• Print
• Share

Thickening Thin Soybean Stands

Approximately 40% of Ontario soybean acreage was planted by May 11th this year. Cold rainy weather followed planting for the next 10 - 12 days. This cold wet weather has slowed emergence and in conjunction with the recent frosts which will cause plant stand losses. Many fields that were planted 3 to 4 weeks ago are just now starting to emerge. The longer soybean seeds are in the ground, the more time insects and diseases have to attack the emerging seedling.

Take the time to carefully assess soybean stands before making replant decisions. Most fields are still expected to have sufficient emergence for optimum yields. Assessment of a poor soybean stand should not occur too quickly. Make sure that all the beans up before making a decision. Fields with a plant reduction of up to 50% do not need to be replanted, if plant loss is uniform and the remaining stand is healthy. Numerous studies and field experience have demonstrated that keeping an existing stand is often far more profitable than replanting.

Replanting gives no guarantee of a perfect stand.

Thickening Stands

If replanting is required, there is usually no need to destroy the existing stand of soybeans. Simply plant the same variety on top of the existing stand. This practice is only recommended with very poor stands or in parts of the field that have very poor emergence, however. Research conducted in Indiana showed that thickening a stand of more than 130,000 seeds/acre did not add to yield. By running a 30" row planter on top of an existing stand, over 2 bu/ac was lost to planter damage (see Chart #1, 133 000 original + 0 Supplemental). Thickening the stand by 67,000 (133 000 original + 67 000 Supplemental) provided no significant increase over leaving the original stand.

With an original seeding rate of 67,000 seeds/acre and adding 133,000 supplemental seeding, a slight increase was realized. This proved superior to replanting the original stand entirely, which produced the lowest yield of all. Replanting often occurs so late that significant yield potential is lost simply due to the later planting date. The replanted beans in these experiments yielded 37.6 bu/ac compared to leaving the original seeding of 67,000 which yielded 50.6 bu/ac. The message is clear: thickening thin stands only produces more yield when the original stand is very poor (under 67 000 seeds/acre).

Original stand planted with a Great Plains Drill. Supplemental stand planted with an International 30” planter at the V2 stage of the original stand. Replanting occurred at the V2 stage of the original crop. Planting rates are in seeds/acre. Dr. Ellsworth Christmas, Purdue University, 2000-2001.

* 0 Supplemental = operating the planter on top of the original stand without planting seed to determine the yield loss from running the planter.

Deciding to Replant

Soybeans have a tremendous ability to compensate for missing plants. Existing plants will branch and produce more pods to fill in gaps. The uniformity of the existing crop is more important than having a high population. Soybean plants can fill spaces up to 30 cm (12 inches) within or between rows with relatively little yield reduction. Ontario research has shown that a 60-70% stand provides 95-100% yield potential under good growing conditions.

Results depend, however, on the remaining plants being healthy and distributed evenly throughout the field. It's also important that plants be kept free of weed competition, or yield reductions will occur. On lighter soils, fields with a plant stand of greater than 100,000 plants/acre should not be replanted, while heavier clay soils will require more plants per acre for success -- a minimum of 100,000 - 120,000. Table #1 shows the yield potential of reduced stands under good growing conditions for soybeans planted at a normal planting time (May 5 - 25)

¹Results of trials conducted at Huron and Kemptville research station,
University of Guelph
1 hectare = 2.47 acres

Ablett, Ridgetown College, University of Guelph

The impact of later planting (June) cannot be reversed, but the effect can be reduced. Planting higher populations (10-15%) in narrow rows helps to offset the disadvantages of June planting. By increasing the population, the number of pods per acre is increased, as well as the height of the lowest pods on the plant. Narrow rows reduce the days necessary to full canopy, which maximizes the amount of sunlight that can be absorbed. Narrow rows will also discourage branching and the plants from setting pods closer to the ground.