Agronomy Journal Journal of Natural Resources and Life Sciences Education
HOME HELP FEEDBACK SUBSCRIPTIONS ARCHIVE SEARCH TABLE OF CONTENTS
QUICK SEARCH:   [advanced]


     

Published online 13 May 2005
Published in Agron J 97:839-846 (2005)
DOI: 10.2134/agronj2003.0241
© 2005 American Society of Agronomy
677 S. Segoe Rd., Madison, WI 53711 USA
This Article
Right arrow Abstract Freely available
Right arrow Figures Only
Right arrow Full Text (PDF) Free
Right arrow Alert me when this article is cited
Right arrow Alert me if a correction is posted
Services
Right arrow Similar articles in this journal
Right arrow Similar articles in ISI Web of Science
Right arrow Alert me to new issues of the journal
Right arrow Download to citation manager
Right arrow reprints & permissions
Citing Articles
Right arrow Citing Articles via HighWire
Right arrow Citing Articles via ISI Web of Science (4)
Right arrow Citing Articles via Google Scholar
Google Scholar
Right arrow Articles by Hashemi, A. M.
Right arrow Articles by Putnam, D. H.
Right arrow Search for Related Content
PubMed
Right arrow Articles by Hashemi, A. M.
Right arrow Articles by Putnam, D. H.
Agricola
Right arrow Articles by Hashemi, A. M.
Right arrow Articles by Putnam, D. H.
Related Collections
Right arrow Maize Management
Right arrow Maize
Right arrow Plant and Environment Interactions
Right arrow Production Agriculture

Production Papers

Yield Response of Corn to Crowding Stress

Abolhassan M. Hashemia,*, Stephen J. Herberta and Daniel H. Putnamb

a Dep. of Plant, Soil, and Insects Sci., Univ. of Massachusetts, Amherst, MA 01003-9294
b Dep. of Agron. and Range Sci., Univ. of California, Davis, CA 95616-8515

* Corresponding author (masoud{at}psis.umass.edu)

Received for publication October 1, 2003.

   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Plants grown at noncompetitive densities (isolated plants) can be used to relate competitive pressure on yield and yield components at high plant densities. The main objective of this research was to quantify the sensitivity of grain yield and its components to manipulation of crowding stress in corn (Zea mays L.). The experiment was conducted in Deerfield, MA (1986, 1987, and 2000), and Shoush, Iran (1998 and 1999). Three single-ear corn hybrids were planted at six densities (0.25, 3, 4.5, 6, 9, and 12 plants m–2), the lowest density being considered an isolated density. The higher three densities (6, 9, and 12 plants m–2) were combined with three removal treatments, consisting of removal of alternate plants in rows at different stages of growth. Intensity of competition was quantified by comparing grain yield and its components of plants in these densities with those of isolated plants. The highest grain yield in all experimental sites was obtained from 9 plants m–2 and for total biomass yield between 9 and 12 plants m–2. Kernel yield per plant decreased linearly in all hybrids as plant density intensified. All yield components had a linear decline in response to increased competition pressure. The reduction in kernel yield was attributed most to the reduction in number of kernels per row. Removal treatments indicated that early competition during vegetative growth had no or little effect on final grain yield. Plant competition between the vegetative stage and anthesis had a large effect on grain yield reduction, which ranged from 8 to 21% in different hybrids and experimental sites. Increased assimilate supply through plant removal again confirmed that adjustments in grain yield occurred primarily through kernel number per row.

Abbreviations: DAE, days after emergence • GDD, growing degree days (base temperature of 10°C) • R0, no removal • R1, R2, and R3, removal during vegetative growth, 50% tassel emergence, and early grain-filling stages, respectively


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
PLANT DENSITY has been recognized as a major factor determining the degree of competition between plants. Yield per plant decreases as the density per unit area increases (Duncan, 1958). The rate of yield decrease is in response to decreasing light and other environmental resources available to each plant. Yield per plant is also affected by soil fertility (Uhart and Andrade, 1995; Katsvairo et al., 2002), planting date (Cirilo and Andrade, 1994; Nafziger, 1994), level of water availability (Herrero and Johnson, 1981; Schussler and Westgate, 1995), and genotype (Hashemi-Dezfouli and Herbert, 1992a; Modarres et al., 1998; Cusicanqui and Lauer, 1999; Widdicombe and Thelen, 2002). Reduction in yield is mostly due to lower number of ears (barrenness) (Bunting, 1973; Hashemi-Dezfouli and Herbert, 1992a), fewer kernels per ear (Baenziger and Glover, 1980; Karlen and Camp, 1985; Tetio-Kagho and Gardner, 1988; Cox, 1996), lower kernel weight (Poneleit and Egli, 1979; Hashemi-Dezfouli and Herbert, 1992b), or a combination of these components. In dense populations, many kernels may not develop. This occurs in some genotypes due to poor pollination resulting from a delayed silking period compared with tassel emergence (Hashemi-Dezfouli and Herbert, 1992b; Otegui, 1997) and/or due to a limitation in assimilate supply that caused kernel and ear abortion (Iremiren and Milbourn, 1980; Karlen and Camp, 1985; Zinselmeier et al., 1995).

Grain yield per unit area is the product of grain yield per plant and number of plants per unit area. The response is usually parabolic with increased density. At low densities, grain yield is limited by the inadequate number of plants whereas at higher densities, it declines mostly because of an increase in the number of aborted kernels and/or barren stalks. Finding the optimum densities that produce the maximum yield per unit area under different environmental conditions and/or genotypes has been the major concern in many investigations. Tollenaar (1989) concluded that hybrids developed in recent years are able to tolerate higher degrees of crowding stress than older genotypes mainly because of lower lodging frequencies. Superiority of new hybrids over old hybrids at high plant densities have also been related to a greater N use efficiency (McCullough et al., 1994), higher leaf photosynthesis rates (Dwyer et al., 1991), and more efficient stomatal conductance and leaf photosynthesis under water stress conditions (Dwyer et al., 1992). Also, a trend to narrow rows (38 vs. 75 cm) has occurred. Several researchers found no density–row width interaction for corn grain (Nielsen, 1988) and corn silage yield (Cox et al., 1998; Cox and Cherney, 2001).

Some research suggested that inadequate assimilate supply, as can occur in short-season regions, may limit final grain yield (Modarres et al., 1998). Light enrichment from reflectors and fluorescent lamps (Tollenaar and Daynard, 1978; Schoper et al., 1982; Ottman and Welch, 1988) and/or plant removal (Baenziger and Glover, 1980; Schoper et al., 1982) has been shown to increase final grain yield. Conversely, artificial shading (Kiniry and Ritchie, 1985; Reed et al., 1988; Hashemi-Dezfouli and Herbert, 1992b; Andrade et al., 1993) and defoliation (Egharevba et al., 1976; Barnett and Pearce, 1983) resulted in significant decreases in final grain yield.

The timing of competitive stress may also be important. Several studies indicated that competition after flowering was more detrimental to grain yield than competitive pressure during vegetative growth. Labeling studies have shown that less than 10% of grain yield is attributable to assimilates formed before silking (Swank et al., 1982; Simmons and Jones, 1985). However, assimilate level before silking may establish sink capacity (Tsai et al., 1978) and thus may be quite important in determining the final grain yield. Similarly, various environmental and plant stresses may increase the contribution of presilking assimilates to yield (Allison and Watson, 1966).

Since current corn hybrids tolerate crowding stress from higher planting densities better than older hybrids, and higher plant densities are being recommended to farmers, the objective of this research was to quantify the sensitivity of grain yield and its components to changes in crowding stress of older and more recent hybrids. Isolated plants and plant removal were used to provide a quantitative estimate of the extent of competition taking place in the crop community.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Cultural Practices
This experiment was conducted in 5 yr: 1986, 1987, and 2000 in the Connecticut River valley (42° N, 73° W) at the University of Massachusetts Agricultural Experiment Station Farm in Deerfield, MA, and 1998 and 1999 in Shoush, Khuzestan Province (32° N, 49° E), in Southwest Iran.

Deerfield, Massachusetts
The soil type was a Hadley fine sandy loam (coarse-silty, mixed, nonacid, mesic Typic Udifluvent). In all years, the experimental site received a basal application of 66 kg N ha–1, 30 kg P ha–1, and 23 kg K ha–1 broadcast after plowing but before secondary tillage (disking) and planting. A further 100 kg N ha–1 was sidedressed when corn was approximately 40 cm high. Weeds were controlled with a pre-emergence application of cyanazine (2-{[4-chloro-6-(ethylamino)-S-triazin-2-yl]amino}-2-methylporpionitrile) and alachlor [2-chloro-2',6'-diethyl-N-(methoxymethyl)-acetanilide] at the rates of 1.8 and 2.2 kg a.i. ha–1, respectively. Plots were planted in a north–south direction on 9 May 1986, 8 May 1987, and 2 May 2000. Corn was not irrigated at this location as is the common practice in Massachusetts because rainfall is normally considered adequate. Soil moisture measured gravimetrically during all seasons showed no differences among treatments, and there were no visual symptoms of moisture stress.

Shoush, Khuzestan
The soil type in this location was silty clay loam (fine loamy clayey, carbonic Typic Torrifluvent), which is representative of a large area of arable lands in the Khuzestan Province. In both years, the experimental site received 170 kg N ha–1 and 140 kg P ha–1 broadcast after plowing, before planting, and a further 115 kg N ha–1 split in half and sidedressed 21 and 40 d after planting. Soil test revealed that no K was required in both seasons of experiments. Plots were planted on 18 July 1998 and 23 July 1999. Weeds were controlled similar to Deerfield, i.e., pre-emergence application of cyanazine and alachlor at the rates of 1.8 and 2.2 kg a.i. ha–1, respectively. Tillage consisted of moldboard plowing to a depth of 20 cm followed by disking. Irrigation was applied during the growing season when required. The total amount of irrigation water per experimental site per growing season was equal to 343 mm in 1998 and 365 mm in 1999.

Experimental Treatments
Deerfield, Massachusetts
The design of the experiments in 1986, 1987, and 2000 was randomized complete block with three (four in 2000) replicates. Agway hybrid 584S [single ear, late maturity, 1425 growing degree days (GDD)] was seeded in 1986 and 1987 and Northrup King Max 21 (single ear, late maturity, 1460 GDD) in 2000 at five plant densities of 3, 4.5, 6, 9, and 12 plants m–2. The higher three densities (6, 9, and 12 plants m–2) were combined with three removal treatments where alternate plants were cut at the soil surface at three critical stages of growth. These treatments consisted of removal during vegetative growth [V5 to V7, 38 to 52 d after emergence (DAE) = R1], removal at 50% tassel emergence (73 DAE = R2), removal at early grain-filling time (87 to 88 DAE = R3). The five plant density plots with no plant removal were designated as R0. Plots were five rows wide with rows 0.76 m apart and 7.30 m long. The final harvest area for grain and stover yields was 3 m2 (2 m2 in 2000) taken from the central row.

All plots in all years were overseeded and hand-thinned using templates that were marked for proper spacing. This initial thinning of all plots to obtain the desired density was done within 10 DAE in all years. One larger plot in each replicate was planted at a wide spacing or as "isolated plants," 2 m between plants (0.25 plants m–2).

Shoush, Khuzestan
A similar experimental design with four replicates, as in Deerfield, was used at this location in 1998 and 1999. Plots consisted of seven rows, 7 m long and 0.75 m apart, and the final harvest area was 3 m2 taken from the two central rows. In both years, the hybrid SC704 (single ear, late maturity, 1850 GDD) was planted on 27 July. Plant densities were similar to those in Deerfield with removal of alternate plants at 35, 69, and 84 DAE for R1 (V5), R2 (50% tassel), and R3 (early grain fill), respectively.

Final Harvest
Grain harvest occurred after physiological maturity (black-layer formation) and was completed 137 and 131 DAE for Agway 584S in 1986 and 1987, respectively. In 2000, harvest was completed 141 DAE for Northrup King Max 21. At the Shoush location, grain harvest was completed 147 DAE in 1998 and 152 DAE in 1999 for hybrid SC704. In all years, the first ear (uppermost) and second ear (when present) of all plants in the final harvest area were hand-harvested. Total weight of ears and stover were measured in the field. All ears were then dried in a forced-air oven at 80°C for 1 wk. Moisture content of stover was determined from a three-plant subsample in each subplot. Number of productive (ears having at least one complete circle of kernels) and nubbin (small, stunted ears) ears were recorded, and also the number of kernel rows in each productive ear was determined. All the ears were shelled using a hand sheller. Cobs and kernels were dried again and weighed separately. Weight per kernel was determined from 1000 kernel subsamples.

All statistical analyses were performed using either GLM or REG procedures of SAS (SAS Inst., 1991). A combined analysis for 1986–1987 and 1998–1999 indicated nonsignificant year x density (except for grain yield in 1986–1987) and year x removal interactions. Therefore, data are presented as the means for these 2-yr experiments. All differences reported are significant at P ≤ 0.05 unless otherwise stated.


   RESULTS AND DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
CONCLUSIONS
 REFERENCES
 
Density Effect
In all experiments, significant differences were detected among plant densities for grain and total biomass yield and all seed yield components except for kernel rows per ear (all years) and harvest index in 2000 (Table 1). The response of total grain yield for nonremoval plots (R0) for all corn hybrids in all locations was quadratic and reached a maximum at the density of 9 plants m–2 (Fig. 1) . The highest grain yield had a range of 9.6 Mg ha–1 for Agway 584S to 10.7 Mg ha–1 for Northrup King Max 21 in 2000. Total biomass yield also displayed a quadratic response to plant density; however, the maximum yield was reached between 9 and 12 plants m–2 (Fig. 2) . A plant density of 8 plants m–2 is currently used for silage production in Massachusetts and the Shoush area. These results suggest that single-ear hybrids similar to those in this study should be planted at higher densities than are now suggested for silage and grain production. Other research has also shown similar trends of higher densities being required of maximum yield of total biomass (corn silage) compared with densities required for maximum production of corn grain (Cox, 1997). Harvest indices (grain yield/total biomass yield) indicated a quadratic relationship with increasing density (Fig. 3) . Increasing density up to 6 or 9 plants m–2 improved harvest index for SC704, but there was little or no effect of density for the other hybrids. For SC704, the improved harvest index was primarily due to a reduction in non-grain-producing tillers. However, when competition pressure exceeded a certain density (6 plants m–2), the efficiency of assimilate distribution to the ears dropped. Corn silage quality also shows a quadratic response to plant density (Cox et al., 1998). The increase in biomass yield with increased plant density is often offset by decreases in grain contribution, thereby lowering corn silage quality.


View this table:
[in this window]
[in a new window]
  		Table 1. Analysis of variance showing the F-test significance for the effect of density and removal density on yield and yield components.

 


View larger version (19K):
[in this window]
[in a new window]
  		Fig. 1. Effect of plant density and removal on total grain yield for three corn hybrids. R0, R1, R2, and R3 are no removal and removal of alternate plants at vegetative, tasseling, and early grain-filling stages of growth, respectively. Data points represent plant densities after thinning. Results are average of 2 yr except for 2000. L, Q, and NS represent linear, quadratic, and nonsignificant response, respectively. **Significant at P = 0.01.

 


View larger version (19K):
[in this window]
[in a new window]
  		Fig. 2. Effect of plant density and removal on biomass yield for three corn hybrids. R0, R1, R2, and R3 are no removal and removal of alternate plants at vegetative, tasseling, and early grain-filling stages of growth, respectively. Data points represent plant densities after thinning. Results are average of 2 yr except for 2000. L and Q represent linear and quadratic response, respectively. *Significant at P = 0.05; **Significant at P = 0.01.

 


View larger version (19K):
[in this window]
[in a new window]
  		Fig. 3. Effect of plant density on harvest index for three corn hybrids. Results are average of 2 yr except for 2000. L and Q represent linear and quadratic response, respectively. *Significant at P = 0.05; **Significant at P = 0.01.

 
Harvest index for SC704 was considerably lower than both Agway 584S and Northrup King Max 21. While the three hybrids were grown in different years/locations, and are genetically different, a contributing reason for a lower harvest index for SC704 may have been the very high temperatures (about 45°C) that normally occur during the time of pollination. This increases the rate of pollen infertility. Also, high night temperatures (average 31°C for entire growing season) at this location in Iran would result in high rates of respiration at night that would have a negative impact on net assimilation rates. Such high day and night temperatures did not occur at the Deerfield location.

The sensitivity of yield components of corn to increasing crowding stress was similar for the hybrids and locations investigated (Fig. 4) . Plants grown in "isolated" densities (0.25 plants m–2) were used as models to determine the relative level of competition for each yield component. The grain yield and yield components of the plant grown in isolation represented the full yield potential of a genotype at a specific location in a specific year. Thus, yield components, as a proportion of the isolated plant yield component, were analyzed to determine how the yield of these hybrids adjusted to increases in plant density or crowding stress.



View larger version (25K):
[in this window]
[in a new window]
  		Fig. 4. Effect of plant density on ear yield components as a proportion of isolated plants for three corn hybrids. Results are average of 2 yr except for 2000. All lines shown were significant with linear trends (P = 0.01).

 
The effect of density on kernel yield per plant and all yield components (except kernel row number in 2000) was significant (Table 1). All components showed a linear decline with increasing plant density (Fig. 4). The relative impact of density on each yield component can be determined by comparing the position and slopes of the regression lines.

The reduction in grain yield was mostly due to the reduction in number of kernels per row. As plant density increased from 3 to 12 plants m–2, the kernel number was reduced linearly by 27, 36, and 46% in Agway 584S, SC704, and Northrup King Max 21, respectively. Other researchers have reported a significant hybrid x plant density interaction (Hashemi-Dezfouli and Herbert, 1992a; Cox, 1996) where ears per plant in prolific and semiprolific hybrids and kernel number per row in single-ear hybrids were the most sensitive components of kernel yield to increased density. In our earlier study (Hashemi-Dezfouli and Herbert, 1992b), we concluded that poor pollination due to a prolonged interval between pollen shed and silking, and also reduced assimilate supply resulting from the light reduction in high densities, would contribute much to the reduction in kernel number per row. In several studies, number of kernels per plant has been related to intercepted radiation at flowering (Kiniry and Knievel, 1995; Andrade et al., 1999). At supraoptimal densities, the efficiency of the crop to convert intercepted photosynthetically active radiation at flowering into grain sink capacity may decrease (Andrade et al., 1993).

Weight per kernel was also reduced with increased plant density, more so than kernel row number per ear but less than kernel number per row and ear number per plant. Reductions in weight per kernel as density increased from 3 to 12 plants m–2 in Agway 584S, Northrup King Max 21, and SC704 were 21, 9, and 10%, respectively. A complex relationship may exist between assimilate supply and weight per kernel; however, adjustments in kernel number per row perhaps compensated for an assimilate reduction in high densities, allowing remaining kernels to grow at higher grain-filling rates.

Removal Effect
The means of unthinned and thinned plants having the same density after thinning as unthinned plants are shown in Fig. 1. From this analysis, the magnitude of competitive pressure between the period of seed emergence and the time of thinning at different levels of crowding stress can be determined.

In Agway 584S in 1986–1987 and for SC704 in 1998–1999, plants that remained after early thinnings (R1), had a similar yield at harvest as unthinned plants (Fig. 1). These results indicate that early competition from crowding stress had little effect on final grain yield in these two hybrids. In 2000, however, competition between plants at this early stage of growth for Northrup King Max 21 caused a 6% reduction in final grain yield (Fig. 1). The difference could be attributed mainly to delay in plant removal in 2000 (V7, 52 DAE compared with V5, 38 DAE in earlier years).

Results showed that the competition of plants between the periods of vegetative stage (R1) and anthesis (R2) had a large effect on grain yield reduction (P < 0.05). In the 1986 and 1987 experiments, the average reduction in yield that occurred between these two stages of growth was 21%. In 2000, the yield reduction for Northrup King Max 21 between the R1 and R2 stages was 11% compared with 21% for Agway 584S at the same location. While the hybrids differed, the result may also be due to the shorter period of time that occurred between R1 and R2 in 2000 compared with 1986–1987. The amount of yield reduction in 1998 and 1999 for a similar stage of growth was only 8%. Again, high temperatures at Shoush location may have compressed the time period between these two stages for accumulating required GDD. Average daily temperature during the R1 to R2 period in Shoush (month of August) is 33°C (average of 33 yr) while for Deerfield (month of June), the average temperature is 20°C (average of 50 yr). The interaction of density x removal was significant for grain and total biomass yields per unit area in 1986–1987 and 1998–1999 but not in 2000 (Table 1). These results indicated that plants in higher densities often benefited more from thinning treatment than plants growing with lower levels of crowding stress.

The competition between R2 and R3 (tasseling and early grain-filling stage) was significant (P < 0.05), and the average reductions in grain yield were 6, 10, and 22 for 1986–1987, 1998–1999, and 2000, respectively (Fig. 1). These results show that the R2 and R3 growth period is also an important contributor to the grain yield. Compared with the R0 treatment, the grain yield reduction (averaged over all plant densities) with the latest plant removal at R3 (early grain filling) was 28, 25, and 29% for Agway 584S, SC704, and Northrup King Max 21, respectively (Fig. 1). The interactive effects of plant density and removal treatment on grain and biomass yield were also significant. Relative yield losses because of competition that occurred before thinning were generally greater with higher levels of crowding stress. For example, in Agway 584S, the competition in highest density for the latest removal (R3) resulted in a 44% reduction in grain yield compared with the same density in nonremoval (R0) treatment. In the lowest level of crowding stress for this hybrid, the reduction was only 12%.

Yield components of the hybrids examined in our studies showed different responses to plant removal (Fig. 5–7) . The component affected most in all hybrids and locations was the number of kernels per row. For example, in highest densities, plants thinned to their final density at anthesis (R2) had about 18% fewer kernels per row (averaged over hybrids) than those in unthinned plots (Fig. 6). Plant removal did not improve the number of ears per plant in these single-ear hybrids; rather, a delay in thinning resulted in a reduced ear number at higher levels of crowding (Fig. 5). Plants in unthinned low-density plots produced some secondary ears and nonproductive tillers while plants in removal plots, which were initially planted twice as dense as unthinned plots, had no or very few secondary ears and no tillers, especially when removal was delayed until silking or early grain-filling stages. The average kernel weight in secondary ears was less than first ears (data not shown). Plant removal had a significant but small effect on number of kernel rows per ear in all experiments. The effect again was mainly due to the adjustments done primarily through number of tillers and secondary ears, which usually had a fewer number of kernel rows per ear. Number of kernel rows in the primary ear is genetically controlled, and cropping management and/or environmental factors have little effect on it. For example, in our previous study (Hashemi-Dezfouli and Herbert, 1992b), we showed that the compounding effect of highest density and artificial shade (50% reduction in ambient light) reduced the row numbers by less than 10%. However, Tetio-Kagho and Gardner (1988) concluded that in prolific hybrids, number of rows in secondary and tertiary ears may have a greater role in adjusting final grain yield.



View larger version (21K):
[in this window]
[in a new window]
  		Fig. 5. Effect of plant density and removal on ear number per plant for three corn hybrids. R0, R1, R2, and R3 are no removal and removal of alternate plants at vegetative, tasseling, and early grain-filling stages of growth, respectively. Data points represent plant densities after thinning. Results and significant trends are average of 2 yr except for 2000. L, Q, and NS represent linear, quadratic, and nonsignificant response, respectively. Significant at P = 0.05; **Significant at P = 0.01.

 


View larger version (24K):
[in this window]
[in a new window]
  		Fig. 7. Effect of plant density and removal on average kernel weight for three corn hybrids. R0, R1, R2, and R3 are no removal and removal of alternate plants at vegetative, tasseling, and early grain-filling stage of growth, respectively. Data points represent plant densities after thinning. Results and significant trends are average of 2 yr except for 2000. L, Q, and NS represent linear, quadratic, and nonsignificant response, respectively. Significant at P = 0.05; **Significant at P = 0.01.

 


View larger version (22K):
[in this window]
[in a new window]
  		Fig. 6. Effect of plant density and removal on kernel number per row for three corn hybrids. R0, R1, R2, and R3 are no removal and removal of alternate plants at vegetative, tasseling, and early grain-filling stages of growth, respectively. Data points represent plant densities after thinning. Results and significant trends are average of 2 yr except for 2000. L, Q, and NS represent linear, quadratic, and nonsignificant response, respectively. Significant at P = 0.05; **Significant at P = 0.01.

 
Frey (1981) reported that thinning at 50% silking had no effect on number of kernels that already showed some growth. However, enhancement of assimilate supply through thinning at this stage of growth resulted in heavier kernels. In our study, average kernel weight was affected by removal treatments in 4 out of 5 yr. Early thinned plots (R1) had a slightly increased weight per kernel compared with unthinned plants (R0) while a delay in thinning resulted in no change or a decrease in weight per kernel. The delay in plant removal reduced secondary ear number and therefore increased average kernel weight in the low-density plants compared with the unthinned plants (Fig. 7). In 1986–1987, weight per kernel declined in each removal treatment with increasing plant density while in 2000, this linear trend was not significant. Other reports have also shown no significant differences for weight per kernel with thinning at different stages of growth (Wilson and Allison, 1978; Baenziger and Glover, 1980).


   CONCLUSIONS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
REFERENCES
 
In all hybrids and experimental sites used in this study, the kernel yield per plant decreased linearly in response to intensifying crowding stress. Use of isolated plants to index the yield relationship with increasing crowding stress showed that all components of kernel yield in single-ear hybrids had a linear response over the density range and that the reduction in total kernel yield per plant was primarily due to the reduction of kernel number per row followed by either the number of productive ears per plant or kernel weight. Results showed that optimum density for grain yield per unit area was lower than that for total biomass and increasing plant density above densities commonly used by farmers would likely improve corn yields.

Plants remaining after early thinning had similar yield at harvest as unthinned plants at equivalent harvest density, indicating that competition during early vegetative stages of growth had no or little effect on final grain yield per unit area. Plant competition occurring between the periods of vegetative stage (V5) and anthesis and between anthesis and early grain filling had the greatest effect (8–21% and 6–22%, respectively) on grain yield reduction. Yield losses due to crowding stress were generally greater in high densities than low densities.


   ACKNOWLEDGMENTS
 
The authors thank Nazer Aryannia for his valuable assistance in conducting the experiments in Shoush and summarizing the data presented in this paper for this location. This research was supported by the Cooperative State Research Extension, Education Service, U.S. Department of Agriculture, Massachusetts Agricultural Experiment Station, and the Department of Plant, Soil, and Insects Sciences under Project no. NE-132.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 




This article has been cited by other articles:


Home page
 Crop Sci.Home page
S. C. Mason, D. Kathol, K. M. Eskridge, and T. D. Galusha
Yield Increase Has Been More Rapid for Maize than for Grain Sorghum
Crop Sci., July 1, 2008; 48(4): 1560 - 1568.
[Abstract] [Full Text] [PDF]

Home page
 Agron. J.Home page
T. Sarlangue, F. H. Andrade, P. A. Calvino, and L. C. Purcell
Why Do Maize Hybrids Respond Differently to Variations in Plant Density?
Agron. J., June 5, 2007; 99(4): 984 - 991.
[Abstract] [Full Text] [PDF]

Home page
 Agron. J.Home page
J.R. Kiniry, L. Echarte, and A. Hashemi
Comments on "Yield Response of Corn to Crowding Stress" by Hashemi et al. (Agron. J. 97:839-846)
Agron. J., September 19, 2005; 97(5): 1472 - 1472.
[Full Text] [PDF]

This Article
Right arrow Abstract Freely available
Right arrow Figures Only
Right arrow Full Text (PDF) Free
Right arrow Alert me when this article is cited
Right arrow Alert me if a correction is posted
Services
Right arrow Similar articles in this journal
Right arrow Similar articles in ISI Web of Science
Right arrow Alert me to new issues of the journal
Right arrow Download to citation manager
Right arrow reprints & permissions
Citing Articles
Right arrow Citing Articles via HighWire
Right arrow Citing Articles via ISI Web of Science (4)
Right arrow Citing Articles via Google Scholar
Google Scholar
Right arrow Articles by Hashemi, A. M.
Right arrow Articles by Putnam, D. H.
Right arrow Search for Related Content
PubMed
Right arrow Articles by Hashemi, A. M.
Right arrow Articles by Putnam, D. H.
Agricola
Right arrow Articles by Hashemi, A. M.
Right arrow Articles by Putnam, D. H.
Related Collections
Right arrow Maize Management
Right arrow Maize
Right arrow Plant and Environment Interactions
Right arrow Production Agriculture

HOME HELP FEEDBACK SUBSCRIPTIONS ARCHIVE SEARCH TABLE OF CONTENTS
The SCI Journals Crop Science Vadose Zone Journal
Journal of Natural Resources
and Life Sciences Education		 Soil Science Society of America Journal
Journal of Plant Registrations Journal of
Environmental Quality		 The Plant Genome