Weed biomass

What is the basis of early vigour, being an
important trait of weed competitiveness in rice
Lammert Bastiaans, Aad van Ast & Dule Zhao

Cultural weed control
Weed control in rice: an excellent example
Standing water: inhibits germination of weed seeds
Use of transplants: provides the crop with a head start

Cultural weed control
Weed control in rice an excellent example
Standing water: inhibits germination of weed seeds
Transplanting: provides the crop with a head start
Developments in S.E.Asia
Limiting resources:
• onfarm labour
• water
(land preparation175750 mm)
→ shift towards direct seeded
aerobic rice

Principle: alteration of competitive relations
YL
Weedcompetitive cultivars make use of the same
principle and might be a good alternative option for
minimizing the negative effect of weeds.
N w
S w
Transplanting
Competitive cultivars

Is there genetic variation for WSA (weed suppressive ability?)
Density: 100 300 500 plants m 2
Variety:
APO
IR60080

Weed biomass used to quantify weed suppressive ability
Weed biomass (g m -2 )
400
300
200
100
0
2003
APO fitted
APO observed
IR60080-46A fitted
0 100 200 300 400 500 600
Plant number m -2
IR60080-46A observed
Weed biomass: Y wc = N w / (b w0 + b wwN w + b wcN c)
2004
b wc → crop competitiveness, time of weeding
N c → seeding rate
Weeding at
3 WAS Weeding at
2 WAS
For both years: APO twice as competitive as IR60080
0 100 200 300 400 500 600
Plant number m -2

Traits that favour weed competitiveness
IR8
Traditional IRtype
Yield ton/ha
6
5
4
3
2
1
0
1 2 3
1 2 3
Mono Mix1 Mix2
IR8
Mahsuri

Analysis using a competition model

Identifying traits with a high potential
Most promising:
early growth
height
characteristics
Relative sensitivity for weed biomass
0.0
-0.5
-1.0
-1.5
-2.0
-2.5
early early growth max.
growth height rate Kdif SLA height

Extensive field study
40 Genotypes of Upland/Aerobic Rice
Germplasm
• Aus (3), Indica (7), Japonica (23), Indica × Japonica (7)
Variety type
• Traditional (4), Improved (36)
Plant type
• Erect ↔ Droopy
Plant height
• 90 – 155 cm
Experiments for determination of:
In weedfree plots: Several (non)destructive plant traits
Weedfree yield
In weedy plots: Weedy yield
Weed biomass

Large genetic variability in WSA (weed suppressive ability)

Indic and aus germplasm most suitable donors
Yield (Mg ha -1 )
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
LSD YF = 0.54
LSD YW = 0.54
LSD W = 39
Indica Aus Indica/
japonica
Short
japonica
Germplasm group
Yield reduction
Weedy yield
Weed biomass
Medium
japonica
Tall
japonica
250
200
150
100
50
0
Weed biomass (g m -2 )

Correlation among traits
Trait Weed Biomass Weed Rating
Early vigour (4 WAS) 0.68 ** 0.82 **
Height (WAS) 0.66 ** 0.70 **
Final height (4 WAS) 0.24 ns 0.18 ns
Crop Biomass 0.33 * 0.43 *
Early vigour and early height are strongly negatively correlated with
weed biomass indication that these traits are related to WSA

Role of plant type
Erect
Droopy

Droopy plant type is not a favourable attribute
Within mixed population of 40 cv’s
Yield w
Weed biomass
Erectness 0.71** − 0.68**
Within japonica germplasm of 23 cv’s
Yield w
Weed biomass
Erectness 0.50* − 0.64*
A more erect plant type was correlated with higher yields (more even light
distribution in the canopy). Surprisingly there was no penalty regarding WSA,
as erectness was associated with a lower amount of weed biomass.

WSA and yielding ability are compatible
Weed biomass
Rice Yield Experiment 1 Experiment 2
Weed free yield − 0.67**
Weedy yield − 0.84** − 0.76**
The highest yielding cultivars, both in weedfree and weedy
conditions, were best able to suppress weeds. This suggests
that there is no tradeoff between WSA and yielding ability

How to select for early vigour?
In field experiment:
Visual score
• Subjective
• Requires sufficient seed
Selection on individual plants
Requirements
• Easy quantifiable \ objective traits
• Nondestructive observation
Additional consideration:
• Are weedsuppressive cultivars characterized by a specific growth
strategy?

How to select for early vigour?
Cultivar
Genotype
group
Early
vigour
score
[ 1 – 9 ]
Weed
biomass
[g / m 2 ]
B6144F Indica 7.2 128 Strong
Vandana Aus 6.9 129 Strong
Weed
suppressiveness
APO Indica 6.1 143 Mediumstrong
IR60080 Tropical
japonica
5.2 183 Medium
Four cultivars that differed in weed suppressiveness and early vigour
were selected for further experimentation

Growth chamber experiment
Germination in coarse sand
Transplanting after 10 days
Growth on water culture

Growth chamber experiment
Frequent harvests
5 8 12 15 19 22 26 dat
Observations
Leaf area
Dry weight of leaf, stem and shoot
Tiller number
Classical growth analysis for 7dayperiods
RGR, NAR, LAR, LWR, SLA

Growth differences similar to field observations
Cultivar
Total
plant dry
weight
[g/plant]
Leaf area
[cm 2 /plant]
Fraction
leaf
Fraction
root SLA
[cm 2 /g]
B6144F 4.67 a 668 a 0.399 bc 0.221 b 314 a
Vandana 5.02 a 521 b 0.402 ab 0.222 b 249 c
APO 2.87 b 322 c 0.411 a 0.193 c 276 b
IR60080 3.37 b 394 c 0.389 c 0.244 a 255 c
The early vigour scores of the cultivars under field conditions were reflected in the
total dry weight and leaf area of the plants in the growth chamber experiment
The allocation pattern of the two slowest growing cultivars was somewhat more
extreme than that of the other two. Is allocation pattern critical for fast growth?

RGR dropped over time for all cultivars
RGR (g g -1 d -1 )
0.35
0.30
0.25
0.20
0.15
0.10
0.05
0.00
B6144-F-MR-6
0 5 10 15 20 25 30
DAT (days)
An indication for intraspecific competition?

Analysis over first 15 days
Cultivar RGR
[d 1 ]
NAR
[mg cm
2 d 1 ]
LAR
[cm 2 g 1 ]
LWR
[g g 1 ]
SLA
[cm 2 g 1 ]
B6144F 0.293 2.06 c 144 a 0.423 b 340 a
Vandana 0.292 2.39 a 122 c 0.382 c 319 ab
APO 0.290 2.27 ab 128 bc 0.393 c 326 a
IR60080 0.296 2.22 ab 133 b 0.444 a 300 bc
• The two fastest growing cultivars were having a completely different growth
strategy:
B6144F invested in quantity (a high LAR)
Vandana invested in quality (a high NAR)
• Fast growth doesn’t seem to be restricted to a single strategy

RGR maintained at higher dry weight
R G R ( g g - 1 d - 1 )
0.35
0.30
0.25
0.20
0.15
0.10
0.05
0.00
0 5 10 15 20 25
DAT
IR60080-46-A B6144-F-MR-6 Vandana APO
IR60080-46-A B6144-F-MR-6 Vandana APO
R G R ( g g - 1 d - 1 )
0.35
0.30
0.25
0.20
0.15
0.10
0.05
0.00
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5
Weight (g)
IR60080-46A B6144-F-MR 6 Vandana APO
APO IR60080-46-A Vandana B6144-F-MR-6
When RGR is plotted against plant dry weight a clear separation
between cultivars with strong and medium WSA becomes apparent

Summary of observations
Total dry weight at 26 DAT varied considerably between cultivars
Differences comparable to differences in early vigour score in the field
Differences not attributable to differences in RGR or any of its underlying
components till 15 DAT
From 15 DAT on, a rapid decline in RGR was observed, most likely due to
intraspecific competition
Comparison of RGR on the basis of plant dry weight revealed clear
differences among cultivars

Hypothesis
Cultivars that were able to maintain a relatively high RGR
were better able to avoid competition
→ Rather than physiology, it might be the morphology that
determines whether a cultivar has a superior weed
suppressive ability
Tiller number provides an indication for this:
B6144F: 7.1 a APO: 4.7 b
Vandana: 7.5 a IR60080: 4.4 b

Current and future research
Focus more on morphology
How does morphology develop over time
What are the morphological differences between cultivars
How does morphology relate to weed suppressive ability

Thank you for your attention