist soil and have no added risk factors; then you may adjust your rate lower by five to ten per cent based on better than average emergence expected or just reap the benefits of a good stand.
To calculate your actual seeding rate based on suggested seeding density, refer to the following:
Seeding rate (kg/ha) = suggested seeding density/m x TKW ÷ 10
If you would like to do your own calculations using your own emergence rates then the equation is:
Seeding rate (kg/ha) = target plant population/m2 x TKW in grams ÷ expected seeding survival
To convert to pounds per acre multiply the seeding rate by 0.89.
Although we can do our best to target a certain plant population, there are many factors that can impact the actual population. Seedbed conditions, moisture, temperature and soil texture play a role and are often out of our control. Factors under our control that can impact establishment include: seeding depth, amount of seed placed fertilizer, and whether or not we use a seed treatment. Insects and disease can negatively affect establishment. Frost is a big player and one that we definitely cannot do anything about. All these factors can influence establishment therefore, it is important that the amount of seed we put in the ground is sufficient to meet target plant populations so that the crop yield potential is not limited before it really gets going.
For more information on this or other crop related topics, contact Sherrilyn Phelps at 306-446-7475 or the Agriculture Knowledge Centre at 1-866-457-2377.
- Phelps is Regional Crops Specialist, North Battleford Regional Services Branch
