Features Agronomy Soybeans
Rethinking planting populations

Seeding rates in corn are increasing, as the newest hybrids have better light utilization traits, with up to 97 percent of light captured by the upper leaves. Photos by Ralph Pearce.

New crop breeding developments bring growers higher yields, better disease resistance, and much more. However, how we manage these crops must keep pace with genetic changes. Using outdated planting recommendations and incorrect advice about other best practices can prevent growers from getting the full benefits that crop advances offer.

Corn densities can be tweaked
Corn planting densities have steadily increased for several years because of the increased light-use efficiency of each new generation of hybrids. “Evidence continues to mount that corn growers can increase seeding rates to approach 35,000 seeds per acre,” says Jonathan Klapwyk, product and agronomy manager at Pride Seeds. “Many growers are now planting at 32,000 or 33,000 seeds per acre.”

New hybrids have featured better canopy architectures from a light utilization standpoint, with the upper leaves becoming more erect over time, says Dr. Elizabeth Lee, professor of plant agriculture at the University of Guelph. “The modern hybrid captures 95-97 percent of the light,” she observes, “so bumping plant densities much higher does not make much sense from the standpoint of light capture and utilization, but there could be some room for tweaking, perhaps in the range of plus or minus 10 percent, with specific hybrids.

Klapwyk agrees. Increasing corn plant populations is an avenue to capture additional yield from genetic gains in stalk strength and stress tolerance, he notes, as well as from additional management tools such as foliar fungicides.

“The growers that will have the most success with adjusting planting rates are those that understand the relation between individual hybrids and the soils that they are planting them on,” Klapwyk says. “It’s an issue that growers should have an in-depth look at with their agronomist or seed company representative.”

Soybean seeding rates decrease
The newest recommendations on soybean seeding rates released in 2008 by the Ontario Ministry of Agriculture, Food and Rural Affairs, are substantially lower than those previously provided. This is because of recent studies headed by provincial soybean specialist Horst Bohner and University of Guelph plant agriculture professor Dr. Hugh Earl.

The current 7.5-inch-row seeding rate recommendation has dropped from 225,000 to 194,000 seeds per acre. For 15-inch rows, it is now 177,000, down from 188,000 seeds per acre. The savings provided by following these new guidelines depend on the cost of seed, says Bohner, but it is reasonable to expect about $8 per acre (7.5-inch rows) and more than $3 per acre (15-inch rows) in savings.

Seeding rates have dropped, observes Dr. Istvan Rajcan, professor of plant agriculture at the University of Guelph, “probably both because of improved genetics and the use of different agronomic practices.” In terms of the latter, some growers are using a lower seeding rate with treated seeds, which have a better survival rate. However, because treated seed is more expensive, any savings gained in using a reduced rate may be nullified.

In Ontario soybean fields, OMAFRA’s Horst Bohner says that seeding rates are going lower but not to the same extent as in parts of the US where the growing season is longer and the soils are more productive.

In a large study conducted by Bohner and Dr. Dave Hooker of the University of Guelph, fungicide-treated seeds resulted in 5000 more plants per acre than untreated seeds using the same seeding rate. With fungicide- and insecticide-treated seed versus untreated seed, the results show that 10,000 more plants per acre emerged. In theory, Bohner says that should mean a grower can plant 5000 fewer fungicide-treated seeds or 10,000 fewer fungicide- and insecticide-treated seeds per acre than untreated seeds. But, he says,“Not all plants are going to make it to harvest, so cutting back too much on the recommended rates when using treated seeds may not be wise. We currently recommend the same seeding rate for both no-till and conventional systems.”

Growers might also choose to use a lower seeding rate if they are seeding earlier in the season because it gives the plants more room to branch over a longer period. However, Bohner stresses, “Seeding earlier also means the plants are facing more stressful cold and wet conditions, so again, cutting back may not be wise. The mortality rate of soybean seeds is high. A seed treatment can help protect the seed when conditions are poor.”

Bohner has seen success in shaving down the seeding rate with the use of a precision planter instead of a drill. “We found this to be the case in 2008 for studies on two sites,” he says, “even though the stand differences were too small that year to make a statistically significant difference on yield. There was a 13 percent plant stand increase when using a planter compared to a drill in 15-inch rows. More studies are underway.”

One of the most common seeding questions Bohner receives is why the seeding rates for US growers are lower. “I can assure growers that we have large amounts of repeated data that US rates are too low for Ontario,” he says. “It’s our growing conditions in this geographic location and our soil. You can get away with a lower rate in the highly productive soils of Iowa. They also have a longer growing season.”

However, some differences in soil conditions among farms in Ontario may affect growers’ choice of row width, which in turn affects the seeding rate. “On heavier soil types such as clay, wider row widths increase the number of seeds per foot of row,” Bohner says, “which can aid in emergence.” The choice of row width should also depend on tillage system, equipment suitability, weed problems, white mould pressure and planting date. However, 30-inch rows are too wide for maximum yield potential in Ontario.

“There seems to be some advantage to using 15-inch rows if the soil conditions are good,” says Rajcan. “The plants will produce side branches with additional pods. However, if conditions are poor, 7.5-inch rows are better because they allow the plants to utilize sunlight more effectively. If 7.5-inch rows are used in good conditions, there’s too much interplant competition, and that will have a negative effect on yield and may represent a waste of seed.”  n