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Friday, July 9, 2021

Soybean Replant Decisions

soybean aphid

Decisions to replant reduced soybean stands can be difficult to make. It is important to understand the cause of an inadequate stand to help prevent a reoccurrence of the problem. A stand count and assessment of plant loss should be made. Consider crop insurance implications, time in the growing season, and the costs associated with replanting such as seed, fuel, labor, and other inputs.

Evaluating the Existing Stand

Factors that can contribute to less-than-ideal soybean stands include planting into a poor seedbed, planter adjustment problems, poor quality seed, soil crusting, inadequate or excessive soil moisture, seedling diseases, herbicide carryover, and numerous environmental issues. Understanding the cause of an inadequate soybean stand is important to help prevent a reoccurrence of the problem if the decision is made to replant. Spotty stand reductions throughout a field can be caused by poorly drained areas, sandy soil patches with inadequate soil moisture, or even soil compaction in certain areas. Before deciding to replant, estimate the uniformity, population, and yield potential of the existing stand. When evaluating soybean stands, only count plants with a good chance of survival. Soybean plants cut off below the cotyledon by hail or other means have no potential for regrowth. However, soybeans with moderate leaf tissue damage can recover with minimal effect on yield potential.

TABLE 1. ROW LENGTH FOR 1/1,000TH ACRE BASED ON ROW WIDTH.
Row Width Row Length 1/1,000th acre
Centimetres Inches Metres Feet-inches
38 15 10.6 34 ft. 10 in.
51 20 7.9 26 ft. 2 in.
56 22 7.3 23 ft. 10 in.
76 30 5.3 17 ft. 5 in.
91 36 4.4 14 ft. 6 in.

To evaluate the plant population for 76 cm (30-inch) rows, count the number of plants in 5.3 m (17 feet 5 inches) of row and multiply the number of plants by 1,000 to determine the number of plants per acre. For 38.1 cm (15-inch) rows, count the number of plants in 10.6 m (34 feet 10 inches) of row and multiply by 1,000 (Table 1).1 Repeat these counts in several locations in the field.

Another method for evaluating soybean stands for any row spacing, and especially drilled soybeans, is to use the hoop method. Measure the diameter of the hoop, toss it in the field and count the number of plants inside the hoop. Do this in at least five to ten locations in the field. Multiply the average number of plants by the appropriate factor listed in Table 2 to determine the number of plants per hectare (acre). Having a hoop diameter of 71.8 cm (28 ¼ inches) allows you to simply multiply by 10,000 to estimate the number of plants per acre. This hoop size can be made by cutting anhydrous tubing to a length of 2.25 m (88 ¾ inches) and joining it to form a circle.

Table 2. Factors to multiply soybean plant count from different sized hoops.
  Factor to multiply the number of plants counted within the hoop
Inside Diameter of Hoop cm (inches) Plants per Hectare Plants per Acre
91 (36) 15,385 6,165
84 (33) 18,182 7,334
76 (30) 22,222 8,874
69 (27) 27,027 10,956
61 (24) 34,483 13,865
Example: 24 plants counted in a 76 cm hoop estimates a stand of 533,328 plants/hectare (212,976 plants/acre).
Adapted from: Bohner, H. 2020. Soybean replant considerations. Field Crop Agriculture, Food and Rural Affairs.
https://fieldcropnews.com/2020/05/soybean-replant-considerations/

Evaluating and Managing the Yield Potential of the Existing Stand

Numerous studies have examined the yield potential of various soybean stands. What appears to be a substantial soybean stand reduction does not automatically translate into a substantial loss of yield potential. Soybean plants can compensate well for gaps in the field. Assuming weeds do not compete, gaps of about 30 cm (12 inches) in diameter can be filled in by branches of adjacent soybean plants. A summary of yield potential from reduced stands is presented in Table 3. Plant stands reduced by 40% of the optimal stand can still achieve the yield of a full stand.

Table 3. Soybean yield estimations in optimal and reduced stands.
% of Full Stand Row Spacing Expected Final
Yield as % of
Optimum
18 cm
(7 inches)
36 cm
(14 inches)
53 cm
(21 inches)
76 cm row
(30 inches)
100% 553,300
plants/ha
(223,900
plants/acre)
402,600
plants/ha
(162,900
plants/acre)
392,700
plants/ha
(158,900
plants/acre)
405,100
plants/ha
(163,900
plants/acre)
100%
80% 442,100
plants/ha
(178,900
plants/acre)
323,600
plants/ha
(131,000
plants/acre)
313,700
plants/ha
(127,000
plants/acre)
323,600
plants/ha
(131,000
plants/acre)
100%
60% 331,000
plants/ha
(134,000
plants/acre)
242,100
plants/ha
(98,000
plants/acre)
237,100
plants/ha
(96,000
plants/acre)
244,500
plants/ha
(98,900
plants/acre)
100%
40% 222,300
plants/ha
(90,000
plants/acre)
160,600
plants/ha
(65,000
plants/acre)
158,100
plants/ha
(64,000
plants/acre)
163,000
plants/ha
(66,000
plants/acre)
87%
20% 111,200
plants/ha
(45,000
plants/acre)
81,500
plants/ha
(33,000
plants/acre)
79,000
plants/ha
(32,000
plants/acre)
81,500
plants/ha
(33,000
plants/acre)
62%
Source: Agronomy Guide for Field Crops - Pub 811. Expected yield of soybeans in optimum and
reduced stands. Table 2-13.

Reduced soybean stands can increase light penetration to the soil surface. This can increase the potential for weed seed germination and weed competition, as well as soil water evaporation and increased soil temperature. These factors can affect nodulation, biological nitrogen fixation, and nutrient and water availability. Weed control should be a high priority, especially in fields with reduced stands, to maximize the yield potential of the existing crop.

Deciding Whether or Not to Replant

  • Determine what caused the stand loss and evaluate the population and uniformity of the remaining stand. Wait several days after soybean emergence or after damage (hail, chemicals, etc.) to determine stand levels, and only count live plants.
  • Estimate the yield potential of the existing stand. With even distribution of plants, a 33% stand loss does not substantially reduce the yield potential.2
  • Determine the full cost of replanting and the yield potential of the replanted crop.
  • For drilled soybeans at 19 cm (7.5 inch) row spacing:
    • On most soil types, plant stands greater than 222,000 plants/hectare (90,000 plants/acre) should not be replanted.
    • On very heavy clay soils, plant stands need to be less than 250,000 plants/hectare (110,000 plants/acre) before replanting should be considered.2
  • Evaluate the current and forecasted weather conditions and consider how the date of replanting can affect yield potential.
  • Determine if recent applications of residual herbicides could prevent replanting. Make sure that you follow replant instructions on the herbicide label.
  • Filling in or interseeding the existing stand is not recommended because this can result in nonuniform plant sizes causing uneven competition for resources.
  • If young stands contain large areas of damaged plants, replanting only into those areas could be an option.
  • Compared to leaving a stand with adequate population and distribution, the least productive replant option for soybeans may be the expense of tillage to destroy the existing stand followed by a replanting date later in the June.
  • If a decision is made to replant, consider using slightly higher seeding rates. This may help increase the soybean plant growth efficiency, reduce weed competition, and potentially result in more pods per hectare.
  • Scout areas of different planting dates in the same field for late-season pest problems. The difference in planting date may cause a pest to be a problem in one part of the field but not in another.

Please consult with your local retailer or Bayer® seed representative to learn about issues and options related to soybean replants.