soil& Tillage Research

Soil & Tillage Research 40 (1996) 73-88

The effects of manure, genotype, seed priming, depth and date of sowing on the emergence and

early growth of Sorghum bicolor (L.) Moench in semi-arid Botswana.

D. Harris ’

SADCC/ ODA Land and Water Management Project, Private Bag, 00108 Gaborone, Botswana

Abstract

Surveys of research reports and farmers’ opinions in semi-arid Botswana suggested that poor stand establishment of cereals was common and a major cause of low yields. Of 146 researcher- managed trials conducted since 1979, 40% failed to germinate, emerge or establish properly. Analysis of a subset of 84 trials for which more detailed data were available showed that the most common reason for failure was ‘soil too dry’ at, or just after, sowing. Therefore we tried to improve post-seeding conditions by agronomic means.

Sorghum seeds were sown on nine planting opportunities during the 1990- 1991 wet season, all after substantial rainstorms. Adding manure did not affect seedling emergence but enhanced growth over the first 25 days after sowing (DAS) when the soil did not dry out rapidly. Two varieties (Segaolane and 65D) differed significantly and consistently throughout the experiment, with Segaolane emerging more quickly and growing more vigorously. The effect of depth of sowing was inconsistent between sowings.

Establishment was more successful if rain fell or if the soil dried out slowly after sowing.

Consequently, both stand count and dry matter (plot-’ and plant-‘) at 25 DAS were directly proportional to the rate at which seedlings emerged. Individual plant vigour in relation to rates of emergence was explained in terms of the interaction between sorghum morphology and rapid soil-drying. Successful establishment was achieved when the soil dried slowly or when seeds germinated and emerged quickly.

Soaking seed in water immediately prior to sowing (seed-priming), as a way of speeding up germination, was explored in detail in controlled environments. The time taken for seeds to germinate at 30°C decreased as the soaking time increased from 0 to lo-12 h, a treatment in which a 50% saving in time could be achieved. Germination of seeds soaked for 16 h or more was

’ Present Address: Centre for Arid Zone Studies, Unversity of Wales, Bangor, Gwynedd LL57 2UW, UK.

Email: D.Harris@Bangor.ac.uk

0167.1987/96/$15.00 Copyright 0 1996 Elsevier Science B.V. All rights reserved.

PII SO167-1987(96)01047-l

14 D. Hurris/Soil & Tillage Research 40 (1996) 73-88

found to continue even after soaking ceased, suggesting that they would be susceptible to damage in the event of any delays in sowing. Emergence from soil at 30°C was significantly hastened by 23% when seeds were pre-soaked for 6 h or longer. The rate of imbibition during soaking was found to be proportional to temperature. The practicality of seed-priming for improving establish- ment in the field is discussed.

Keywords: Sorghum; Stand establishment; Manure; Sowing date; Seed priming; Botswana

1. Introduction

More than 850000 km2 of the SADCC (Southern African Development Coordinating Conference - now SADC, Southern African Development Community) region, includ- ing virtually all of Botswana, receive only 400-600 mm rain per annum (World Bank, 1981). An area of comparable size receives more rain, but crop production is still constrained by the erratic distribution within the growing season. Many of these areas are considered marginal for sustainable crop production, particularly since most of the rain falls in the summer months when rates of potential evapotranspiration are high. Nevertheless, ever increasing numbers of people live and farm under these conditions.

Good crop stand establishment is considered to be essential for the efficient use of resources like water and light (Monte& and Elston, 1983). This is true in temperate regions or under irrigation or in the humid tropics where, within broad limits, yields are often proportional to plant population density. In the rainfed semi-arid tropics, however, the balance between water supply and demand is critical and smaller, more conservative population densities are often required (Jones, 1987). Nevertheless, uniform stand establishment is still a pre-requisite for cropping success because, under adverse conditions, crowding should be avoided in order to minimise competition for limited soil water. A high percentage germination and emergence are the key to controlling stand establishment. Similarly, vigorous early growth is often associated with greater yields (Okonwo and Vanderlip, 1985; Austin, 1989; Carter et al., 1992). Delayed emergence can reduce the subsequent relative growth rate of the seedling and, in general, healthy plants with well-developed root systems can withstand adverse conditions better than plants whose development and growth have been interrupted at an early stage.

Observations of crops in farmers’ fields in Botswana between 1987 and 1991 suggested that crop establishment is often poor (L&WMP, 1989; L&WMP, 1990; L& WMP, 1991) and Harris (1992) has proposed that a major reason for this is the poor access to timely draught by resource-poor farmers. Crops must be sown when ‘windows of opportunity’ present themselves after a rainstorm. Depending on the waterholding characteristics of the soils and the weather following the storm, these planting ‘ windows’ can be as short as 2 days or as long as a week. Crops are often sown too late after suitable rainfall, when the soil has become too dry (and hot) to permit fast emergence of a large proportion of the seeds, many of which fail to emerge. Support for these observations is provided by the results of a survey of the opinions of 92 farmers in eight villages in Botswana between 1985 and 1987 (DAR, 1988). Thirty-two percent of farmers cited poor plant establishment as a serious constraint to crop production, with more mentioning ‘drought’ effects in general.

D. Harris/ Soil & Tillage Research 40 (1996) 73-88 15

In order to quantify the extent to which poor stand establishment is a problem, a review was made of research reports in Botswana since 1979, the results of which are presented in this paper. On the strength of this review it was deemed essential to investigate in detail the principal causes of poor establishment. Recent work has shown that additions of kraal manure to the lighter soils of Botswana can result in larger yields (Carter et al., 1992) and there is a suggestion that this effect was associated with vigorous growth of seedlings. Consequently, during the 1990-91 season, an experiment was conducted to assess the effect of manure, variety, sowing depth and date of planting on the emergence and early growth of grain sorghum (Sorghum bicolor (L.) Moench). Analysis of the results of this experiment, reported below, led to further experiments to test seed priming (soaking seed in water prior to sowing) as a possible way of overcoming environmentally-induced constraints on stand establishment. These experi- ments are also reported below.

2. Materials and methods

2.1. Survey

One hundred and forty-six researcher-managed field trials (cereals only) from Botswana between 1979 and 1991 were identified for which the success or failure of stand establishment had been reported. Three types of trial were considered: farming systems investigations, on-farm fertiliser trials and general agronomy experiments, and the proportion of failures calculated. A subset of 84 agronomy experiments was analysed in more detail and reasons for failure tabulated.

2.2. Field Experiment

Six main plots (15 m X 5 m>, two in each of three blocks, were located at Sebele Agricultural Research Station, near Gaborone (24” 34’S, 2.5” 57’E, elevation 990 m). The soil was a Ferralic Arenosol of granitic origin. Depth to the C horizon was 1.0 m and the loamy sand topsoil had the following properties: CEC, 3 meq (100 g)-’ ; pH (CaCl), 4.9; organic carbon content, 2 g kg-‘; available P (Olsen/Bray), 1 p.g g- ‘. The overall slope of the field was 0.5%. Kraal manure was spread evenly over one of each pair of plots (chosen at random) at a rate equivalent to 20 Mg ha-’ on 21 August 1990. All plots were ploughed on 10 October 1990.

Seeds were sown by hand on nine occasions, under optimum conditions of soil moisture following substantial rainstorms, between 26 November 1990 and 11 February 1991. Exact dates and the size of each preceeding rainstorm are shown in Tables 1 and 2. On each of eight dates, one randomly-located sub-plot of 2 m X 1 m in each main plot was hand-dug and sown with Segaolane and 65D (the two most widely grown varieties in Botswana) sorghum seed at target depths of 3 cm and 6 cm. Two replicate sowings were made on 06 January 1991 (dates 3 and 4) as a check. Each variety X

depth treatment combination was sown as 30 seeds spaced evenly in a 2 m length of row, each row being 0.25 m apart. Carbofuran granules were applied to the sowing

Tab

le

I M

ain

eff

ect

of s

owin

g da

te

on v

aria

bles

m

easu

red

2.5

days

af

ter

sow

ing

(exc

ept

rate

s of

em

erge

nce)

. D

iffe

renc

es

sign

ific

ant

at P

< 0

.001

fo

r al

l va

riab

les.

P

Var

iabl

e D

ate

$ I

2 3

4 5

6 7

8 9

SE

D

a 2.

Dat

e an

d am

ount

of

pre

ceed

ing

rain

26

/11

23/1

2 06

/O

I 06

/01

IO/O

1 24

/01

25/0

1 29

/Ol

I l/O

2 ;

38 m

m

24 m

m

36 m

m

36 m

m

32 m

m

30 m

m

l2m

m

22 m

m

38 m

m

Num

ber

of l

eave

s (p

lan-

’ )

5.

27

4.03

6.

39

6.61

5.

56

6.40

6.

69

6.19

5.

12

0.33

;

Hei

ght

(cm

pl

an-

’ )

5.18

3.

80

9.54

10

.07

7.20

10

.85

12.7

4 10

.37

5.01

1.

09

Y

9=:

Mes

ocot

yl

leng

th

(cm

) 2.

42

2.85

3.

23

3.15

2.

85

2.97

3.

50

3.29

3.

77

0.1

I 2

No.

of

root

ax

es

(pla

n-

’ )

4.52

2.

24

8.75

9.

30

5.04

7.

51

8.74

6.

32

3.60

0.

86

? Sh

oot

dry

wei

ght

(g p

lot-

’ )

2.

52

1.03

-

b 14

.53

4.7

I 16

.21

19.1

7 15

.11

2.53

3.

04

(28)

2 2

Shoo

t dr

y w

eigh

t (m

g pl

an-

’ )

341

155

_b

722

337

746

875

679

188

I30

(28)

$

Em

erge

nt

seed

lings

(p

lot-

’ 1

2

7.62

7.

87

18.6

7 19

.37

13.4

6 20

.46

21.5

4 21

.33

12.8

8 I .

39

s

Rat

e to

50%

em

erge

nce

(h-

’ , X

IO

31

0.63

0.

10

9.40

9.

48

2.19

8.

12

10.1

6 8.

26

5.14

8

0.88

Rat

e to

20%

em

erge

nce

(h-l

, X

IO

’)

4.50

2.

79

I I.

51

1 I.

56

7.17

10

.44

12.7

0 11

.33

8.42

0.

62

? 2 a

SE

D

= st

anda

rd

erro

r of

dif

fere

nce;

er

ror

degr

ees

of

free

dom

=

36 e

xcep

t w

here

in

dica

ted

in p

aren

thes

es.b

D

ata

mis

sing

.

D. Harris/Soil & Tillage Research 40 (1996) 73-88 77

Table 2

Significant (P < 0.05) effects of factors and interactions on variables at each date of planting.

Variable Date

1 2 3 4 5 6 7 8 9

Date and amount of preceeding

ram

Number of leaves (plan- ’ ) Height (cm plant- ’ ) Mesocotyl length km)

No. of root axes (plan- ’ ) Shoot dry weight (g plot- ’ ) Shoot dry weight (mg plan-

Emergent seedlings (plot- ’ ) Rate to 50% emergence

(h-l, X 103)

Rate to 20% emergence

(h- ‘, X 103)

‘1

V d

d m.v

d

m.v.d

m.v.d

06/01

36mm

m

m,v.d

d

- ’

_’

V V

v.d

06/01

v,d v,d v.d

v.d

V

V

V

V

IO/O1

32mm

d

d

d

d

d

d

v.d

24/01 25/01 29/01 11/02

30mm 12mm 22mm 38mm

v.d

d v.d

v,d v.d d v,d V v.d

v.d V m.d

V v.d V V

d v.d d.m.v m.d

v.d v.d m.v.d d

m, manure; v, variety; d, depth of planting (e.g. m.v represents a significant interaction between manure and

variety).’ Data missing.

furrow at a rate of 1 g m- ’ of row to minimise seed predation. Rows were randomised within each sub-plot at each date. The experimental design was thus a split-split plot design with manure as the main plot treatment, sowing date as the first split and a 2 X 2 factorial combination of variety and sowing depth as the second split.

Regular (twice daily) counts were made of the number of plants emerging, and rates of emergence were calculated as the recriprocal of the time taken for 20% and 50% of the seeds sown to emerge. All plants were harvested 25 days after sowing (DAS). Plants were dug up with as much of the root system as possible intact and the numbers of leaves, tillers and root axes (with and without secondary branches) were counted. Plant height and mesocotyl length were measured and the dry weight of the seedlings was measured after 3 days in an oven at 85°C (for eight dates only - material from date 3 was lost). Rainfall and ‘A’ pan evaporation data were recorded at a nearby meteorologi- cal station throughout the period of the experiment.

2.3. Seed priming

2.3.1. Experiment I. Samples of 100 sorghum seeds (cv Segaolane) were soaked in distilled water at 30°C

in an incubator for 0, 1, 2, 3, 4, 5, 6, 8, 10, 12, 16, 20, and 24 h. At the end of the soaking period all seeds were blotted carefully between paper towels and then either set to germinate on moist filter paper in petri dishes at 30°C or left to dry for 8 h at 30°C then set to germinate in the same fashion. The delay was designed to simulate the time which might elapse between soaking and sowing in the field. There were three replicate seed lots for each soaking time X delay treatment combination. Germination was assessed every 4 h throughout the experiment. Seeds with radicles exceeding 5 mm in length were recorded and removed. Water was added to dishes as necessary.

78 D. Hurris/Soil & Tillage Research 40 (1996) 73-88

2.3.2. Experiment 2. A small piece of the land used in the field experiment was irrigated uniformly and

allowed to drain for 2 days during the winter when evaporation rates (‘A’ pan values) were less than 2 mm day-‘. The soil was then mixed thoroughly to a depth of 20 cm, and 1000 cm3 samples put into shallow trays which were immediately wrapped in plastic to minimise water losses while in transit. Three replicate lots containing 50 sorghum seeds (cv Segaolane) were soaked in distilled water for 0, 3,6, 12, 18, and 24 h in an incubator at 30°C. After soaking, seeds were dried by rubbing them vigorously with a coarse towel in order to simulate abrasion during the sowing operation. Observations at this stage showed that any radicles protruding were broken off during this treatment. Seeds were then sown at 1.5 cm depth in the soil trays at 30°C. Seedling emergence was assessed every 4 h throughout the experiment. Emergent seedlings were noted but were not removed, in order to avoid disturbing other seeds in the tray. The rate at which the soil dried was calculated after weighing each seed tray after each assessment and adjusting for the weight of the seeds.

2.3.3. Experiment 3. Additional weighed seed samples were included in experiments 1 and 2, and seed

water contents were measured by weighing samples after soaking and after oven-drying (3 days at 85°C). Rates of imbibition by soaked seeds were also measured in the same fashion at 20°C and 40°C. In experiment 2 the rate of imbibition in moist soil was calculated after recovering seeds at various times after sowing, weighing, drying and re-weighing them.

3. Results

3.1. Survey

Results of the review are shown in Table 3. Poor establishment was observed in about 40% of cases, with remarkably good agreement between the three different categories of trial (Table 3a). Further analysis of the subset of 34 out of 84 agronomy experiments conducted by the Land & Water Management Project supports the view that lack of soil moisture is the principal reason for failure (Table 3b). Reasons for crop failure are not often reported in detail in the literature, so many opportunities are lost for further analysis of causes and mechanisms. In addition, experience suggests that constraints on early growth of seedlings are often not expressed clearly and will not be detected unless some measurement of early growth rate is attempted. This is not common in ‘classical’ trials. Consequently, 40% is likely to be an underestimate of the importance of poor establishment as a serious problem.

3.2. Field Experiment

The results of the overall analysis of variance concerning main effects of manure, variety, sowing depth and date of sowing are summarised in Tables 4-6 and 2,

D. Harris/ Soil & Tilluge Research 40 (1996) 73-88 79

Table 3

The incidence of, and reasons for, poor establishment of cereal crops in 146 field trials in Botswana 1979-91.

(a) Percentage of all cereal sowings resulting in poor establishment. (b) Reasons for ‘failure’ in 34 out of 84

agronomy experiments.

(alType of trial Total no. of trials ‘Failures’ (o/o)

Farming systems 17 41

Fertiliser trials 45 40

Agronomy 84 40

Total 146

(b)Reason for ‘failure’

Soil too dry

Pests (including cattle)

Flooding

Storm damage

Poor sowing

Soil chemical

Total

Total no. of trials

34

‘Failures’ (o/o)

62

23

6

3

3

3

100

respectively. Adding manure at a rate of 20 Mg ha-’ tended to increase all growth and development variables, although none of the differences were significant (Table 4). Data in Fig. 1 suggest that early growth benefits from manuring are only gained when other constraints (e.g. lack of water) are not present. There appears to be no evidence of any overall effect of manure application on emergence.

There were clear differences between the two varieties tested for all variables except number of leaves plant-’ and plant height (Table 5). Segaolane emerged faster and more completely, was heavier and had produced more root axes by 25 DAS. Segaolane is more broadly-adapted than 65D and generally grows larger, although 65D has a slightly shorter crop duration.

Sowing depth had a significant effect on the number of leaves plant-‘, plant height, mesocotyl length, the number of emergent seedlings plot-’ and the rate to 50% emergence (Table 6). Overall, shallow sowing resulted in slightly faster and better

Table 4

Main effect of manure on variables measured 25 days after sowing (except rates of emergence). No differences

were significant (P < 0.05).

Variable No manure Manure SED a

Number of leaves (plan- ’ ) 5.54 6.07 0.20 Height, (cm plan- ’ ) 7.42 9.20 1.14 Mesocotyl length (cm) 3.08 3.15 0.08 No. of root axes (plan- ’ ) 5.53 6.92 0.74 Shoot dry weight (g plot- ’ ) 7.73 11.22 3.32 Shoot dry weight (mg plant ’ ) 407 604 149.7 Emergent seedlings (plot- ’ ) 16.19 15.63 0.47 Rate to 50% emergence, (h- ‘, X 10’) 5.97 5.91 0.33 Rate to 20% emergence, (h- ’ , X 103) 9.10 8.77 0.33

a SED = standard error of difference; error degrees of freedom = 2.

80 D. Harris/ Soil & Tilluge Research 40 (1996) 73-88

Table 5

Main effect of variety on variables measured 25 days after sowing (except rates of emergence).

Variable Variety Significance ’ SED b

Number of leaves (plan- ’ ) Height (cm plan- ’ ) Mesocotyl length (cm)

No. of root axes (plan- ’ ) Shoot dry weight (g plot- ’ ) Shoot dry weight (mg plan-‘)

Emergent seedlings (plot- ’ ) Rate to 50% emergence, (h- ’ , X 10’)

Rate to 20% emergence, (h- ’ , X 103)

Segaolane 65D

5.77 5.84

8.03 8.59 3.32 2.91 6.68 5.77

11.04 7.91

537 473

17.44 14.38 6.78 5.11

9.29 8.58

NS 0.10

NS 0.20 *** 0.05 *** 0.20 *** 0.58 (32) ** 22 (32) *** 0.51 *** 0.28 ** 0.21

a NS, not significant, * , * * and * * * significantly different at P < 0.05, 0.01 and 0.001, respectively.b SED

= standard error of difference; error degrees of freedom = 36 except where indicated in parentheses.

emergence, whereas deeper sowing was associated with a faster rate of leaf production and taller plants at 25 DAS.

Sowing date was the most important influence on emergence and seedling vigour in this experiment (Table 1). Effects on all variables were highly significant ( p < 0.001) and rate of emergence at any date was linked to subsequent seedling growth and development, presumably because of environmental factors. There was no pattern with respect to the progress of the season. Establishment was highly variable between planting dates and Table 2 summarises the significant main effects of, and interactions between, the experimental factors for separate analyses of variance at each date. Adding manure affected emergence significantly only on date 1, reducing it from 30% to 20%, whereas it increased the number of leaves plan-’ and plant height on date 3. Although not statistically significant, there was a trend (data not shown) for adding manure to increase the number of leaves plant- ’ and number of root axes plant- ’ (all nine dates); to increase plant height (seven dates out of nine); and to increase shoot dry weight plot-’ (six dates out f o eight). However, the presence of manure tended to decrease the

Table 6 Main effect of sowing depth on variables measured 25 days after sowing (except rates of emergence).

Variable

Number of leaves (plant ’ ) Height (cm plant- ‘) Mesocotyl length (cm)

No. of root axes (plant- ‘)

Shoot dry weight (g plot- ’ )

Shoot dry weight (mg plant ’ )

Emergent seedlings (plot- ’ ) Rate to 50% emergence (h- ‘, 10’)

Rate to 20% emergence, (h- ‘, X 103)

Shallow Deep Significance a SED b

5.66 5.95 * * * 0.08

8.03 8.59 * * * 0.13

2.39 3.83 * * * 0.06

6.09 6.36 NS 0.16

9.63 9.32 NS 0.56 (32)

492 519 NS 24 (32)

16.44 15.38 * 0.44

6.36 5.53 * * 0.25

8.96 8.92 NS 0.21

a NS, not significant, * , * * and * * * significantly different at P < 0.05, 0.01 and 0.001, respectively.b SED = standard error of difference; error degrees of freedom = 36 except where indicated in parentheses.

D. Harris/Soil & Tillage Research 40 (1996) 73-88 81

Shoot dry matter minus manure (g per plot)

Fig. 1. Total shoot dry matter, g plot-‘, 25 DAS in plots with and without manure for eight sowing dates (data

from date 3 were lost). The line is the 1:l ratio.

final emergence in six out of nine sowings, although there was no discernible influence on rate of emergence. Varietal effects were entirely consistent across dates, with Segaolane emerging faster and more vigorously than 65D.

All seeds were sown in what were judged to be optimal conditions of soil moisture, but emergence and early growth varied widely between dates, although there were no significant differences in any variables between sowings 3 and 4 on 6 January 1991. Thus it is assumed that conditions after sowing affected establishment success. Data from all 24 plots at each sowing date were pooled and overall means were calculated. Fig. 2a, b and c show these mean values for emergence rate, final emergence and total shoot dry matter plot-’ at 25 DAS, respectively, plotted against a moisture stress index, calculated as the cumulative rainfall for 5 days after sowing minus the cumulative potential evapotranspiration (‘A’ pan values X 0.75) over the same period. This index represents the degree to which soil moisture in the seedbed was retained or replenished during the time seeds would be expected to germinate and emerge and is a proxy for direct measurements of soil water content or potential in the surface layers of the seedbed. More negative values represent drying conditions, while rainfall and humid weather after sowing are indicated by more positive numbers.

Emergence was slower and less successful when the soil dried quickly than when it dried slowly or when more rain fell. It is unclear, however, what the form of the relation is between these establishment characteristics and soil drying, but quadratic curves fit the data best. An alternative interpretation might split the data into ‘successful’ and ‘unsuccessful’ sowings, although a more continuous relation with environmental condi- tions seems intuitively more correct.

Seed lots which emerged fastest also produced more complete stands (Fig. 3a) and grew most vigorously over 25 DAS (Fig. 3b). The positive y-intercept in Fig. 3a is an artefact of the way in which rate of emergence is calculated, i.e. individual plots contributing to these mean values can have an emergence rate of zero if final emergence was less than 50%. A second flush of seedlings after initial germination had been

82 D. Harris/Soil & Tilluge Research 40 (1996) 73-88

-40 -20 0 20 40 60 80

Ramfall minus PET for 5 DAS (mm)

- + 0' y 56.1 0 66 x 0.008 Y’

-40 -20 0 20 40 60 80

Ramfall minus PET for 5 DAS (mm)

25 z

CC)

= 20. .

-40 -20 0 20 40 60 80

Ramfall minus PET for 5 DAS (mm)

Fig. 2. Relationship between (a) emergence rate, the inverse of the time to 50% emergence, (b) final

emergence and (c) total shoot dry matter as a function of conditions after sowing (cumulative rainfall minus

cumulative potential evapotranspiration, PET, for 5 days after sowing). Points are means of 24 plots at each

date.

curtailed was observed only on dates 2 and 5, when 53% and 22%, respectively, of all emergent seedlings appeared late.

3.3. Controlled environment experiments I-3

Fig. 4 shows how the time required for 50% germination of seed samples at 30°C decreased as the amount of pre-soaking increased. When seeds were soaked then dried

and left dry for 8 h before being set to germinate those soaked for less than about 12 h did not continue to germinate in the absence of external water (Fig. 4). Thus 12 h would appear to be the maximum ‘safe’ soaking period (at 30°C) if accidents due to unforseen delays in sowing are to be avoided. Germination time (under ideal conditions) was reduced from 29 h to 15 h, a saving of almost 50%.

Soaking for 12 h was associated with a water content of about 50% of oven-dry weight (Fig. 5). This shows how the rate of imbibition depends on temperature and how

D. Harris/Soil & Tillage Research 40 (1996) 73-88

IOO- (a)

80-

y = 26.95 + 4391 x : R’ = 0.90

0~000 0.002 0.004 0.006 0.008 0.010 0.012

83

Rate of emergence, per hour

0~000 0.002 0.004 0.006 0.008 0.010 0.012

Rate of emergence, per hour

Fig. 3. (a) Final emergence 25 DAS and (b) total shoot dry matter 25 DAS as a function of the rate for 50%

safe soaking periods might be estimated for other temperature regimes, being longer or shorter at lower or higher temperatures, respectively. This approach, however, does not take into account any differences in rates of metabolic activity at these temperatures which might affect subsequent viability. Seeds in soil at 30°C imbibed water more

c g 25

g 'Z 20 .9 E 2 15 00 9 k IO

,o

g5 i=

0 0123456 8 IO 12 16 20 24

Soaking time, hours

Fig. 4. Time for 50% germination of sorghum seeds held at 30°C immediately after various periods of soaking

in water at 30°C (solid blocks) and after being dried and held for 8 h at 30°C before being set to germinate

(stippled blocks).

84 D. Harris/Soil & Tillage Research 40 (1996173-88

25 30 _j,

Soaking time, hours

Fig. 5. Seed moisture content of sorghum, as a percentage of oven-dry weight, after soaking in distilled water

at 20°C (closed circles), 30°C (open circles) and 40°C (closed squares). For comparison, imbibition of seeds in

moist soil at 30°C is also shown (open squares).

slowly than seeds soaked at the same temperature, presumably because of the higher potential of the free water, and also the less complete seed surface contact with soil.

In experiment 2, initial water content of the soil was 6.63% by weight and declined in a linear fashion at a rate equivalent to 0.44 mm day-‘. This drying pattern did not simulate what would happen in the field where fast drying would take place initially, followed by slower, steady-state rates of evaporation (Idso et al., 1974). Nevertheless, the soil had dried to well below the sowing depth after 77 h. Final emergence ranged between 36% and 70% (Table 7) but was highly variable, and there were no significant differences between soaking treatments although seeds soaked for 0 and 3 h never achieved 50% emergence. In order to compare rates the time for 25% emergence was used because 25% and 50% emergence times were highly correlated in experiment 1 (to,5 = 1.06 r0,25 + 2.67; R2 = 0.96). Rates of emergence are also shown in Table 7. Analysis of variance showed that seeds soaked for 6 h or more took significantly less time to emerge - 36 h instead of 47 h, a saving of 23%. Thus soaking seed avoided the risk associated with 11 h of soil-drying.

Table 7

Final mean emergence (measured after 77 h) and time for 25% emergence from initially moist soil at 30°C.

Soaking treatment at 30°C (h) Final emergence (%) Time for 25% emergence at 30°C (h)

0 45.3 b 46.9 b 3 36.3 b 47.6 b

6 70.0 b 35.6 c 12 54.3 b 36.1 ’ 18 53.0 b 39.2 ’ 24 57.7 b 36.1 ’

SED a 14.6 3.76

’ SED = standard error of difference; error degrees of freedom = IO. bx Within a column, values with the

same letter do not differ significantly at P < 0.05

D. Harris/ Soil & Tillage Research 40 (1996) 73-88 85

4. Discussion

The suspicion that poor crop establishment is widespread in semi-arid Botswana was confirmed by the review presented in this paper. Farmers and researchers all seem to recognise its importance as a constraint to crop production. Good and timely manage- ment are also recognised as ways of minimising establishment failures (Harris, 1992). Sowing good quality seed of an appropriate variety at the right time into a seedbed with adequate moisture will maximise the probability of obtaining a good stand of healthy plants.

Even so, after four out of nine sowings during this season, subsequent lack of rainfall or fast rates of evaporation resulted in poor establishment (Fig. 2). In these cases poor emergence was associated with slow seedling growth to 25 DAS, both on a per plot and a per plant basis. This link between the rate at which a population of seedlings emerges and subsequent early growth is clear (Fig. 3b) but why this should be so for growth expressed per plant (Table 1) requires explanation. Observations of sorghum crops in farmers’ fields (L&WMP, 1991) showed that emergence during hot, dry periods resulting in establishment failures was associated with the non-formation of, or damage to, adventitious root tips. Emergent seedlings were often found to be surviving on water taken up by the primary root alone, from moist soil at depth. Because all other root initials at the base of the stem were surrounded by hot, dry soil, no adventitious roots were able to reach this moisture. This is likely to be a problem peculiar to cereals, particularly sorghum, which relies heavily on nodal, adventitious roots initiated from the base of the stem close to the soil surface.

Fig. 6 shows how the rate of root system development increased with more favourable conditions - a familiar pattern (see Fig. 2) - and these data appear to confirm that the production of root axes per plant is inhibited when the soil dries out quickly. Root and shoot production is, of course, highly correlated (compare Fig. 2c and Fig. 6, and see data in Table 1) and there is much debate on the nature of the mechanisms regulating the balance between them (e.g. Sharp and Davies, 1989). Nevertheless, the spatial separation between moist soil and sites of formation of adventitious roots in cereals as the soil dries rapidly is likely to be a constraint to rapid

y = 6.26 + 0.08 x - 0.00074 x2: R2 = 0.81 ,

-20 0 20 40 60 80

Rainfall - PET for 25 DAS, mm

Fig. 6. Mean number of adventitious root axes plant-‘, measured 25 DAS as a function of conditions after

sowing. Points are means of 24 plots at each date.

86 D. Harris/ Soil & Tillage Research 40 (1996) 73-88

early growth. Blum and Ritchie (1984) showed that when the top 0.3-m soil layer was wet (70% field capacity) initiation and growth of nodal roots proceeded at the maximum potential rate. If moisture around the seed is deficient, however, the development of the secondary root system can be prevented or delayed (Comish et al., 1984). Soman and Seetharama (1992) showed that genetic variation for rate of initiation and growth of nodal roots exists in sorghum, and they suggested that breeding for this trait is possible.

Management factors, such as manure and sowing depth, had a variable effect on establishment success, depending on subsequent conditions. On balance, manuring was beneficial, whereas seedlings emerged more quickly from shallow sowings when conditions allowed them to do so. Seedlings from shallow sowings often did not grow as vigorously as those from deeper sowings, however. Soman and Seetherama (1992) demonstrated that variation in sowing depth did not greatly affect nodal root growth because sorghum can vary the mesocotyl length to bring the crown to the same approximate depth. In the field experiment described here deep sowings produced significantly longer mesocotyls than shallow sowings at all dates (Table 2), but the effect of sowing depth on other growth and development variables was more complex. Given the unpredictability of the post-sowing environment and the ability of sorghum to emerge from up to 10 cm depth, (Carter, 1990) shallow sowings should be avoided if possible.

Apart from the direct effects of soil drying on germination and emergence, indirect effects (e.g. rate of crust formation, or rate of increase in soil temperatures to supra-optimal levels) may be just as important in determining establishment success, and genetic variation exists for traits to ameliorate these conditions (e.g. Agrawal et al., 1986; Harris et al., 1987). Nevertheless, these indirect effects are all time-dependent, and fast germination and emergence are still desirable under those circumstances. The various means by which fast germination can be promoted are not mutually exclusive. Indeed, their effects are likely to be additive, e.g. the use of mulch with pre-soaked seed of a fast-germinating genotype would be an excellent option.

There are many seed pre-treatments used to enhance germination and, sometimes, crop performance (Evenari, 1964; Heydecker and Coolbear, 1977; Hayman and Yokoyama, 19901, but most are far too complicated or costly for use by low-input farmers in the semi-arid tropics. Soaking seed in water for a short period of time prior to sowing seems not to have been tested systematically for small-grained cereals, although the practice is often used with maize (e.g. in Zimbabwe, personal observation, and in Malawi, Z. Kasomekera, personal communication). Pre-soaking seed in a controlled fashion seems to offer benefit, with the advantage that the technique requires no expensive inputs. There are a number of possible drawbacks, although the osmotic shock damage reported in hard-seeded legumes subjected to rapid rehydration (Ellis et al., 1982; Abdullah et al., 1991) has not been observed in sorghum.

In sorghum soaking for too long will render the seed liable to damage during planting if radicles begin to protrude through the seed coat. In the soil tray experiment reported above, any protruding radicle tips were removed prior to sowing. Observations of seedlings at the end of the experiment suggested that the primary root was shorter in treatments where radicle tips had been removed. On the other hand, secondary branching from the mesocotyl region was much more prolific. Since the soil in this region will dry

D. Harris/ Sod & Tillage Research 40 (1996) 73-88 87

rapidly in the absence of rain, enhanced root production is probably of limited value. This hypothesis should be tested in the field at the earliest opportunity.

A number of practical problems are apparent. The data presented here are only of direct relevance to hand-sown seed. Since soaked seeds swell up, planting rates and the efficiency of delivery will change when mechanical planters are used. In simple planters it may be just a matter of changing the size of the controlling aperture, but in planters supplied with standard plates or discs more complicated modifications may be needed to achieve comparable sowing rates. Similarly, relatively soft pre-soaked seed might be more susceptible to physical damage in planters which use ‘knockers’ to eject seed.

The question of how soaked seed reacts to a delay in sowing is very important. Unpredictable weather patterns might easily produce a succession of wet days following the ‘planting’ rain and access to the land might not be possible. Even if seed were surface-dried thoroughly after soaking, a delay of several days is somewhat analogous to the process of controlled deterioration used extensively in the seed industry to assess seed vigour (Matthews, 1980). This is particularly so if seed is stored at high tempera- tures and high humidity. Gurmu and Naylor (1991) showed that storage of imbibed sorghum seed at 45°C for 24, 48 and 72 h significantly reduced both the emergence and subsequent elongation of radicles and coleoptiles. Although an 8-h dry delay caused no ill effects in experiment 1, the possibility obviously exists for deterioration of seed in practice. Careful drying and storage of pre-soaked seed, as advocated by Kathiresan and Gnanarethinam (19851, is obviously required to minimise this, especially since soaking seed removes any seed dressing present, although it may be possible to re-apply dressings after soaking.

The principal difficulty in demonstrating real benefits from the technique is that adverse conditions do not occur after every sowing date and it is likely that previous attempts to test this technique in Botswana (DLFRS, 1980) failed to take this into account. It is essential that the technique is tested over as many sowing dates as possible to increase the chance that some adverse conditions will be sampled.

Acknowledgements

I wish to thank Douglas Thamage for technical support for this work, Boingotlo Sebolai for her invaluable advice and guidance concerning statistical procedures, and the Botswana Department of Agricultural Research for the loan of an incubator and for the provision of seed. The work was funded by the UK Overseas Development Administra- tion.

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