August 8, 2007

Nebraska growers use seeding rates for winter wheat that vary from 30 to 180 pounds 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.

UNL recommends wheat seeding rates based on seeds per acre not pounds per acre. The change can result in as much as a 200% difference in seeding rate.

Winter Wheat Seed Size Varies

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 can range from more than 20,000 to less than 10,000, depending on the variety and the year it was produced. In the 2006 Nebraska Winter Wheat Variety Evaluation for 16 trials, winter wheat yields ranged from a low of 37 to a high of 101 bushels per acre. Seed size ranged from a low of 11,860 seeds per pound to a high of 24,790 seeds per pound.

When weight is used for wheat seeding rates, a seed size difference of this magnitude can result in a 209% difference in seeding rate.

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. Panhandle 600,000 to 900,000 seeds/acre Southwest 700,000 to 900,000 seeds/acre Central 800,000 to 1,350,000 seeds/acre Southwest 900,000 to 1,350,000 seeds/acre Irrigated 1,000,000 to 2,500,000 seeds/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. Row spacing (inches) Estimated yield potential (bu/ac) 6 153 8 115 10 92 12 77 14 66

A review of seedling rates vs. yield potential is helpful. On the 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 percent germination and emergence has a yield potential of approximately 1.5 bushels per acre. Seeding 40 pounds of seed with a weight of 15,000 seed per pound has a yield potential of 60 bushels per acre.

Test Weight

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 seed test weight above 57 pounds 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.

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 pounds to the seedlots 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.

Planting Rate

Table 3. Seeding rate for winter wheat to achieve 18 seeds per foot of row.   Row spacing (inches) Feet of row/acre Wheat seeds/lb 12,000 13,000 14,000 15,000 16,000 17,000 18,000     lb/ac of seed     6 87,120 131 12 112 105 98 92 87 8 65,340 98 90 84 78 76 69 65 10 52,272 78 72 67 63 59 55 52 12 43,560 65 60 56 52 49 46 44 14 37,337 56 52 48 45 42 39 37

How many winter wheat seeds should be planted per acre? There are several options on this. 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 what row spacing from 6 to 18 inches. This is for winter wheat seeded at an optimum planting date. 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 Drill

With seeding rate determined, how can we be sure the drill plants the desired amount? First, use the operator's manual to set the drill. Then use one of the several items available to help calibrate the drill. For example, see NebGuide 1511, Calibration of Sprayers (Also Seeders), specifically Problem 10 which illustrates how to calibrate a seeder.

Robert N. Klein
Cropping System Specialist