August 2011
Growth and yield response to plant population of two cotton
varieties with different growth habits
Guangyao (Sam) Wang, Ruth K. Asiimwe, and Pedro Andrade
Maricopa Ag Center, University of Arizona
Summary
When determining desirable cotton plant population, plant growth habits need
to be considered since they affect plant interactions directly. A field experiment
was conducted at Maricopa Agricultural Center to investigate the effects of
plant population on cotton growth and yield using a columnar type variety
DP161 and a bush type variety ST4498. Three plant populations (25K, 45K, and
60K plants/acre) were tested. The lowest density reduced cotton growth and
yield significantly in the columnar type variety DP161 but not in the bush type
variety ST4498 compared to the 45K plant population treatment. The bush type
variety ST4498 was able to compensate for the low plant population better than
the columnar type variety DP161 by increasing growth rate and reducing dry
matter partitioning to stems. This study suggests that growth habits should be
considered in deciding target plant populations.
Introduction
Seed costs in cotton production have increased dramatically in the past decade and have now become a significant
portion of total input costs. For a planting rate of 10 lb/acre, the cost of the seed and technology fee is about 70
dollars/acre, a 10-fold increase from 1996. Refining the recommended plant population to maintain crop yield with
lower planting rates, thus reducing input costs, could be significant to increased Arizona growers’ profitability.
Currently the recommended cotton plant population in Arizona is in the range of 25-50K/acre (Silvertooth, 2001).
This recommendation varies with a number of factors such as field location, planting date, soil type, and cotton
varieties (Silvertooth, 1999). Thus it is possible for growers to narrow down the target population by their specific
farming conditions and management practices.
Crop growth habit affects plant-to-plant interactions, and therefore needs to be accounted for in recommending plant
population. For example, varieties with a bush-type growth habit might have a different optimal plant population
compared to a columnar-type variety. Thus, a field experiment was conducted in 2010 to investigate the effects of
plant population on cotton growth and yield using two varieties with different growth habits.
Materials and Methods
The field experiment was conducted at the University of Arizona, Maricopa Agricultural Center, Maricopa in the
2010 growing season. Cotton was planted in a single row in dry soil on 40-inch beds at a rate of 80,000 plants/acre
on April 7, 2010 and watered up the next day. Two cotton varieties were used in this study: a bushy type Stoneville
ST-4498-B2RF (ST4498) and a columnar type Delta Pine 161-B2RF (DP161).
The experiment was a randomized complete block design with four replications. Each plot was eight rows wide and
50 feet long. Cotton was thinned to three final plant populations on May 6: low density 25K (LD), regular density
Arizona Cotton Report (P-161) August 2011
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45K (RD), and high density 60K (HD) plants/acre. Plant were sampled on June 1, June 10, June 21, July 1, July 12,
July 23, August 3, and September 10 during the growing season. At each sampling date, plant height was measured
for five randomly-chosen plants per plot and cotton plants in a 3.3 feet row length were cut were cut at the soil
surface and the number of plants recorded. From the sampled plants, the number of nodes for five plants was also
counted. Leaves, stems, and reproductive structures (if any) from the sampled plants were separated. The area of
fresh green leaves was then measured with an optical leaf area meter. Due to the large biomass, a subsample was
used to measure and estimate leaf area for the last three sampling dates. Leaves, stems, and reproductive structures
were dried at 70oC with ventilation until a constant weight was reached, their dry mass was recorded.
The crop was managed following practices common to cotton production in the Central Arizona. Pix Plus
(Mepiquat Chloride, 7969-173, BASF) was applied at 16 ounces/acre on June 30 to suppress vegetative growth. The
last irrigation was applied on September 2 and cotton was defoliated in late September. Cotton plants in one of the
middle two rows in each plot were harvested in November using a one-row cotton picker and weight of seed cotton
recorded.
Results and Discussion
Cotton growth response to plant population and growth habit
Early in the season, plant was significantly taller in the higher plant population treatments (45K and 60K) compared
to the 25K treatment (Table 1), which may be due to inter-plant competition for nutrients and light. However, the
differences were less significant later in the season. As expected, the columnar variety DP161 was taller than bush
type variety ST4498 over the entire growing season.
The 25K treatment produced lower biomass early in the season compared to the 45K and 60K treatments (Table 2).
With the columnar type variety DP161, the differences were still significant on the July 1 and August 3 sampling
dates. However, with the bush type variety ST4498, the differences in biomass among plant population treatments
were not significant after the June 1 sampling date, indicating a higher compensatory growth response to low plant
population than the columnar type variety DP161.
Early in the growing season, biomass was partitioned to cotton leaves more than other plant structures by both
varieties. However, the percentage of leaf biomass to total biomass decreased from about 75% at germination to
about 20% at the end of the growing season. Stem weight was about 25% of the total biomass at cotton emergence,
but peaked to 55% between first bloom and peak bloom, and then decreased to 20-30% at the end of growing season
(Data not shown). The weight of bolls, flowers, and squares (BFS) increased from 0% at about 700 heat units (HU)
to 50-60% of the total biomass at the end of the growing season (Figure 1). Not surprisingly, the taller columnar
type variety DP161 partitioned more biomass into the stems than the bush type variety ST4498 at all three plant
populations throughout the growing season. Generally, in a similar pattern to total biomass production, cotton plants
from the 25K treatment produced significantly less leaf mass and stem mass during the early growth stages than the
higher plant population treatments, which caused a slower reproductive structure (BFS) increase per unit area for the
low density treatment during the growing season (Figure 1). The two varieties showed significant differences in
biomass growth. The differences among the plant population treatments were smaller in the bush type variety
ST4498 than in the columnar type variety DP161.
Cotton yield response to plant population
Columnar type variety DP161 produced higher crop yield at a density of 45K than the 25K low population treatment
(Figure 2). With the bush type variety ST4498 there was no difference in cotton yield caused by plant population
treatments. The smaller differences in yield among the plant population treatments indicate greater ability to
compensate plant population changes with the bush type variety ST4498.
Cotton in low densities will grow into a closed canopy later in the growing season and produce acceptable yields
(Galadima et al., 2003). However, for the columnar type variety DP161, space and sunlight were not fully utilized
under lower population densities, resulting in reduced yield compared to the higher plant population treatments. The
bush type variety ST4498 was more likely to compensate for the low plant populations and produce yields
comparable to the higher plant populations.
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References
Galadima A., S.H. Husman, and J.C. Silvertooth. 2003. Plant population effect on yield and fiber quality of three
upland cotton varieties at Maricopa Agricultural Center, 2002.
http://www.cals.arizona.edu/pubs/crops/az1312/az13121e.pdf
Silvertooth, J.C. 1999. Row spacing, plant population, and yield relationships.
http://ag.arizona.edu/crops/cotton/comments/april1999cc.html
Silvertooth, J.C. 2001. Plant population evaluation/management for cotton.
http://cals.arizona.edu/pubs/crops/az1203.pdf
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Table 1. Cotton plant height (Inches) of a columnar type variety DP 161 and a bushy type variety ST 4498 at three
plant populations (LD, RD, and HD) during the 2010 growing season.
Date
Jun 1
Jun 10
Jun 21
Jul 1
Jul 12
Jul 23
Aug 3
Sept 10
Variety
Heat units (F)**
845
1074
1333
1602
1913.0
2254
2562
3613
5.9
9.8 b* 15.6 b
23.7 b
29.0
32.2
34.9
42.4
LD (25K)
6.6
11.2
a
18.4
a
24.3
ab
29.3
32.6
37.6
44.5
DP161
RD (45K)
6.7
11.4 a
18.0 ab 26.2 a
28.4
36.1
35.6
46.7
HD (60K)
5.3 b
9.1 b
15.2
20.4
24.7
27.1
26.8 b
35.1
LD (25K)
5.5
ab
8.9
b
15.6
21.7
23.1
27.7
30.7
a
37.5
ST4498
RD (45K)
5.9 a
10.1 a
15.8
19.3
23.8
26.4
28.5 ab
33.9
HD (60K)
* Means followed by the same letter within each variety and sampling date are not statistically different according to
Fisher's least significant difference mean separation tests.
** Cotton heat units (86/55F as the ceiling/base temperature) were obtained from The Arizona Meteorological
Network (http://www.cals.arizona.edu/azmet/).
Table 2. Plant biomass (lbs/acre) of a columnar type variety DP 161 and a bushy type variety ST 4498 at three plant
populations (LD, RD, and HD) during the 2010 growing season.
Date
Jun 1
Jun 10
Jun 21
Jul 1
Jul 12
Jul 23
Aug 3
Sept 10
Variety
Heat units (F)**
845
1074
1333
1602
1913.0
2254
2562
3613
124
b*
292
b
873
c
1714
b
2978
5261
6974
b
13704
LD (25K)
DP161
239 a
485 a
1285 b
2108 b
3457
6811
8938 a
16563
RD (45K)
238 a
597 a
1531 a
2585 a
3463
6616
8470 a
16654
HD (60K)
177 c
369
1164
1835
3228
6555
6962
12881
LD (25K)
ST4498
241 b
410
1273
2195
3238
5962
7934
15003
RD (45K)
284 a
591
1274
2206
4000
5833
7104
12376
HD (60K)
* Means followed by the same letter within each variety and sampling date are not statistically different according to
Fisher's least significant difference mean separation tests.
** Cotton heat units (86/55F as the ceiling/base temperature) were obtained from The Arizona Meteorological
Network (http://www.cals.arizona.edu/azmet/).
Arizona Cotton Report (P-161) August 2011
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Leaf
BFS
10
Stem
Dry matter (1000 lbs/A)
Dry matter (1000 lbs/A)
10
8
6
4
2
Leaf
BFS
Stem
8
6
4
2
DP161-LD
Leaf
BFS
10
Stem
8
6
4
2
0
500
10
Dry matter (1000 lbs/A)
1000 1500 2000 2500 3000 3500 4000
Heat unit (F)
DP161-RD
1000 1500 2000 2500 3000 3500 4000
Heat unit (F)
6
4
2
2
1000 1500 2000 2500 3000 3500 4000
Heat unit (F)
ST4498-RD
1000 1500 2000 2500 3000 3500 4000
Heat unit (F)
Leaf
BFS
Stem
8
6
4
2
DP161-HD
0
500
Stem
4
Stem
8
Leaf
BFS
6
10
Leaf
BFS
1000 1500 2000 2500 3000 3500 4000
Heat unit (F)
8
0
500
Dry matter (1000 lbs/A)
Dry matter (1000 lbs/A)
10
ST4498-LD
0
500
Dry matter (1000 lbs/A)
0
500
ST4498-HD
0
500
1000 1500 2000 2500 3000 3500 4000
Heat unit (F)
Figure 1. Cotton dry matter partitioning in columnar type variety DP 161 and bush type variety ST 4498 planted at
three densities (LD – 25K, RD – 45K, and HD – 60K) during the growing season. BFS refers to the total dry mass
of bolls, flowers, and squares. Cotton heat units (86/55F as the ceiling/base temperature) were obtained from The
Arizona Meteorological Network (http://www.cals.arizona.edu/azmet/).
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6000
a
Seed cotton yield (lbs/A)
ab
b
a
a
a
5000
4000
3000
25K
45K
DP161
60K
25K
45K
60K
ST4498
Figure 2. Seed cotton yield response to plant population for a columnar type variety DP161 and a bushy type variety
ST4498. Means followed by the same letter within each variety are not statistically different according to Fisher's
least significant difference mean separation tests.
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