How low can you go with Roundup Ready soybean seeding rates?

Technology is a wonderful thing, but it comes at a price, and for soybean growers, a high price for Roundup Ready seed. With a record supply of soybeans in the pipeline and rumors of prices falling to $4 per bushel, how low to go on number of beans planted per acre may be the key to making or losing money in 2006.

In 2005, the top three yielding fields of soybeans, judged by the North Carolina Soybean Producers Association, averaged slightly over 78 bushels per acre and averaged slightly over 56 pounds of seed planted per acre.

The most efficient crops judged by the Association averaged using 50 pounds of seed per acre, all produced over 60 bushels per acre and at an average total production cost of slightly under $3 per bushel.

The top yield and efficiency winners verified results of a research study conducted by North Carolina State University Soybean Specialist Jim Dunphy. In 32 tests, conducted statewide in 2005, Dunphy’s research team found that in early May-planted, early maturity beans, there was no yield increase when seeding rates exceeded 50,000-60,000 seed per acre.

When the seeding was moved up to June, the optimum seeding rate was 70,000 per acre and in late planted, double-cropped beans the optimum was 100,000 seed per acre.

“Regardless of time of planting or variety planted, seeding rates of 50,000 to 100,000 are going to be most profitable,” Dunphy says. “With the high cost of seed, it is critical for growers to know as much as they can about the seed they plant,” he continues.

It is a delicate balance for soybean growers to keep production costs down, but to also put enough seed in the ground to supply a uniform stand that is capable of high yields. Too few seed are clearly a yield robber. In 2005 tests, Dunphy’s research team saw yield losses that averaged eight bushels per acre when seeding rates were dropped from 50,000 seeds per acre to 25,000 seed per acre.

On the other side of that balance, the North Carolina researchers found $36 per acre savings when seeding rate was reduced from 200,000 per acre to 50,000 seed per acre.

Dunphy notes that many factors affect the ideal number of seed to plant. High among the influencing factors is variety.

In statewide testing, the North Carolina researchers compared varieties from Maturity Group IV up to Group VII beans. In each test, yield was averaged for each maturity group to determine high, low and average varieties.

In Group IV Roundup Ready varieties, 4401RR and 4804RR were the only varieties to produce better than a 10 percent yield increase above average. Both these varieties are from Progeny Seed in Arkansas.

In Group V, nine out of 60 Roundup Ready varieties tested produced nine percent or better yield above the average. These include: AG 5605, HBK R5924, RT5450N, USG 7553nRS, V58n3RR, AG 5903, DP 5915RR and USG 7582nRR.

In Group VI maturity beans, only USG 620nRR produced yields at least 10 percent higher than average. Among the non-Roundup Ready varieties tested in Group VI varieties tested, NC Roy at 7 percent above average, was the only one above average in yield.

In Group VII maturing varieties, 34J71, DP 7220, and USG 7732nRR were 10 percent or better above average in yield. NC Raleigh, a non-Roundup Ready variety, provided yields better than 10 percent above average.

“We also looked at determinant versus non-determinant varieties for the first time in 2005. “This year the indeterminant varieties did not hold up as well at lower seeding rates, which was expected because the indeterminants diverted some energy to reproduction,” Dunphy explains.

“We also looked at 15-inch versus 30-inch rows, but saw little difference in the pattern of seed populations,” he says. The North Carolina researcher notes that the narrower rows did produce higher yields than wider row spacings, which, he says, was no surprise.

Fertility is another critical factor influencing performance of soybean seed. In consecutive years, Dunphy says, they have seen a slight yield increase from using an inoculant, plus nitrogen. “We don’t have a good explanation of these findings, but they occurred all over the state, two years in a row,” he explains.

The North Carolina State Soybean Specialist says they have not seen significant yield increases from the use of fungicides or insecticides alone. However, he does note a slight yield increase when using the fungicide Quadris in combination with the insecticide Warrior. Whether this combination is economically feasible depends on disease and insect pressure, he concludes.

If statewide yield and profitability are indications, how low to go on Roundup Ready seed seems to be closer to 50,000 seeds per acre than 100,000 seeds per acre. By knowing all the performance features of the seed variety planted and the soil dynamics, growers can adjust the rate up from 50,000 seed per acre to the ideal rate for a particular field, according to North Carolina State researchers.

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