Vern Hofman, Extension Agricultural Engineering

Grain drills are designed to plant seed at the proper rate and with precision if the proper adjustments are made to the seed metering system. Seeding at the proper rate and desired depth are essential to get a good stand and maximize yield. This makes planting one of the most important jobs in the crop production process.

Seed size varies between varieties and the size of seed of the same variety will vary from one year to the next. This requires that drill metering systems must be calibrated so the correct amount of seed is placed in the soil.

The seeding rate tables found in the operators manual or on the inside of the seed hopper lid are based on a standard weight per bushel for various crops. Wheat has a standard weight of 60 pounds per bushel. Due to differences in seed varieties, the seed size, and weight may vary from the standard.

Seed metering systems are based on a volume displacement. Therefore, if one lot of seed varies in size and weight from another, two different amounts or number of seeds will be metered if the drill setting is not changed. For this reason, metering systems should be calibrated when seeding in pounds per acre to obtain a particular plant population per acre.

To determine the amount of seed to plant, you need to decide what plant population is desired at harvest time. This has a major affect on yield. Figure 1 lists desired plant populations that should be strived for if yields are to be maximized. For example, a spring wheat plant population of 1,250,000 plants per acre at harvest is desired in eastern ND. A slightly lower plant population will depend on more plant tillers to produce yield, but main stem heads will usually yield better than heads on tillers. The little extra seed will usually be easily recovered in higher yields.

Drill Calibration Procedure -To arrive at a particular plant population, an estimate on the field stand lost must be made. This can often be as low as 10 percent or as high as 40 percent. An estimate needs to be made based upon previous years crops. Often, a 10 to 20 percent loss occurs due to disease organisms, weed competition, and seed damage during handling.

Usually all seed does not grow due to less than 100% germination. A germination test should be done on all seed so the amount planted can be increased to account for this loss.Figure 1 Plant Populations at Harvest Time Needed to Maximize Yield in North Dakota

Crop	West	East
Spring Wheat	1,000,000	1,250,000
Winter Wheat	800,000	1,000,000
Durum	1,000,000	1,250,000
Barley	800,000	1,000,000
Oats	1,000,000	1,250,000
Soybeans		150,000
Sunflower
Oil	20,000	20,000
Corn.	17,500	17,500
Corn	hybrid specific	hybrid specific
Dry Beans
Pinto		60,000
Navy		75,000 - 85.000

Calculate the number of seeds to plant by:

A. Determining the seeds in a pound.

1. Count out 100 seeds. (Count out a larger amount if your scale has marginal accuracy.)

2. Weigh on a gram scale (Some elevators have a gram scale).

3. Calculate seeds per pound.

Example: 100 seeds weigh 2.89 grams

[453.6 gm/lb / 2.89 gm] x 100 seeds = seeds/lb

156.95 x 100 = 15,695 seeds/lb

Note: There are 453.6 gm/lb and there are 7,000 grains in one pound.

B. Desired stand at harvest is 1,250,000 plants per acre.

Field stand loss is 10 percent

Germination is 95 percent

A 10% field loss plus a 5% germination loss requires a 15% increase in seeding rate.

To determine the number of seeds to plant:

Multiply 1,250,000 plants per acre x l.15 / 1,437,500 seeds to plant

C. Determine pounds of seed to plant per acre:

1,437,500 seeds to plant / 15,695 seeds/lb = 91.6 lb./ac or 92 lb./ac

D. How many seeds per square foot must be planted to be seeding 92 lbs. per acre?

1. Seed count is 15,695 seeds/lb

15,695 seeds/lb x 92 = 1,443,940 seeds/acre

2. There are 43,560 square feet/acre

1.443.940 seeds/acre / 43,560 sq. ft/ac = 33 seeds/sq. ft

E. Check your drill.

A simple way to check calibration is to count the number of seeds dropped in a foot of drill row. To do this:

1. Operate your drill on a firm soil surface at or near your normal operating speed.

2. Count the seeds dropped in one foot of drill row. (Make several counts to obtain an average.)

3. Multiply the single row seed count by the following drill row adjustment factor:

Drill Row Spacing		Adjustment
(Inches)		Factor
6		2
7		1.7
8		1.5
10		1.2
12	1

4. Compare the seeds counted to the values found in the following table, "Wheat Seeding Plant Population Per Square Foot," does not allow for a reduction in germination.

5. Make adjustments if necessary and repeat your calibration.

Wheat seeding plant population per square foot*
	Durum	Spring Wheat
	10,000 seeds/lb	13,000 seeds/lb	15,000 seeds/lb	17,000 seeds/lb
(pounds/acre)	seeds/square foot
60	14	18	20	23
70	16	21	24	27
80	18	24	27	31
90	21	27	31	35
100	23	30	34	39
110	25	33	38	43
120	28	36	41	47

*The numbers in the chart are based on seeds planted per square foot and does not consider stand reduction from less than 100 percent germination.

F. Air Seeder Calibration.

All air seeder manufacturers include a calibration method in the operators manual. These units usually require collecting seed from the air delivery tubes over a distance of travel. The seed collected at the openers is weighed and the seeds per square foot or seeds per lineal foot of drill row can be calculated. Spreading a tarp under the seeder collecting the seed and weighing will give the total amount of seed metered.

1. Start by determining the circumference of the seed meter drive wheel. Measure the wheel diameter in inches and use this formula:

C(ft)= [diameter in inches/ 12] x 3.14 = circ. in ft.

2. Determine the drive wheel revolutions to equal 1/10 acre. Look in the following chart to find the distance of travel for your air seeder width. Divide the travel distance for 1/10 acre by the metering wheel circumference. This is the number determined in step 1. Remember this must be in feet.Travel Dis ance for 1/10 Acre Drill Width (Ft~ Distance (Ft~

Travel distance for 1/10 acre
Drill width (ft)	Distance (ft)
16	272
20	218
24	181
28	156
32	136
36	121
40	109
44	99
48	91

Example: An air seeder has a metering wheel diameter of 28 inches and is 32 feet wide. How many revolutions must the metering wheel be turned?

C(ft) = [28 X 3.14] / 12 = 7.33 ft.

Look in the adjoining chart and find the travel distance for a 32 ft. unit. A 32 ft. air seeder must operate over 136 feet for 1/10 acre.

No. of revolutions of the drive wheel = 136 / 7.33 = 18.6 rev.

3. Place bags over the outlets of each air delivery tube. A cloth bag or a paper bag can be used. The cloth bag should be tied around the shank but the bag must allow the air to flow through. A paper bag can be set on the ground under each air tube and the top of the bag must extend up above the air tube outlet and the top must be left open to allow the air to escape. A bag over the outlets is best so the flow from each outlet can be measured. An alternative is to lay a tarp under the entire air seeder so it can collect all the seed metered.

4. Place seed in the tank and start the air delivery fan. Place a mark on the metering system drive wheel and rotate the drive wheel the number of turns calculated in Step 2.

5. Weigh the seed in each bag to be sure all tubes are delivering equal amounts. Use a gram scale. After checking for equal amounts from all seed tubes, add all samples together and weigh. This is the weight for 1/10 acre.

Or, collect the seed on a tarp and weigh. The total is the pounds metered over 1/10 acre. To determine the amount for an acre, multiply by 10. Compare this to the total pounds determined in an earlier step.