Plant Population and Planting Geometry

An optimum plant population is a prerequisite for realizing high seed yields. If the plant population is below the optimum mark, high seed yields cannot be obtained with any measures. The optimum plant population may be ensured by using an adequate quantity of good-quality seed. Furthermore, seed treatment against seed-borne diseases prior to sowing helps to check plant loss due to diseases.

The optimum seed rate of soybean varies with the seed size, plant type and the maturity period of the genotype. Generally, 62.5-75.0 kg seed ha-1 is considered optimum (PAU, 2009). Chauhan and Joshi (2005) summarized the information on soybean production technology for different agroclimatic zones of India and reported optimum plant populations of 0.4 million plants ha-1 for the North Hill and North Plain zones and 0.4 to 0.6 million plants ha-1 for the Central, Southern and North-Eastern zones. Furthermore, row and plant spacings can be maintained at 45 x 5 cm in the North Hill zone, 45-60 x 5-8 cm in the North Plain zone and

30-45 x 5 cm in the Central, Southern and North-Eastern Zones. In Pune, Maharashtra, India, a planting geometry of 30 x 10 cm (0.333 million plants ha-1) has been found to provide the highest oil and seed yields of soybean, followed by 45 x 5 cm (0.444 million plants ha-1) (Table 7.6). With constant row spacings but increasing plant spacings, plant height tends to decrease while branches per plant, pods per plant, seeds per plant and 100-seed weight increase.

Soybean is generally sown in rows, and the spacings between rows could vary depending upon the growth habit, sowing time, soil type and so on. In the early soybean production system, which is practised in some parts of the USA, yields are higher with a narrow row spacing of 23 cm (Holshouser and Whittaker, 2002) or <40 cm (Bowers et al., 2000). Many other studies have also shown that soybean planted at narrow row spacings provides higher yields than that planted at wider row spacings, such as 38 versus 76 cm (De Bruin and Pedersen, 2008c), 19 versus 57 cm (Andrade et al., 2002), 19 versus 38 cm (Kratochvil et al., 2004) and 23 versus 46 cm (Holshouser and Whittaker, 2002). Soybean in narrow rows exhibits higher yields than that in wide rows in general and in late-planting dates in particular. Increased seed yields in response to closer rows could be due to an improvement in light interception during the critical period for seed set (Andrade et al., 2002) or late pod fill (i.e. stages R6-R7) (Bennie et al., 1982), increased leaf area index (Holshouser and Whittaker, 2002) and higher photosynthesis (Bennie et al., 1982).

Row spacing may influence both main-stem and branch seed yields. In one study (Norsworthy and Shipe, 2005), when soybean was grown in narrow (19 cm) and wide (97 cm) rows at the recommended seeding rates, main-stem yields accounted for 45% and 69% of the total seed yield in wide and narrow rows, respectively, whereas branch seed yields accounted for 55% and 31%. Therefore, only genotypes with more branching should be selected for wider rows, whereas less-branching genotypes should be preferred for narrow rows.

Table 7.6. Effect of planting geometry and plant density on growth, yield attributes and yield of soybean in Pune, Maharashtra, India (adapted from Halvankar et al., 1999).

Planting

Plant density

Plant

Pods

Seeds

100-seed

Seed

geometry

(million

height

per

Branches

per

weight

Oil yield

yield

(cm)

plants ha-1)

(cm)

plant

per plant

plant

(g)

(kg ha-1) (kg ha-1)

30 x 5

0.666

58.9

27.7

3.0

47.1

12.2

571

3239

30 x 10

0.333

51.4

47.0

5.0

83.4

12.5

623

3529

30 x 15

0.222

48.9

69.5

6.6

122.8

12.8

574

3234

45 x 5

0.444

54.1

36.6

4.0

67.3

12.4

599

3391

45 x 10

0.222

49.3

61.8

6.3

116.8

12.5

543

3048

45 x 15

0.148

46.3

85.1

6.8

154.6

13.0

522

2932

CD (P = 0.05)

-

1.8

2.9

0.3

5.0

0.3

42

237

CD, critical difference.

CD, critical difference.

As the late-planted crop has shorter plants with lower plant biomass, the plant population may be increased to realize higher seed yields from late-planted crops. In the mid-south of the USA, the optimum plant population has been reported to range from 108,000 to 232,000 plants ha-1 for May-sown crop compared with 238,000-282,000 plants ha-1 for June-sown crop (Lee et al., 2008). Row spacings may also influence the required plant population. Plant populations of 194,000-290,800 plants ha-1 and 157,300-211,800 plants ha-1 are required for locations where soybean is sown at 38 and 76 cm row spacings, respectively (De Bruin and Pedersen, 2008b).

Soybean yields do not continue to increase at high plant population densities due to decreased radiation-use efficiency (Purcell et al., 2002). In the case of glyphosate-resistant genotypes of soybean, similar seed yields with 20% reduced seeding rates (345,800 seeds ha-1 for full season and 444,600 seeds ha-1 for double crop production) than standard have been obtained (Kratochvil et al., 2004), indicating that seed rate may be reduced with an additional profit to the range of $14.30-27.72 ha-1. In an earlier study, glypho-sate-resistant soybean at 370,000 seeds ha-1 and 620,000 seeds ha-1 produced similar yields when sown in narrow rows without moisture stress (Norswor-thy and Frederick, 2002). Since high plant populations involve high seeding rates and consequently high production costs, it is not the seed yield but the net returns that matter most to farmers.

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  • TODD
    What is seeding geometry plant science?
    1 year ago

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