Even More Soybean Problems?
Soybean Seed is Small!

Dry! Hot! Aphids! These factors proved to be too much for last year's soybean crop. Along with harvesting half the yield seed quality and size were reduced tremendously. How will 2002 soybean growth and final yield be affected by planting small seed?

August environmental conditions are critical for soybean development. Seed size is determined by both genetics and the environment in which the seed develops. Severe stress on a soybean plant during flowering and seed fill can drastically reduce seed size and quality. Seed companies are reporting that soybean clean out is up to 50% this year. Soybean seed size this spring will be smaller than normal.

Seed Germination

Seed from soybean plants that experienced stress during seed fill often have reduced germination. Research indicates a 5% drop in germination when plants are exposed to drought during seed fill. High air temperature during seed fill also substantially reduce germination. In one experiment seeds that developed at a day/night air temperature of 27/22 C had 84% germination, but those exposed to high air temperature stress (32/28 C) for 35 days had only 73% germination. The largest reductions in germination occur when high air temperatures and severe water stress occur at the same time. In one study the average germination dropped from 91% to 69% when plants were exposed to high air temperatures along with severe water stress!

Seed Vigour

As seed quality declines, seed vigour drops off more rapidly than germination. Even if germination is relatively good, vigour may be considerably lower. Soybeans exposed to water stress during seed fill show a 12% drop in the seedling axis dry weight (a measure of vigour) compared to a 5% drop in germination. Figure 1 shows that with increased stress, vigour declines more rapidly than germination.

Figure 1. Germination and seedling vigour response to seed maturation under increased periods of high temperature. (Keigley and Mullen,1986)

Not only was 2001 extremely dry, but it was unusually hot. Ridgetown reported 11 days above 30° C in June and July, and perhaps even more importantly, the first 9 days of August were all above 30° C. Soybeans can withstand heat stress when they have adequate moisture, but heat coupled with drought leads to significant declines in seed quality.

Emergence

Depth of seeding, weather, genetics, vigour and seed size all interact during emergence. Seed size is the least important of these factors. Some studies indicate slightly reduced emergence of the smallest seeds within a seedlot. However the seeds tested in this research were extremely small and would be removed during seed cleaning. Studies show that across a broad range of seed sizes, effects on emergence are insignificant at normal seeding depths.

Plant Height

Some differences in final plant size have been reported when small seed is planted. One reason may be the greater amount of stored energy in a larger seed (used by the growing seedling) allows the seedling a better start and results in a slightly larger plant later in the season. Table 1 gives the results of a comparison of 10% of the largest and 10% of the smallest seeds from a seedlot. The smallest seed produced slightly smaller soybean plants.

 

Table 1

Seed Size	Plant Height
Largest 10%	89.4 cm (35.2")
Smallest 10%	84.6 cm (33.3")

These differences are minor and would not have a significant impact on yield.

Final Yield

The impact of seed size is minimal on final yield. Remember, large or small seed contains the same genetic material; therefore the genetic potential of the plant remains the same. When purchasing soybean seed, the quality of the seed is for more important than the actual size of the seed.

How much will last year's conditions affect this year's crop? Only germination and vigour are affected significantly. Get a germination and vigour test! Certified seed will state the germination on the bag, but you should ask for vigour test results. If you're planning on using bin run seed, get a certified lab to check both germination and vigour.