Source: University of Nebraska Lincoln / CropWatch News

Smaller soybean seeds in 2004 offer savings potential

by Roger Elmore
Extension Crops Specialist

Soybean seed weights are lower again this year, continuing a trend toward growers getting more soybean seeds per pound than they once did. The graph below shows the change with seed from University of Nebraska soybean variety performance trials for the last nine years. In 2003 soybean seeds from these trials weighed about 3,400 seeds per pound, while in the two previous years seeds weighed 3,000 to 3,200 seeds per pound. Seed weights in the UNL trials in 2000 were similar to those in 2003. This trend is typical for commercial seed producers, especially following drought years. Seed imported from eastern states is also likely to be smaller than that produced in Nebraska in 2003.

Smaller seed size is not indicative of reduced quality or germination potential and should offer an economic advantage for producers (see story, below). For example, at the pre-drought average of 2,500 seeds per pound, the normal seeding rate would be 60 pounds per acre, or about 150,000 seeds. However, a seed lot with 3,500 seeds per pound would have a planting rate of about 43 pounds per acre. That would be a 28% savings of seed costs relative to normal seed weights and a normal year. At today’s seed prices, that would be equal to about a $9 per acre savings.

To garner this savings, growers will need to carefully estimate their seed needs and calibrate their planters and drills for the smaller seed. (CropWatch will feature more details on adjusting planters and drills for this year’s soybean seed in an upcoming issue.)

Growers will want to review the seed weight listed on the bag to calibrate their planters correctly and avoid overplanting. Soybeans planted too thick will be tall, spindly, and more prone to lodging. This could reduce yields. We recommend planting soybeans at 150,000 live seeds per acre regardless of whether the field is irrigated or dryland, in wide rows or narrow rows, no-till or tilled, or if the seed is large or small. We’ve seen consistent results with different varieties across a wide range of planting practices and environments in Nebraska with this seeding rate. However, if seed-soil contact is less than ideal for seed, you may need to increase seeding rates accordingly. A stand of 100,000 to 120,000 plants in the field at harvest is ideal.

Make sure to adjust this recommendation based on the germination rate of the seed you plant. If the seed weight and germination is not listed on the seed tag, contact the seed dealer for further information. Fortunately, seed quality and germination rates for this year’s seed are excellent.

Possible causes for the trend

Several factors may be contributing to the trend toward reduced soybean seed size. Certainly, the recent dry summers could have an impact. Soybean seed weight is determined in August when conditions are often at their driest. (This is even more typical in seed production further east where there is less irrigation and seed size may be even smaller -- with some companies estimating 4,000 soybean seeds per pound this year.) At first glance, this is the most likely explanation; however, the linear trend is similar for both dryland and irrigated experiments in Nebraska (see graph above). I expected little or no trend over years with irrigation since we are able to optimize seed size with proper late-season irrigation. Are we perhaps reducing seed size and thus yield potential by shutting off the water too early in the fall?

Another factor to consider might be whether the varieties being planted now are inherently smaller than they were seven to nine years ago. A longer term, more focused analysis of the data is necessary to answer these questions. Please let me know if you have any insight on these trends.

For more information about seeding rates, consult NU Cooperative Extension NebGuide Soybean Seeding Rates, G99-1395, and “Plan for smaller soybean seed at planting” in the January 2004 Web-only Crop Watch story by Sandi Alswager.

Soybean seed quality high; germination over 94%

by Larry Prentice, R.S.T.
Assistant Manager, Nebraska Crop Improvement Association

Tests at the Nebraska Crop Improvement Association indicate soybean seed quality and germination are very high this year. In 25 years with NCIA, I have never seen the average germination on soybeans exceed 90%; however, this year the average is above 94% and we haven’t seen the typical midwinter decrease in germination or vigor. In addition, Nebraska has maintained its high mechanical quality and germination standards while some other states were not able to this year. Buyers need to be cognizant of the analysis and test date for the seed they’re buying.

The seed size is slightly smaller this year than the average for the past seven years, averaging about 3200 seeds per pound compared to an average of 3100 seeds per pound. The smaller seed size will not affect emergence or establishment of the young plants, and in fact, research has shown that the photosynthetic ability of the first leaves on smaller sized soybean seed is higher than it is for larger sized seed. This suggests that the plant can compensate for the reduced amount of stored carbohydrates in the cotyledons when it is first emerging from the soil. Further research has indicated that there is no effect on eventual yield potential for fields planted with smaller seeds versus larger seeds of the same variety.

Generally, smaller soybean seed will have less mechanical damage than larger seed, a fact reflected in the germination of the seed lot.

In some other crops seed size correlates to yield potential but this has not been shown to be the case for soybeans with only slightly smaller than average size.