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a National Corn and Sorghum Research Center, Kasetsart Univ., Klangdong, Pakchong, Nakhonratchasima 30320, Thailand
b USDA-ARS Plant Genetics Research Unit and Dep. Agronomy, Univ. of Missouri, 110 Curtis Hall, Columbia, MO 65211 USA
c Dep. Statistics, Univ. of Missouri, 105 Math Science Building, Columbia, MO 65211 USA
d USDA-ARS Plant Genetics Research Unit retired and Dep. Entomology, Univ. of Missouri, 204 Curtis Hall, Columbia, MO 65211 USA
darrahl{at}missouri.edu
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ABSTRACT |
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 TOPNOTES ABSTRACT INTRODUCTIONMaterials and methodsResults and discussionREFERENCES |
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Abbreviations: ACC-1E, [MoSQA(S7-H)C8 x Georgia Cow Corn](H-1Ear)C8 • ACC-2E, [MoSQA(S7-H)C8 x Georgia Cow Corn](H-2Ear)C8 • BCC-1E, [MoSQB(S8-H)C8 x Georgia Cow Corn](H-1Ear)C8 • BCC-2E, [MoSQB(S8-H)C8 x Georgia Cow Corn](H-2Ear)C8 • SI171-1E, SI171(H-1Ear)C8 • SI171-2E, SI171(H-2Ear)C8 • N, nitrogen
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INTRODUCTION |
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TOPNOTESABSTRACTINTRODUCTIONMaterials and methodsResults and discussionREFERENCES |
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There are a number of reports which indicate the association of greater prolificacy with higher grain yields (Mareck and Gardner, 1979; Moll and Hanson, 1984; Singh et al., 1986; Subandi, 1990; Maita and Coors, 1996). Some researchers reported that grain yields of prolific genotypes were more stable than those of non-prolific genotypes over different environments (Collins et al., 1965; Russell and Eberhart, 1968; Barnes and Woolley, 1969; Miller et al., 1995). Prolific genotypes tended to produce fewer barren plants at higher plant densities than non-prolific genotypes (Russell, 1968; Duvick, 1974). Prior and Russell (1975) found that prolific hybrids out-yielded non-prolific hybrids at low plant density, but they were lower yielding than non-prolific hybrids at high plant density. The data suggested that prolific genotypes compensated with more grain per unit area at low plant density. Inconsistency of grain yield superiority of prolific vs. non-prolific genotypes over different environments might be attributed to genetic background differences of parents or environmental effects.
One concern among maize breeders selecting for prolificacy is that prolificacy seems to have an association with poor stalk strength and plant standability (Lonnquist, 1967; Duvick, 1974; Motto and Moll, 1983; Thomison and Jordan, 1995; Carena et al., 1998). Poor root and stalk strength for a prolific genotype might be caused by competition between root–stalk and ear sinks for photosynthate. The prolific genotype needs more photosynthate delivered to the grain as a major sink. Reduced supply of photosynthate to roots and early remobilization of photosynthate from the stalk to the grain results in earlier root and stalk senescence followed by root and stalk lodging (Duvick, 1974; Dodd, 1977).
Most previous studies that compared prolific with non-prolific genotypes used genotypes with different genetic backgrounds. This likely confounded results of the effect attributed to prolific vs. non-prolific types. To avoid genetic background differences, our study used three populations divergently selected for one- and two-eared types. The objective of this study was to compare stalk strength and other agronomic characters from one- and two-eared selection in three maize populations evaluated at different nitrogen levels and plant densities.
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Materials and methods |
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TOPNOTESABSTRACTINTRODUCTIONMaterials and methodsResults and discussionREFERENCES |
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The MoSQA and MoSQB stalk quality populations were developed by M.S. Zuber, USDA-ARS, at the University of Missouri in 1958. MoSQA is a white endosperm synthetic composed of nine parental inbred lines. MoSQB is a yellow endosperm synthetic including 12 inbred lines as parents. Both underwent eight cycles of S0 recurrent selection for high stalk crushing strength (Zuber, 1973). These were used as sources of one-eared genotypes with high stalk strength in this experiment. `Georgia Cow Corn' was used as a source of prolificacy. It is a late maturing, open-pollinated, prolific cultivar. Georgia Cow Corn might have six or more small ears per plant with average kernel weight of 179 mg (Poneleit et al., 1980). It has a white dent kernel, small red cob, is easily shelled, and is considered poor in stalk strength. SI171 is a yellow endosperm, semiprolific genotype adapted to Missouri that includes various sources of prolificacy (Brotslaw et al., 1988).
Three populations, MoSQA(S7-H)C8 x Georgia Cow Corn, MoSQB(S8-H)C8 x Georgia Cow Corn, and SI171, were initially selected in 1979 with the idea of producing new germplasm with prolificacy and good stalk strength. Phenotypic mass selection for one- and two-eared types was subsequently performed in each population for eight cycles. To obtain one- and two-eared subpopulations, each population was grown in two separate nursery blocks with approximately 500 to 700 plants and a medium plant density (43 000 plants ha-1). Selection for the one-eared subpopulations was done by bulking pollen from 50 to 60 plants appearing to be one-eared, the bulked pollen was used to pollinate 100 to 120 shoots of apparent one-eared plants. At harvest, pollinated plants with a second ear were discarded. An equal number of seeds of selected ears was bulked for the next cycle of improvement. For the two-eared subpopulations, shoot bags were placed only on the second ear of prolific plants before flowering. At pollination time, bulked pollen from 50 to 60 prolific-appearing plants was used to pollinate 100 to 120 of the shoot-bagged plants. At harvest, selected ears were taken from plants that produced a second ear nearly equal or equal in size to the first ear. An equal number of seeds from selected pollinated ears was bulked for planting the next cycle. After two cycles of selection, little separation between one- and two-eared subpopulations was observed, with the majority of the plants bearing two or more ears on a stalk. Beginning with Cycle 3, low (23 800 plants ha-1) and high (71 400 plants ha-1) densities were used with the one- and two-eared subpopulations, respectively, to create environments that better distinguished ear types among segregating plants within each subpopulation.
The Experiments
Experiments were conducted in 1995 and 1996. In 1995, the experiments were grown in two Missouri environments: Hinkson Bottom at Columbia, on Freeburg silt loam soil and the Agronomy Research Center near Columbia, on Mexico silt loam soil. Only the experiment at Hinkson Bottom was successfully harvested in 1995; the other was discarded because of severe damage by fall armyworm [Laphygma frugiperda (A.&S.)] during the vegetative stage. In 1996, experiments were carried out in four Missouri environments: Hinkson Bottom (two environments: early planting on 25 April and late planting on 24 May), Agronomy Research Center, and South Poultry Farm near Columbia, both on Mexico silt loam. All four environments were successfully harvested. A total of five environments was used for data analyses. In order to create a range of different environmental conditions within each location-year combination, three N levels and three plant densities were applied in this experiment. The experimental design was a randomized complete block with treatments in a split-split-plot arrangement and three replications. Three levels of N application (90, 180, and 270 kg ha-1 N) were main-plots. Three plant densities (35 800, 47 800, and 59 800 plants ha-1 which were appropriate for this germplasm) were sub-plots. The six entries were sub-sub-plots. Sub-sub-plots included three rows 6.9 m long with 0.9 m between rows. Each sub-sub-plot was planted with two seeds per hill with a hand planter and plants were thinned to a single plant per hill at the six- to eight-leaf stage. To obtain plant densities of 35 800, 47 800, and 59 800 plants ha-1, each sub-sub-plot was planted using 1.9-cm-diam aluminum pipes marked with tape at 18.3, 22.9, and 30.5 cm apart for the hills, respectively. For the main-plots, ammonium nitrate was used as N fertilizer. For accuracy and uniformity of application, a manual fertilizer spreader calibrated to 90 kg ha-1 N for each pass was used. Plots with 90, 180, and 270 kg ha-1 N received one, two, or three passes at planting or before plants were 10 cm high, respectively. Ninety kg ha-1 N and 70 kg ha-1 K2O were applied preplanting as basal fertilizer at Hinkson Bottom in 1995. Phosphorous (P2O5) and potash (K2O) were applied preplanting as basal fertilizer at a rate of 110 kg ha-1 at the South Poultry Farm in 1996. A rate of 2.2 kg ai ha-1 of atrazine (2-chloro-4-ethyl-amino-6-isopropylamino-s-triazine) and metolachlor [2-chloro-N-(2-ethyl-6-methylphenyl)-N-(2-methoxy-1-methyl-ethyl) acetamide] was applied as pre-emergence herbicide in 1995. Atrazine and metolachlor were applied at rates of 1.8 and 1.1 kg ai ha-1, respectively, at all locations in 1996. Carbofuran (2,3-dihydro-2,2-dimethyl-7-benzofuranol methylcarbamate) was used to control European corn borer (Ostrinia nubilalis Hübner) when necessary.
Data Collection
For stalk crushing strength measurement, stalk sections including the second elongated internode above ground and the attached nodes were harvested 2 to 3 wk after flowering from 10 competitive plants in the center row of each plot. Whole stalk sections were slowly dried at room temperature for at least 3 mo. After drying, internode sections of 5.1-cm length were cut from the middle of the second elongated internode with a double-bladed electric saw. The 5.1-cm stalk sections were then vertically crushed with an automated hydraulic press (Enerpac, Butler, WI)1
and stalk crushing strength was recorded and converted from load-pounds per plant to load-kilograms per plant (Zuber and Grogan, 1961).
Rind penetrometer resistance was used to measure rind strength. Measurements were done about 10 to 14 d after silking before stalk harvesting for crushing strength. Ten competitive plants from the center row of each plot were probed in the middle of the flat side of the internode immediately below the primary ear node with a modified electronic rind penetrometer based on an Accuforce Cadet digital force gage, 18-kg capacity, powered by a 9-V alkaline battery (Ametek, Hunter Spring Division, Hatfield, PA) (Sibale et al., 1992). Rind penetrometer resistance observations were converted into load-kilograms per plant.
Vertical root pulling resistance was evaluated 2 to 3 wk after flowering. All plants in the right row of each plot were cut off 40 to 50 cm above the ground. Ten alternate competitive plants (pulling roots from one plant affects the rooting area of adjacent plants) were pulled vertically from the ground with a Kellem wire puller (flexible-eye wire pulling grip). The wire puller was attached to a chain which was connected to a load cell and the boom of a small skid-steer loader (Model 1816C, J.I. Case, Racine, WI). The skid-steer loader pulled on the Kellem until the plants broke free of the ground and the peak force was sensed by the load cell and read in millivolts on a Polycorder (Model 516, Omnidata International, Logan, UT). Millivolt readings were converted to load-kilograms per plant by the regression of millivolts (X) on known weights (Y) to generate the following regression equation for prediction of peak force:
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Root and stalk lodging counts were recorded on the first row of each plot just before ear harvest. A root lodged plant was one that leaned 30° or more from vertical. A plant was counted as stalk lodged if it was broken at, or below, the top ear node. Root and stalk lodging counts were expressed as a percentages of total counted stand. Top and second ear grain yields in megagrams per hectare at 155 g kg-1 moisture in grain were obtained from the left row of each plot. Each plot was harvested placing the top and second ears in separate fiber bags. Second ears included all ears below the top ear. Ears from each bag were shelled separately, weighed, and sampled for moisture content. Plant height was measured on 10 competitive plants as the distance from the ground to the joint of the lowest tassel branch. Ear height was measured on the same plants as the distance from the ground to the top ear node.
Statistical Analysis
A combined analysis of variance was performed for all traits using plot means. Environments (location–year combinations) were considered as random effects while N levels, plant densities, and entries were considered as fixed effects. Although responses to N levels and plant densities were examined, these variables primary purpose was to generate different "environmental conditions" within a single location–year combination to allow greater generalization of results.
The sum of squares due to entries with five degrees of freedom was partitioned into two degrees of freedom for a comparison among the three populations and one degree of freedom each for comparisons between the one- and two-eared selections within the ACC, BCC, and SI171 populations. Interactions among N levels and plant densities, and among genotypes and environments were also obtained.
Tests of significance for all main effects, interactions, and partitions in the combined analysis of variance were made against their respective interactions with environments. All interactions with environments were tested against the respective pooled error from the combined analysis of variance.
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Results and discussion |
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TOPNOTESABSTRACTINTRODUCTIONMaterials and methodsResults and discussionREFERENCES |
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only for stalk crushing strength (Table 1)
. That significant linear effect on stalk crushing strength was negative (Tables 1 and 2)
. Results from this study agreed with a report published by Moentono et al. (1984) who found that stalk crushing strength tended to decrease with increased N application, but their differences were not statistically significant.
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, or highly significant, overall effect for most characters, except for root lodging and total grain yield (Table 1). A negative linear effect of plant density was highly significant for stalk crushing strength, rind penetrometer resistance, vertical root pulling resistance, and second-ear grain yield (Tables 1 and 2). And, a significant, or highly significant, positive linear effect of plant density was found for stalk lodging, root lodging, top-ear grain yield, and plant and ear heights (Tables 1 and 2). This study confirmed other reports that indicated an increase in stalk lodging was observed when plant density increased (Zuber et al., 1960; Twumasi-Afriyie and Hunter, 1982; Moentono et al., 1984; Thomison and Jordan, 1995). The highly significant, negative linear effect for second-ear grain yield found in this experiment demonstrated that increasing plant densities had a detrimental impact on second ear development. Non-synchronization between silking and anthesis (Harris et al., 1976) and lack of photoassimilate during the grain filling period due to mutual shading (Bauman, 1960) may cause abortion of lower ears. In contrast to second-ear grain yield, increasing plant densities had a beneficial impact on top-ear grain yield (Table 2). Our results verified that grain yield improvement can be achieved by selecting genotypes that respond to higher planting densities. Greater plant and ear heights with increasing plant densities were generally observed. There were highly significant differences among entries for all characters (Table 1). Significant, or highly significant, differences among the three populations were found for all characters, except for stalk crushing strength and ear height. Differences between one- and two-eared subpopulations of ACC (ACC-1E vs. ACC-2E) were significant, or highly significant, for all characters except total grain yield. All characters for the contrast between one- and two-eared subpopulations of BCC showed highly significant differences. For SI171, significant, or highly significant, differences were found for the majority of characters, except for stalk crushing strength, vertical root pulling resistance, root lodging, and ear height.
Pairwise comparisons of all characters for the one- and two-eared selections of ACC, BCC, and SI171 averaged over all N levels and plant densities showed that stalk crushing strength was significantly higher for the one-eared subpopulation than for the two-eared subpopulation in ACC, but the result was opposite for the BCC population (Table 3) . Stalk crushing strength tended to be higher in the one-eared subpopulation than the two-eared subpopulation for SI171, but the result was not significant. Brotslaw et al. (1988) evaluated three prolific maize populations for stalk crushing strength and found that there were no significant differences between one- and two-eared plants within each population for stalk crushing strength. Rind penetrometer resistance was significantly higher for one-eared selections than for two-eared selections in our study for all three populations (Table 3).
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Significant differences between the one- and two-eared subpopulations for total grain yield were only found for the BCC and SI171 subpopulations. The two-eared subpopulations had greater total grain yield than the one-eared subpopulations for all populations (Table 3). Total grain yield of two-eared selections was also higher than for one-eared selections at the higher plant densities when evaluated at 180 kg ha-1 of N fertilizer (combined N level effects were not significant; Table 1) (Table 4). The results from this study demonstrated that two-eared plants were more efficient in conversion of photosynthate to grain than one-eared plants, resulting higher total grain yield for the two-eared subpopulations of BCC and SI171. Brotslaw et al. (1988) included the effect of prolificacy on grain yield and root and stalk strength, and also found the same result; two-eared plants had significantly higher mean grain yields than one-eared plants for their three prolific populations. There are a number of reports which indicated prolificacy was associated with higher grain yield (Lonnquist, 1967; Mareck and Gardner, 1979; Moll and Hanson, 1984; Singh et al., 1986; Coors and Mardones, 1989; Subandi, 1990; Maita and Coors, 1996). Russell and Eberhart (1968) speculated that continued breeding efforts for the two-eared type would give greater grain yield improvement than for the one-eared type. Their prediction was based on the simple fact that much more work has been done with the one-eared type when compared to the two-eared type.
Top-ear grain yield for the two-eared selection was significantly lower than that for the one-eared selection for all populations, which was expected because photosynthate produced by the two-eared plants must be shared between the first ear and the second ear, while the one-eared plants moved all photosynthate into the first ear (Table 3). Tetio-Kagho and Gardner (1988) also reported that ear priority for photosynthate was in the order of Ear 1 > Ear 2 > Ear 3. In contrast, selection for two-eared genotypes resulted in significantly higher second-ear grain yield than selection for one-eared types for all three populations, which indicated the effectiveness of selection in increasing the number of ears per plant for the two-eared subpopulations.
Selection for a two-eared plant type resulted in significantly greater ear height than selection for a one-eared plant type for all populations except for SI171 (Table 3). There were greater differences between ear heights of the two-eared selection and one-eared selection of ACC and BCC than for SI171, indicating the effect contributed by the prolific parental source, Georgia Cow Corn, which was used as a parent in the ACC and BCC populations. Georgia Cow Corn typically produces several small ears with ears positioned above the middle of the stalk. The other possible explanation was the effect of high plant density in the selection experiments for developing two-eared types resulting in a competitive disadvantage for prolific plants with lower ear height due to effects of shading. Increases in ear height also have been shown in other reports on selection for prolificacy (Brotslaw et al., 1988; Subandi, 1990). However, Maita and Coors (1996) reported that ear height was reduced by 0.3 cm cycle-1 after 20 cycles of biparental mass selection for prolificacy in the open-pollinated maize population Golden Glow. They explained the reason for the reduction in ear height; it was probably due to selection for earliness at least during the first 12 cycles of selection.
Differences between the one- and two-eared subpopulations of each population for plant height were all highly significant. Plant heights for two-eared selections were significantly greater than for one-eared selections (Table 3). This likely occurred because of the high plant density (and resulting taller plants) utilized for developing the two-eared plant populations. A tall plant had a competitive advantage over a short plant for greater light interception for photosynthesis.
Prolificacy associated with higher grain yield was demonstrated in this study. However, selection for only prolificacy had adverse consequences for other important agronomic characteristics, especially root and stalk strength, which are major factors that limit use of prolific genotypes in commercial hybrids. Concurrent improvement for grain yield, prolificacy, and root and stalk strength by using a standardized, weighted selection index could eliminate undesirable effects which occur with selection for prolificacy alone.
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ACKNOWLEDGMENTS |
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NOTES |
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TOPNOTESABSTRACTINTRODUCTIONMaterials and methodsResults and discussionREFERENCES |
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1 Mention of a trademark or proprietary product does not constitute a guarantee, warranty, or recommendation of the product by the U.S. Department of Agriculture or the University of Missouri and does not imply its approval to the exclusion of other products that may also be suitable. 
Received for publication August 9, 1999.
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REFERENCES |
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TOPNOTESABSTRACTINTRODUCTIONMaterials and methodsResults and discussionREFERENCES |
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