Time of Sowing

Time of sowing is a non-monetary input that influences the productivity of soybean to a great extent. Both too early and too late sowings result in drastic reductions in yields. Soil and air temperatures of 13-16°C are necessary for germination and seedling growth of soybean, but further increases in temperature up to about 32°C are better (Christmas, 2008). The optimum time of sowing is determined on the basis of various factors such as weather parameters (e.g. minimum and maximum temperatures, photoperiod, relative humidity, rainfall) during the crop growing season, maturity duration of the genotype, soil type, moisture availability at sowing and so on.

The optimum time for soybean sowing may vary at different locations due to different climatic conditions. In India, mid-June to the first week of July is the optimum sowing time for the North Hill and North Plain zones, whereas mid-June to mid-July is optimum for the North-Eastern and Central zones and mid-June to end of July is optimum for the Southern zone (Chauhan and Joshi, 2005). In Wisconsin, USA, crop sown in early May has been found to produce a higher yield than that sown in late May (Table 7.2), with the early planting producing a higher seed number, pod number and harvest index than the late planting (Pedersen and Lauer, 2004). Culti-vars may show differential responses to sowing time. For example, in Arlington, USA, cultivar CX 232 yielded 7% higher when sown in early May

Table 7.2. Influence of date of sowing on seed yield of soybean at Arlington and Hancock,

Wisconsin, USA (adapted from Pedersen and Lauer, 2003).

Seed yield (kg ha-1)

Location

Sowing date

1997 1998 1999 2000

Arlington Early May (3-6 May)

3330 3470 130 3630 3510 NS

4590 4430

4300 4010 110 4580 2930 820

3750 3490 210 3790 3330 360

Hancock Early May (8-13 May)

5250 5120 NS

LSD, least significant difference; NS, not significant.

(4370 kg ha-1) than in late May (Pedersen and Lauer, 2003), whereas no sowing-date effect was observed for two other cultivars. In another study, the yield was decreased from 3000 to 2900 to 2800 kg ha-1 when sowing was delayed from an early (6-21 May) to intermediate (20-27 May) to late (4-11 June) planting date, respectively (Perez-Bidegain et al., 2007).

In Lincoln, Nebraska, USA, delayed sowing after 1 May led to a significant seed yield decline of 17 kg ha-1 day-1 in 2003 and 43 kg ha-1 day-1 in 2004 (Bastidas et al., 2008). In Hyderabad, Andhra Pradesh, India, a delay in planting in the winter season caused a progressive decline in soybean yield from 2573 to 2396, 2193 and 1975 kg ha-1 when sowing was delayed from 15 October to 4 November, 24 November and 14 December, respectively (Mur-thy et al., 2001). In the summer season, the crop yielded 1425, 1538, 1295 and 1141 kg ha-1 when sown on 5 January, 25 January, 14 February and 6 March, respectively. In Ludhiana, Punjab, India, 25 May, 10 June and 25 June sowings yielded 1685, 2210 and 1909 kg ha-1 (Singh et al, 2000). Lower yields from 25 May and 25 June sowing dates were attributed due to very high and very low dry matter accumulation, respectively, on the two sowing dates. In another study conducted at Ludhiana, crop sown on 24 May, 8 June, 24 June and 8 July yielded 1414, 1419, 1363 and 723 kg ha-1 (Singh and Jolly, 2004b). Other researchers have also reported lower yields of soybean with delayed planting (Oplinger and Philbrook, 1992; Egli and Bruening, 2000; De Bruin and Pedersen, 2008a).

A timely sown crop generally results in higher yields than late-sown crop, unless there is a specific problem such as drought, waterlogging, high incidence of insect pests and disease or lodging. Higher yields in the timely sown crop may be due to better plant growth and yield attributes, longer maturity duration and higher agroclimatic indices such as growing-degree days, heliothermal units and photothermal units (Table 7.3). Lower yields in late-planted crops could be due to a variety of reasons, including shifting of the reproductive phase into less favourable environment (shorter days, lower temperatures and insolation), less availability of soil moisture and a shorter growth period.

Table 7.3. Agroclimatic indices at physiological maturity, total dry matter and seed yield of soybean cultivar PK 416 sown on different dates at Ludhiana, Punjab, India (adapted from Hundal et al., 2003).

Accumulated

Accumulated

Accumulated

Total dry

Seed

growing-degree

heliothermal

photothermal

matter

yield

Sowing date

days (°C day)

units (°C day h)

units (°C day h)

(kg ha-1)

(kg ha-1)

6 June 1997

2448

19001

32201

7348

1632

23 June 1997

2301

18390

29913

6116

1462

7 June 1999

2669

21085

35843

6163

1723

21 June 1999

2544

20097

33888

5862

1528

Table 7.4. Effect of planting date and maturity group on vegetative growth and reproductive growth characteristics, yield and yield components of soybean in Lexington, Kentucky, USA (average across years) (adapted from Egli and Bruening, 2000).

Maturity group

Table 7.4. Effect of planting date and maturity group on vegetative growth and reproductive growth characteristics, yield and yield components of soybean in Lexington, Kentucky, USA (average across years) (adapted from Egli and Bruening, 2000).

Maturity group

Character

Sowing date

I

II

III

IV

Node number (no. per m2)

Early (mid May)

827

850

1071

1578

Late (late June)

662

638

755

798

Above-ground vegetative

Early (mid May)

566

535

821

815

mass (g per m2)

Late (late June)

513

533

633

656

Length of flowering and pod

Early (mid May)

26

32

37

37

set (R1-R5) (days)

Late (late June)

21

23

25

26

Seed-filling period (R5-R7)

Early (mid May)

31

31

31

37

(days)

Late (late June)

31

33

35

35

Yield (g per m2)

Early (mid May)

343

403

410

400

Late (late June)

291

312

313

300

Seeds per m2

Early (mid May)

2141

2164

2575

2391

Late (late June)

1881

1604

1940

1850

Seed size (mg seed-1)

Early (mid May)

161

186

159

171

Late (late June)

155

195

162

162

Early flowering and a shorter vegetative growth phase in late-planted soybean results from the combined effect of photoperiod and temperature. In late-planted crop, due to early flowering, plants are shorter and have fewer nodes, resulting in fewer seeds per unit area, lower seed size and ultimately lower seed yields (Table 7.4). Delayed planting shortens the flowering and pod set period, but not the seed-filling period. Under late planting, soybean yield may be increased to some extent by the use of a high plant population and narrow rows.

Planting date not only influences the seed yield, but also the quality of soybean oil. The quality of soybean oil can be improved by reducing palmitic acid (16:0) and linolenic acid (18:3). Early planting (24-29 May) has been found to decrease linolenic acid, while late planting (22-28 June) decreased palmitic acid levels in modified fatty acid breeding lines of soybean (Ray et al, 2008).

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