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Agronomy update: Economic optimum soybean plant density for irrigated soybean

In southern Alberta, the development of very early maturing varieties in the MG 00 and MG 000 maturity groupings made soybean production under irrigation possible. However, very little research had been conducted on target plant populations and row spacing in southern Alberta’s cool climate. 

Research was conducted by Agriculture and Agri-Food Canada research scientist Francis Larney (retired) and MSc student Tram Thai. One study compared agronomic practices for soybean investigating the relationships between plant density and soybean yield for different genotypes, row spacing, and growing environments. The other study looked at these factors to estimate the Economic Optimum Plant Density (EOPD; stand establishment), for irrigated, early maturity soybean in southern Alberta.

The research was conducted at Bow Island and Lethbridge, Alta., from 2014 through 2016 under irrigation using two glyphosate-resistant varieties rated as MG 00.4 with a CHU rating of 2400 (NSC Tilston RR2Y) and MG 00.8 with a CHU rating of 2525 (Podaga R2). The varieties were seeded on either narrow row spacing of seven inches (17.5 cm) or wide row spacing of 14 inches (35 cm). Seeding rate was 30, 50 or 80 seeds/m² (three, five or eight seeds/ft²). This translates into approximately 130,000, 218,000, and 350,000 seeds per acre. The Lethbridge 2016 site was lost due to hail, leaving five site-years for analysis. 

Average plant emergence was 74 per cent for the MG 00.4 and 80 per cent for the MG 00.8 variety. Overall plant emergence ranged from 75 to 79 per cent, but was 82 to 84 per cent when 2016 Bow Island was excluded due to soil crusting issues. These are typical emergence percentages for soybean under good soil moisture and temperature conditions.

As a result, plant density at establishment was slightly greater for MG00.8 at 4.3 plants/ft² compared to 3.9 plants for MG00.4. There was a slight but significant increase in plant density with wider row spacing. As expected, plant density increased as seeding rate increased with an average of 2.3 plants/ft² at three seeds/ft², 3.8 plants at five seeds, and 6.2 plants at eight seeds/ft². Days to 95 per cent maturity ranged from 114 to 132 days, and one out of six site-years (Lethbridge 2014) experienced a killing frost prior to maturity. 

Increasing seeding rate from three to eight seeds/ft² resulted in an increase in grain yield from 31.4 bu/ac (2107 kg/ha) at the lowest rate, increasing to 36 bu/ac (2416 kg/ha) at the medium rate and 41.6 bu/ac (2,793 kg/ha) at the highest seeding rate. Wide row spacing yielded 37 bu/ac (2,494 kg/ha), which was 1.7 bu/ac (111 kg/ha) higher than the narrow row spacing at 35.5 bu/ac (2,383 kg/ha). Overall, wide row spacing increased grain yield five per cent to 20 per cent in four out of six environments compared with narrow rows.

The Economic Optimum Plant Density (stand establishment) was calculated to determine the optimum seeding rate that delivers the best economic returns. This takes seed costs, yield, and commodity price into consideration as they relate to seeding rate and row spacing. For example, the highest seeding rate might result in the highest yield, but the added seed cost might be more than the added revenue from the higher yield. 

The EOPDs for genotype varied by location, with two of four sites having a higher EOPD for MG 00.4 compared to MG 00.8, but lower at two other sites, and similar at the 5th site. When compared across environments and row spacings, the EOPD difference between varieties was almost identical at 4.6 plants/ft² for MG 00.4 and 4.7 plants/ft² for MG 00.8. This suggests that seeding rate may be the same for different early season soybean varieties, specifically for 00 MG soybeans .

The EPODs for row spacing also varied between sites, with wide rows having a higher, similar and lower EPOD than narrow rows depending on sites. When combining variety and sites, the EOPD gap between row spacings was small, with the EOPD for narrow rows at 4.5 plants/ft² and 4.8 plants/ft² for wide rows. Similarly, grain yields and revenue at the EOPD were very similar, with wide rows having a $483/ac ($1195/ha) return compared to $477/ac ($1180/ha) for narrow rows. 

Overall, the gaps in EPOD were minimal between environments, genotypes, and row spacing. This gap was only 0.1 to 0.3 plants/ft², and only resulted in 1.3 per cent to two per cent higher grain yield, and an increase in gross revenue of $6.47/ac to $9.72/ac ($16 to $24/ha). The overall EOPD was estimated at 4.6 plants/ft² regardless of environment, genotype and row spacing. 

As a result, the Economic Optimum Seeding Rate was estimated to be 58 seeds/m² or 5.8 seeds/ft², assuming 80 per cent plant emergence. This is equivalent to 252,000 seeds/acre seeding rate – substantially higher than Manitoba seeding rate recommendations of 175,000 to 180,000 seeds/acre, which targets 140,000 to 160,000 stand establishment assuming 80 per cent plant emergence.  

Bruce Barker divides his time between CanadianAgronomist.ca and as Western Field Editor for Top Crop Manager. CanadianAgronomist.ca translates research into agronomic knowledge that agronomists and farmers can use to grow better crops. Read the full Research Insight at CanadianAgronomist.ca.