Wheat seeding rate recommendations in Kansas are in pounds of seed per acre and vary according to precipitation zone. However, seed size can have an impact in the final number of seeds actually planted per acre. A variety with larger kernels, when planted in pounds per acre, will result in less seeds planted per acre and possibly thinner stands. If the weather and soil fertility during the growing season are not favorable for fall tiller formation and survival, grain yields may be reduced due to the thinner stand. Examples of varieties with large kernels include WB4458 and Ruby Lee. On the other extreme, a variety with small kernels can result in above-optimal stand establishment, increasing plant-to-plant competition for available resources such as water, nutrients, and incident solar radiation. Additionally, planting in pounds of seeds per acre can reduce seed costs when wheat kernel size is relatively small.
Seed size can be measured in terms of the number of seeds per pound. The “normal” range is about 14-16,000 seeds per pound, but it can range from 10,000 seeds per pound to over 18,000 seeds per pound. Although seed size is specific to each individual wheat variety, it can vary within variety depending on seed lot and seed cleaning process. Figure 1 compares three different wheat varieties and the seed size as affected by seed cleaning. For this simple study, the varieties Everest, WB Grainfield, and SY Wolf were evaluated at different times during the seed cleaning process, hereafter referred to as ‘Unclean’ (harvested seed before cleaning), ‘Air screened’ (seed following air cleaning or the blower), ‘Mid gravity’ (seed from the low end of the gravity table), and ‘Top gravity’ (the seed from the top end of the gravity table). It is clear from Figure 1 that wheat variety plays a major role in determining wheat kernel size as does the quality of seed cleaning. Overall, the number of seeds per pound decreased (or individual seed size increased) as the quality of the seed cleaning process increased.
Figure 1. Effects of wheat variety and seed cleaning on final number of seeds per pound. Seed for this research provided by Ohlde seeds, research by Romulo Lollato.
Figure 2 highlights the two most contrasting treatments from the above study, the ‘Unclean’ WB-Grainfield (top figure, 17,335 seeds per pound) versus the ‘Top-gravity’ SY Wolf (bottom figure, 12,427 seeds per pound). To achieve the same number of seeds per acre, ‘Top-gravity’ SY Wolf would require a 39% increase in pounds per acre planted when compared to ‘Unclean’ WB-Grainfield. In other words, if both varieties are planted at a seeding rate of 75 pounds/acre, final number of seeds planted per acre will be 1.3 million seeds/acre for ‘Unclean’ WB-Grainfield and 930,000 seeds/acre for ‘Top-gravity SY’ Wolf. If the goal was to achieve 1.2 million planted seeds per acre, wheat would be over-seeded at about 8% for the smaller seed and under-seeded in about 22.5% for the larger seed. This assumes the same emergence rate for the cleaned and uncleaned seed, which would not necessarily be expected.
Figure 2. Differences in seed size between treatments ‘Unclean’ WB-Grainfield (17,335 seeds per pound; top photo) and ‘Top-gravity’ SY Wolf (12,427 seeds per pound; bottom photo). Photos by Romulo Lollato.
If planting occurs in seeds per acre instead of pounds per acre, we might see the opposite results where seed cleaning will actually increase stand establishment. The seeds above were no-tilled in heavy corn residue in an experiment during the 2015-16 growing season, with final seeding rate established in seeds per acre. The resulting stand counts are shown in Figure 3. These results indicate that the seed cleaning process increased stand establishment. These results were possibly due to better seed quality as the cleaning process removed small and shriveled grains that may have lower vigor than larger, healthier grains. Regardless of planting in seeds per acre or pounds per acre, these results highlight the importance of measuring wheat seed size before planting to avoid the final amount of seeds planted per acre being too far away from the original target.
Figure 3. Final wheat stand establishment as affected by seed cleaning process. Plots were sown in seeds per acre, and the improved seed quality resultant from the cleaning process increased final stand establishment. Research by Romulo Lollato.
Certified seed, or seed submitted for germination testing, will have seeds per lb information available. However, an easy on-farm method to estimate the average seed weight of a seed lot is to collect several representative 100-seed samples and weight each 100–seed sample in grams. To calculate seeds per lb, divide the conversion factor 45,360 by the average weight the 100-seed samples. Samples should be collected from the lot as is, including large and small kernels in the same proportion as found in the seed lot. The targeted number of seeds per acre is then divided by the number of seeds per pound to determine the number of pounds to be planted per acre. The following table is a quick reference guide to adjust the planting rate in pounds per acre based on seed size and the targeted number of seeds planted per acre:
Table 1: Reference guide to adjust planting rate in pounds per acre
|
|
Target planting rate (seeds per acre) |
||||
|
600,000 |
750,000 |
900,000 |
1,200,000 |
1,500,000 |
|
|
Seeds/lb |
Pounds of seed per acre |
||||
|
10,000 |
60 |
75 |
90 |
120 |
150 |
|
12,000 |
50 |
63 |
75 |
100 |
125 |
|
14,000 |
43 |
54 |
64 |
86 |
107 |
|
16,000 |
38 |
47 |
56 |
75 |
94 |
|
18,000 |
33 |
42 |
50 |
67 |
83 |
|
20,000 |
30 |
38 |
45 |
60 |
75 |
How to use Table 1:
A dryland wheat producer in western Kansas whose target may be 750,000 seeds per acre has a seed lot with large kernels, averaging 12,000 seeds per pound. Seeding rate in pounds per acre for this seed lot for a final placement of 750,000 seeds per acre should be ~63 lb/ac. The same producer, planting a different lot with smaller seeds averaging of 16,000 seeds per pound, should plant ~47 lb/ac to achieve the same final seed placement of 720,000 seeds per acre.
A wheat producer in eastern Kansas whose target may be 1.2 million seeds per acre has two seed lots, the first averaging 14,000 seeds per pound and the second, with slightly smaller kernels, averaging 16,000 seeds per pound. This producer should use a seeding rate of 86 lb/ac in the first seed lot and 75 lb/ac in the second seed lot to achieve the same final seed placement. In this case, both seed lots were in the “normal” range of about 14,000-16,000 seeds per pound, and a simple ±10% adjustment on the seeding rate should compensate for differences in seed size.
Romulo Lollato, Wheat and Forages Specialist
lollato@ksu.edu
Lucas Haag, Northwest Area Agronomist
lhaag@ksu.edu
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