When soybean planting time arrives, doing it right could pay big dividends. The 2008 soybean seed supply is critically tight and expensive. Some of the seed supply is of marginal quality with less than 80-percent germination and more vulnerable than usual to planting time stresses. There won't be much opportunity for replanting, so doing it right will be crucial to this year's success.
Seed germination and vigor are highest at physiological maturity which may or not be good depending upon weather, planting date, variety and other factors.
Seed germinability declines with aging after this time. The rate of decline is influenced by seed moisture (the higher the faster), seed temperature, handling, seed decay organisms, etc.
Last year was a time when soybean germination and vigor were less than ideal at physiological maturity. The hot, dry mid-season weather followed by rains near maturity resulted in many soybeans having relative high levels of free fatty acids. These are components of fat, but when not attached to the fat molecule can be a significant part of the seed decay process, either by the physical effect they have on germination or their vulnerability to decay by microbial organisms.
This year, a lot of soybean seed have this problem — in spite of the best efforts of seedsmen to gather, handle, store and process seed for planting.
Usually, it's not difficult to produce planting seed with 80 percent-plus germination. But because of 2007 weather conditions, a lot of 2008 soybean planting seed will have marginal quality, 70 to 80 percent germination.
In light of this, handling and planting procedures this spring will be especially important for getting acceptable soybean stands. The need for getting good stands is also paramount, in that there won't likely be soybean seed available for replanting.
Handling and planting procedures to consider:
Handling soybean seed needs to be as gentle as possible. Rough handling can fracture soybean seed internally, especially when seed are at low moisture. Seed need to be gently lifted or lowered when handled. Seed plates on some planters are major culprits for cracking seed, especially if planting at fast speeds. If seed cracking is noticeable, change seed plates or slow down planting speed.
Storage of soybean seed needs to be under cool, dry conditions. If seed are picked up now and not planted until June, storage conditions could be an issue of concern. Ideally, soybean seed should be stored between 40 and 65 degrees F. Avoid situations where seed might accumulate moisture above 13 percent or be subjected to changing temperature or hot temperatures, above 80 degrees F., for an extended period of time.
Use seed treatments, but don't overdo it. Three seed treatments — fungicide, inoculant, and molybdenum — are commonly used on soybeans. Each can have a special benefit but the chemical affects of these treatments can be adverse to soybean germination if soil moisture is limited and/or if soil temperature is high, above 95 degrees F.
Use seed treatments as needed, but planting when soil moisture and temperature are favorable will help these treatments be beneficial for soybeans without reducing soybean emergence. This same advice would also apply to soil pesticides and fertilizers. Use them as needed, but don't concentrate them in the seed bed.
Planting in moist soil is essential for getting a good soybean stand. Marginal soil moisture usually results in spotty and poor soybean stands.
The soybean seed needs to absorb 50 percent-plus of its weight in moisture for the germination process to begin. If moisture absorption is less than this amount, seed decay rapidly takes place.
If irrigation is used to enhance stand emergence, it should be applied ahead of, not after planting. Doing so will give better soybean stands.
Planting depth needs to be deep enough to plant in moist soil. When soil moisture is good the ideal depth will be 1 to 1.25 inches deep. Planting deeper than this can result in poor emergence, especially for low vigor seed.
Plant in a clean seed furrow, one that is free of trash and/or old crop litter in the immediate area of the placed soybean seed. Litter near the placed soybean seed can help increase seed decay or interfere with good seed-soil contact.
Try to avoid soil crusting. This is most likely to occur when planting in wet soils and/or with excessive firming of soil over the seed bed. Allow soil to adequately drain after rains or irrigation before planting. Set the planter press wheel to lightly firm the soil over the planted seed, but make sure that it is not excessive. Make planter adjustments as needed to get desired soil firming. If crusting does occur, use a rotary hoe or irrigation to reduce its stress on germinating soybean seedlings.
Use PPI or PE herbicides only as needed for control of glyphosate-resistant or tolerant weeds. These herbicides tend to have some negative effect on soybean emergence even in ideal conditions. Use of these herbicides at high labeled rates could result in poor stands, especially if soybean seed quality is marginal.
Their detrimental effects on soybean emergence can be reduced if they are applied at moderate rates, in split applications, or two to three days after planting when soybean germination has begun. When PPI or PE herbicides are needed, it would probably be wise to increase the seeding rate about 10 percent to help compensate for any soybean stand reduction they might cause.
What about soybean seeding rate?
There is a lot of interest in reducing soybean seeding rate, especially in light of current high seed costs. This strategy has merit in that soybean yield is usually not affected much by population density within limits of 50,000 to 200,000 soybean plants per acre.
Planting at low populations (seven to nine plants per foot of row for 36-inch spacings) has some merit for saving on seed costs but only if high quality seed are used.
For marginal quality seed, the emphasis should be on planting more, not less, seed to help insure getting a good soybean stand.
Even with good seed and good planting conditions, only 50 to 60 percent of soybean seed typically emerge to a stand.
For early planting, aim for five soybean plants per foot (36-inch rows). With good soybean seed and good planting efforts, it will usually take eight to 10 seed per row foot to achieve this.
For marginal quality seed, it will likely take 10 to 12 seed per foot to get the same results. Soybeans planted late after June 15 tend not to grow and branch as much as early plantings. For these plantings, it would also be prudent to increase the soybean seeding rate by 10 to 15 percent.
These are some of the “pot holes in the road” as we approach the 2008 season. Planning efforts to avoid them could be the difference between success and failure for 2008 soybeans.