By Robert Klein, Emeritus Extension Professor
Winter Wheat Seed Size Varies
Nebraska growers use seeding rates for winter wheat that vary from 30 to 180 lb per acre (320,000 to almost 3,000,000 seeds per acre). The lower rates are most common in drier areas. The higher rates are used for irrigated wheat which often is seeded in narrower rows and later in the season since it often follows another crop rather than fallow. This later seeding date reduces tillering and requires higher seeding rates to compensate for the reduction. Also, higher yield potential requires higher seeding rates.
Historically, wheat growers often calculated their seeding rate based on pounds of seed per acre; however, seed size varies and it's now recommended that growers calculate their seeding rate using seeds per acre to provide more accurate seeding results. The number of winter wheat seeds in one pound varies depending on the variety and the condition under which it was produced. In the 2019 Nebraska Winter Wheat Variety Evaluation for 16 trials, winter wheat yields ranged from a low of 30 to a high of 123 bushels per acre. Seed size ranged from a low of 9,000 seeds per pound to a high of 14,100 seeds per pound.
When weight is used for wheat seeding rates, a seed size difference of this magnitude can result in up to a 62% difference in seeding rate. With the good conditions for seed fill in 2019, there was less variation in seed size than in previous years when the seed size in the variety test ranged from a low of 12,000 seeds per pound to a high of 23,000 seeds per pound, a 92% difference in seed size.
Selecting a Seeding Rate
Winter wheat is capable of compensating among yield components, which often results in similar grain yields being produced across a fairly wide range of seeding rates. However, using seeding rates that are too low can lead to excessive tillering. It also may delay maturity, increase weed competition, and fail to make use of the plant's full yield potential. Using rates that are too high may increase costs, result in increased lodging, and possibly reduce yields.
Too much competition, even among small grain plants, may lead to fewer kernels per head and lower kernel weight. The key is to get an optimum plant population with uniform distribution for efficient use of available resources.
Table 1. Range of optimum seeding rates for areas of Nebraska.
|600,000 to 900,000 seeds/acre
|700,000 to 900,000 seeds/acre
|800,000 to 1,350,000 seeds/acre
|900,000 to 1,350,000 seeds/acre
|1,000,000 to 2,500,000 seeds/acre
A review of seedling rates vs. yield potential is helpful. On average, there are 22 seeds per head and 5 heads per plant, or 110 seeds per plant. With an average seed size of 15,000 seeds per pound or 900,000 seeds per bushel, a pound of average-sized seed with 80% germination and emergence has a yield potential of approximately 1.5 bushels per acre. Seeding 40 lb of seed with a weight of 15,000 seed per pound has a yield potential of 60 bushels per acre.
Table 2. Yield potential in bushels/acre of a seeding rate of 18 seeds per foot of row with 80% germination and emergence and seeded on the recommended seeding date for the area.
|Estimated yield potential
Large, dense seeds are considered to be of better quality than low test weight seeds. Large seed tends to tiller more than small seeds; however, small dense kernels are better than large, light kernels. In the seed-cleaning process, a gravity table will remove the light seed. Another factor affecting seed quality is the protein content of the seed. The amount of protein in the seed, not the protein percentage, is very important to early seedling vigor. Large seed may have a lower protein percentage than small, shriveled seed, but because it is larger, it may have more total protein per seed.
Grain test weight often is used as a measure of seed quality, but test weight is a bulk density or a weight-per-volume measurement, and small seed that packs well can have a high test weight. If producers use test weight as a seed quality measurement, they should use a seed test weight above 57 lb per bushel. Actually, a high thousand kernel weight (TKW) is a better measurement of seed quality. A 30 gram (1.05 ounce) TKW, which translates to 15,200 seeds per pound, is an appropriate minimum TKW for a seedlot. A few varieties grown in Nebraska have small seed with a lower TKW, but their seed is still suitable for planting. With these varieties, producers should use the largest seed they can obtain.
Table 3. Seeding rate for winter wheat to achieve 18 seeds per foot of row.
Seed Cleaning and Germinability
Seed cleaning and sizing is essential to remove straw, chaff, dirt, stones, weed seeds, and broken, diseased or small shriveled kernels. Generally, seed cleaning will add 1 to 2 lb to the seedlot's test weight by removing the small kernels. Taking a germination test is essential to determine the seed viability. After seed germinability has been determined, the seeding rate can be determined. Seed for planting should be above 85% germination.
How many winter wheat seeds should be planted per acre? There are several options. Floyd E. Bolton, crop scientist at Oregon State University, says 18 seeds per foot of row seems to be the point of diminishing yield increases, no matter the row spacing from 6 to 18 inches. This is for winter wheat seeded at an optimum planting date. Recent research by Cody Creech, Nebraska Extension Dryland Cropping Systems Specialist resulted in 16 to 18 seeds per foot of row achieved the greatest yields across the locations evaluated. Winter wheat seeded late because of weather or following harvest of another crop may require a 30%-60% increase in seeding rate.
For dryland winter wheat in western Nebraska, row spacings of 10 to 14 inches are recommended. Weed competition can be reduced by using narrower row spacings. For irrigation, and especially with late seeding dates, row spacings of 6 to 8 inches are preferred. Table 1 lists optimum seeding rates for Nebraska.
Table 2 shows the yield potential of winter wheat seeded on the recommended seeding date at 18 seeds per foot of row with 80% germination and emergence (stand establishment). Table 3 provides the pounds of seed needed per acre for 6- to 14-inch row spacings and seed sizes of 12,000 to 18,000 seeds per pound, based on 18 seeds per foot of row.
Calibrating the Seeder
With seeding rate determined, how can we be sure the seeder plants the desired amount? First, use the operator's manual to set the seeder. Then, use one of the several items available to help calibrate the seeder. For example, see NebGuide G2044, Calibration of Sprayers (Also Seeders), specifically Problem 10 which illustrates how to calibrate a seeder.