University of Nebraska-Lincoln Extension, Institute of Agriculture and Natural Resources

March 31, 2006

Rate of 12,500 seeds per acre performs well in southwest Nebraska

Research suggests seeding rate for rainfed corn

Differences in climate between Lincoln and Scottsbluff are as great as from Lincoln to the east coast of the United States. These differences naturally affect the seed rates for rainfed corn in Nebraska.
Recommended seeding rates for rainfed corn vary across Nebraska, depending on average precipitation, the field's microclimate, soil type and condition and the total growing degree days required for the hybrid being planted.

Given the drought conditions in some areas, a field's soil moisture profile and the likelihood of filling it before summer starts are key considerations. The mean summer water balances -- the cumulative differences between monthly precipitation and potential evapotranspiration for June, July, and August -- across Nebraska range from a deficit of 4 inches in areas of southeast Nebraska to a deficit of as much as 10 inches in portions of northwest Nebraska. This assumes a 100% effective rainfall for June through August. Often, however, warm season rainfall on bare soil can be as low as 20% efficient due to evaporation, runoff, and weed use. (See maps illustrating the mean summer water balances and accumulated growing degree-days for Nebraska in NebGuide 1528, Recommended Seeding Rates and Hybrid Selection for Rainfed (Dryland) Corn in Nebraska.

At planting, the only real guides to potential moisture for the crop year are the amount of stored water in the soil profile and the amount of crop residue and planned tillage and cultivation, all of which affect stored moisture. While the amount of stored water can't be controlled, you can control the use of tillage and cultivation and provide for surface residue. Crop residue can reduce soil evaporation and wind speed, increase water infiltration, and suppress weed growth.

Seed rates for rainfed corn in Nebraska have ranged from about 8,000 to 24,000 plants per acre. Water requirements for corn only decrease when populations are less than 18,000 plants per acre. Generally, corn yields will increase from 6 to 12 bushels per acre for each additional inch of soil water beyond the 8-11 inches needed for initial grain development.

One study conducted for four years at four sites across southwest Nebraska (near North Platte and McCook and in Hayes and Perkins counties) found that the most cost efficient seeding rate in these trials was 12,500 seeds per acre. While one higher planting rate yielded more, the extra seed cost outweighed the small increase in yield.

In good production years there has been a tendency for increased yield with higher plant populations, although these levels also may create a greater potential for disaster in case of drought. If it appears that water may be limited, reducing the seeding rate likely won’t result in a proportionate decrease in the amount of water needed for the crop. As plant population increases, leaf canopy increases, which reduces the amount of direct sunlight reaching the soil surface. Wind movement within the canopy also is reduced as population increases. In addition, the shade created by the higher population density reduces soil temperature which in turn reduces surface evaporation. The higher population also creates some mutual shading, which cools the leaves and reduces the transpiration rate.

Research, models test rates against various moisture levels

Results from several years of field research and a model using 53 years of data supports a recommendation of planting 8,000 to 16,000 plants per acre in 30-inch rows in western Nebraska. Simulations had one of three levels of available soil water at planting, either 3.1, 6.3 or 9.4 inches in the top 5 feet of a loam soil, representing one-third, two-thirds, and full soil water profiles, respectively. Median yields were maximized at 8,000, 12,000 and 16,000 established plants per acre for starting available water levels of 3.1, 6.3 and 9.4 inches. Gross margins were maximized at 12,000 established plants per acre when starting available water was 6.3 or 9.4 inches. The probability of a financial loss at this population was reduced from about 10% at 6.3 inches to 0% at 9.4 inches. Perhaps the greatest benefit of additional soil water at planting was the reduced risk of financial loss.

Table 1 shows recommended average planting rate, seed spacing in inches and projected harvest populations for rainfed corn in Nebraska. Note that while lower populations may seem more viable, they are not as competitive with weeds and there may be increased moisture loss as compared to higher planting rates.

Drew Lyon
Extension Dryland Crops Specialist
Bob Klein
Extension Cropping Systems Specialist

Table I. Average planting rate, seed spacing in inches and projected harvest populations for corn.
Planting rate/acre
Row width
20 inches 30 inches 36 inches
Harvest population
10% Loss 15% Loss
Inches between seeds
Plants/A
6,000
52.3
34.8
29.0
5,400
5,100
8,000
39.2
26.1
21.8
7,200
6,800
10,000
31.4
20.9
17.4
9,000
8,500
12,000
26.1
17.4
14.5
10,800
10,200
14,000
22.4
14.9
12.4
12,600
11,900
16,000
19.6
13.1
10.9
14,400
13,600
18,000
17.4
11.6
9.7
16,200
15,300
20,000
15.6
10.5
8.7
18,000
17,000
22,000
14.3
9.5
7.9
19,800
18,700
24,000
13.1
8.7
7.3
21,600
20,400
26,000
12.1
8.0
6.7
23,400
22,100
28,000
11.2
7.5
6.2
25,200
23,800
30,000
10.5
7.0
5.8
27,000
25,500

You've got mail!
To receive Email notification when the latest Crop Watch is posted to the Web, use this form.

Copyright 2006 by the University of Nebraska Board of Regents. All rights reserved.
Published by University of Nebraska-Lincoln Extension in the Institute of Agriculture and Natural Resources Cooperating with the counties and the U.S. Department of Agriculture.
The University of Nebraska-Lincoln does not discriminate on the basis of gender, age, disability, race, color, religion, marital status, veteran's status, national or ethnic origin, or sexual orientation.