The short answer is yes; you will most likely need to change your seeding rate, but this is not just because you are planting a different wheat variety. Rather, seeding rates should be adjusted annually to reflect seed source characteristics (germination, thousand kernel weight [TKW]) and the environment the seed is being planted into, to ensure you can achieve your target plant population.  

Let’s dig into why this is. For spring wheat, provincial target plant population recommendations are between 23-28 pl/ft², with many producers targeting the upper end of this recommendation. Achieving your target plant stands sets your crops up for success, as crop uniformity is improved, weed pressure is combatted and resources are optimized.  Seeding rates should be calculated to achieve your target plant stand, which means accounting for germination percentage, expected mortality and, importantly, your TKW. TKW changes year-to-year and from variety to variety.

Let’s consider an example to illustrate the relationship between kernel weight and seeding rate. For this example, let’s call our two varieties variety one and variety two. When comparing these two varieties we need to consider:

• TKW: The weight (g) of 1,000 kernels of wheat from a specific seed lot.
• Germination: If using bin-run seed, a seed test is needed to determine germination percentage. If purchasing certified seed, then your seed provider will be able to provide you with this information.
• Expected mortality: The percent of seed/seedlings that won’t produce a plant due to unfavourable conditions or biotic stress.
• Expected seed survival: Germination – Expected Mortality.

Example #1:

Using the formula provided by Manitoba Agriculture (below) we can see the difference in seeding rates and therefore seed costs.

Seeding by plant population formula:

Seeding rate (lb/ac) = Target Plant Population (pl/ft²) x TKW (g)

                                              Expected Seed Survival (decimal) x 10

 Variety 1:

Seeding rate (lb/ac) = Target Plant Population (pl/ft²) x TKW (g)

                                              Expected Seed Survival (decimal) x 10                                          

                                                         = 27 (pl/ft²) x 42 (g)

                                                                     0.91 x 10

                                                         = 125 lb/ac

Variety 2:

Seeding rate (lb/ac) = Target Plant Population (pl/ft²) x TKW (g)

                                              Expected Seed Survival (decimal)

                                                         = 27 (pl/ft²) x 34 (g)

                                                                     0.91 x 10

                                                          = 101 lb/ac

This example shows that there is roughly a 24 lb/ac difference between the two seeding rates to achieve the same desired plant population. If you seeded variety 2 at the same rate as variety 1, then you would have over seeded, which could result in a thicker canopy, bringing challenges like an increased lodging and disease development risk.

Also, over seeding would have cost you money. Using a standard seed cost of 0.27 ¢/lb of seed (Manitoba Cost of Production Guide), then variety 2 at your normal rate would cost an additional $6.5/ac of seed that is probably not needed to reach your desired plant population.

Now, if the weather, disease, insects and equipment co-operate, you’ll achieve your target plant population. But it’s always best practice to do plant counts to get an understanding of the crop establishment and uniformity of emergence. More information on plant stand counts can be found here: Plant Stand Counts in Spring Cereals | Manitoba Crop Alliance.