Re-Thinking Soybean Row Widths

In the early years of narrow row adoption, drills (7.5 inch row spacing) became the planting implement of choice across much of Ontario. Intermediate rows (15-inch spacing) were not as widely promoted or adopted. In some parts of the US intermediate row widths have become more popular compared to 7 inch rows. Several factors have influenced the switch away from narrow to intermediate row widths:

1. High cost of glyphosate tolerant seed
2. One piece of equipment being able to plant both corn and soybeans
3. Better emergence in wider rows
4. White mould
5. Little yield benefit comparing 7 to 15 inch rows

Yield Impacts

As row widths are narrowed, yield increases follow the law of diminishing returns. As you move from 30 to 20 inch rows the yield increases are the highest, but as you move from 20 to 15 inch rows the benefits are less evident. Moving from 15 to 7 inch rows shows little or no benefit. Ontario research has shown a yield increase of over 5 bu/ac between 30 inch rows and 15 inch rows, but as little as 0 bu/ac or slightly over 1 bu/ac between 7.5 and 15 inch rows. Refer to Table 1. Narrow rows would likely show more benefit in lower CHU zones (less than 2600) and late planting. It’s also important to note that these research trials don’t contain white mould. Better emergence with wider rows and the presence of white mould may actually favour 15 over 7 inch rows.

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Table 1 - Soybean Yield Response Under Various Tillage Systems

Where the difference between 2 treatments exceeds 2.4, there is a less than 1 in 20 chance that it is due to random variation. University of Guelph (1998-2000). Average of 9 sites per year. Trials were conducted on clay loam, silty – clay loam, silt loam, and Guelph loam soil types.

Studies in the US have shown similar results, of little or no benefits of 7 inch over 15 inch rows. Refer to Table 2.

Table 2 - Adjusting Management Practices Using Glyphosate-Resistant Soybean Cultivars

G. Bertram, P. Pedersen, 1997-1999

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Switch From 7” to 15” Rows?

About 60% of the Ontario soybean acreage is now planted with glyphosate tolerant varieties. The cost of Roundup Ready seed continues to increase, and is now in excess of $50.00 per acre. Can seeding costs be significantly lowered by switching from 7 to 15 inch rows?

If a producer would switch from 7 inch rows to 15 inch rows, they would save $7.00 per acre in seed costs (reducing seeding rates from 225,000 to 200,000 seeds per acre). This assumes a $37.00 per unit cost of Roundup Ready seed with 2600 seeds/pound.

Seed Treatment Increases Plant Stand

Seeding rates could likely be reduced even further if the seed is treated with a seed treatment. Recent Ontario research has demonstrated a stand increase of between 5 to 25% when using a fungicide seed treatment. Although soybean fungicide seed treatments don’t always show a yield advantage, plant stand increases are consistently demonstrated. With the use of a fungicide seed treatment it may be possible to reduce seeding rates while maintaining the number of plants per acre at harvest time.

Seeding Rates

Ontario Ministry of Agriculture, Food and Rural Affairs row width/seeding rate recommendations are:

• 225,000 seeds/acre in 7 inch rows,
• 200,000 seeds/acre in 15 inch rows,
• 170,000 seeds/acre in 22 inch rows,
• 160,000 seeds/acre in 30 inch rows.

However, the above recommendations are based on research that was conducted with conventional varieties, untreated seed, and with less precise planting equipment than is now available. Several studies have shown that for most soil types there is little economic benefit to having more than 150,000 plants/acre at harvest time for maximum economic yields. If a fungicide seed treatment is used this stand is often achievable with a seeding rate of 175,000 seeds/acre in 15 inch rows. Many US states recommend a seeding rate of 175 000 seeds/acre in 15 inch rows.

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In-field Spraying Trips

With the introduction of soybean aphids to Ontario in 2001 and the possible invasion of soybean rust in 2005, more in-field spray trips will be required in the future. This is another reason to consider moving away from narrow rows to intermediate row spacing.

Is a 15” Planter better than a 15” Drill?

No Ontario data is available on this subject. However trials conducted in Indiana have shown that if a drill is driven at the same speed as a planter (i.e. not at 8 MPH) yields are the same between the two pieces of equipment. If drills are pulled too quickly, seed placement becomes less accurate and may have a negative impact on yields.

Considering the cost of Roundup Ready seed and the widening popularity of fungicide seed treatments on soybeans, it is necessary to reconsider the most cost-effective use of that seed. For many Ontario producers, that may mean planting 15-inch rows instead of 7-inch rows.

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