Jochum Wiersma, Small Grains Specialist, University of Minnesota Extension
Each year questions arise about the correct seeding rate for hard red spring wheat. ‘Is a bushel and a peck enough?’ is a question I have been asked more than once. Research in the mid nineties demonstrated that - on average - an initial stand of 30-32 plants/ft2 maximized grain yield. As planting was delayed past the optimum, the initial stand needed to be increased by ~ 1 plant/ft2 for each week of delay to maximize grain yield. With this number in mind and assuming a stand loss between 10-15% one can calculate a seeding rate using the following formula.
Repeated research, however, has demonstrated that the optimum seeding rates differ for individual varieties. To determine this optimum seeding rate the variety in question is tested at a wide range of seeding rates. The seeding rate for maximum grain yield can be derived from the parabolic response curve of grain yield versus number of plants per unit area, which increases quickly to a maximum and slowly decreases at higher plant densities.
The same research that yielded the current general recommendation actually tested 7, then commonly grown, wheat varieties. Some of the key findings were that:
1. The initial plant population increased as seeding rate increased, but the proportion of seedlings to live seed planted decreased with seeding rate, regardless of the planting date. This increase in stand loss was found for each planting date and year the trials were conducted.
2. The stand loss that corresponded was, on average, 10% higher than the previously assumed 10-15% stand loss that is used in the seeding rate calculation.
3. The optimum seeding rate that maximizes grain yield varied between 29 and 37 plants/ft2 when seeding early and 29 and 38 when planted late for individual varieties.
Since none of the varieties tested at that time are grown anymore it is of little use to actually share all the results with you.
How should we then individualize the seeding rate recommendations for more modern varieties? First, repeating the optimum seeding rate trials is probably worthwhile. Meanwhile we may be able to approximate a recommendation – varieties that tillered well tended to have lower optimum plant densities when planted early, while varieties that didn’t tiller as much required higher plant densities both early and late planted. Thus if you’re growing some varieties that tiller well - such as Faller - you can probably be near the lower limits of the current recommendation while varieties that do not produce as many tillers - such as Granite - should probably be seeded at, if not beyond, the current upper limits of the recommendations.