Cut Soybean Seeding Rates with Caution

Seed costs have become one of the largest input expenses for soybean production. Glyphosate tolerant varieties cost in excess of $50.00 per acre. This has lead to reduced seeding rates by some producers. But do lower seeding rates sacrifice yield?

Current Ontario Ministry of Agriculture Food and Rural Affairs seeding rate recommendations are based on research conducted with conventional varieties, untreated seed, and with less precise planting equipment than is now available. Current OMAFRA seeding recommendations are:

In 2005 large scale field trials were initiated to assess the most profitable seeding rates when taking into account new technology such as glyphosate tolerant varieties, seed treatments, soil types, and precision seeding equipment.

If recommended seeding rates could be reduced by 12-15% (for example from 225 000 to 200 000 seeds/acre in 7.5 inch rows) and still achieve maximum yields, a significant savings could be achieved. This reduced seeding rate would represent an approximate $6.75 per acre savings, assuming a $35.00/unit cost with 2600 seeds/pound. If a producer could switch from 7.5 inch rows to 15-inch rows, that producer would save $13.50 per acre in seed costs (reducing seeding rates from 225 000 to 175 000 seeds per acre).

Figure1.
Seeding of Various Row Widths and Seeding Rates

Figure 2.
Soybean Emergence of 7.5 Inch vs 15 Inch Rows

Results from 2005:

Table #1: Average Soybean Plant Stands (plants/acre)

Table #2: Planting Rate Yield Results (bu/ac)

*Values followed by the same letter are not significantly different at the 5% level. Trials were planted no-till with a John Deere 1560 drill.

The spring of 2005 was relatively dry throughout much of southwestern Ontario. This was beneficial for timely planting, but it was so dry that seed germination and emergence was reduced by lack of moisture. On average only 73% of the seed planted emerged in this study. Field experience has shown that at least an 80% emergence rate can be expected in no-till soybeans fields in an average year. These reduced emergence rates in 2005 must be taken into consideration when interpreting the results.

The first year of this study indicates that seeding rates of 200 000 seeds/acre in either 7" rows or 15" rows are sufficient. However, lowering seeding rates any further sacrificed yield. (2 bu/ac or more) Remember, that even though this data is from 12 large scale trials it's only one years' data. These trials will be continued for at least two more years.

2005 average trial results showed no yield difference between 7.5" rows and 15" rows, regardless of the Crop Heat Unit area or planting date. No yield differences could be shown between a planter unit and a drill from the first year of this study.

When are Higher Seeding Rates Critical?

What really counts for yields is not the seeding rate but how many plants survive to harvest and how tall those beans will grow. The mortality of soybeans from seeding time through to harvest can be huge. In a normal no-till field we typically lose 15-20% before the seed even makes it out of the ground. This is due to seed quality (germination), insects, diseases, crusting and plant competition. The real question we need to answer is: how many plants per acre are required to maximize yield and how many seeds do we need to plant to achieve that plant stand? For most situations at least 150 000 plants/acre must survive to maturity for maximum yields. In fields that are planted late or on heavy soil types this number is higher. The reason heavy soil types and late planted fields need more plants per acre is that the beans do not grow as tall and have fewer pods closer to the ground so more plants/acre are required to achieve maximum yields.

Do Fungicide Seed Treatments Increase Plant Stands?

Fungicide seed treatments protect the seedling from damping off and will increase the number of plants that survive to harvest. Trials have shown significant (5-20%) plant stand increases with the use of a fungicide seed treatment depending on the site and the year. Please see results from trials conducted by Ridgetown College and OMAFRA in the table below.

Table 3: 2004-05 Seed Treatment Results (Plants/Acre)

*Values followed by the same letter are not significantly different at the 5% level. 2004 = 12 sites X 3 replications, 2005 = 11 sites X 3 replications

Table 4: 2004-05 Seed Treatment Results (Bu/Acre)

*Values followed by the same letter are not significantly different at the 5% level. 2004 = 12 sites X 3 replications, 2005 = 11 sites X 3 replications

A seeding rate of 200 000 seeds/acre is sufficient for most soil types if the number of plants that survive to fall is above 150 000/acre. Factors that will help to reduce plant losses are seed treatments, rotation, tillage, and planting into fit soil conditions.

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