Published online 27 April 2005
Published in Agron J 97:823-831 (2005)
DOI: 10.2134/agronj2004.0237
© 2005 American Society of Agronomy
677 S. Segoe Rd., Madison, WI 53711 USA
Corn
Early-Season Defoliation Effects on TopCross High-Oil Corn Production
T. F. Mangena,
P. R. Thomisona,* and
S. D. Strachanb
a Hortic. and Crop Sci. Dep., The Ohio State Univ., Columbus, OH 43210
b Pioneer Hi-Bred Int., P.O. Box 1004, Johnston, IA 50131
* Corresponding author (thomison.1{at}osu.edu)
Received for publication September 7, 2004.
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ABSTRACT
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High-oil corn (Zea mays L.) produced with the TopCross grain production system is planted as a physical mixture (a TC Blend) containing 91% high-yielding, male-sterile hybrid (grain parent) seed and 9% pollinator seed. Field experiments were conducted at two locations in Ohio in 1999 and 2000 to determine if high-oil corn (HOC) TC Blends are more sensitive to defoliation damage than their normal-oil corn (NOC) hybrid counterparts consisting of 100% male-fertile plants. Corn plants were 100% defoliated at V5 and 50 and 100% defoliated at V13. Defoliation similarly affected grain yield and oil concentration of HOC TC Blends and NOC hybrids. Complete defoliation at V13, averaged across years and hybrids, was the most damaging defoliation treatment, reducing grain yields 29%. Defoliation did not significantly alter grain oil concentration of HOC grain parents (74.5 g kg–1) or NOC (34.5 g kg–1). Defoliation caused similar responses in ear weights and oil concentrations in both components (the pollinator and the grain parent) of the HOC TC Blends. The lack of hybrid x defoliation interactions for ear yield components and various agronomic traits indicates that HOC TC Blends and their respective NOC counterparts responded similarly to early-season defoliation over a range of environmental conditions. Guidelines for assessing early-season leaf injury in conventional corn may be suitable for assessing leaf injury in HOC TC Blends.
Abbreviations: cms, cytoplasmic male-sterile • GDD, growing degree day • HOC, high-oil corn • NOC, normal-oil corn • 7L, seven-leaf stage • 14L, 14-leaf stage
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INTRODUCTION
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HIGH-OIL CORN (HOC) typically contains from 60 to 80 g kg–1 oil on a dry weight basis. Some livestock producers prefer HOC as a feed source because the grain contains approximately 4.5% more energy per kilogram than conventional corn grain and can replace more expensive dietary sources of fat and protein (Alexander, 1988). Feeding trials with HOC for various livestock indicate that HOC improves feed efficiency and increases rate of gain over conventional corn (Lambert, 2001). Contract production of HOC grain may offer growers higher profits through premiums (U.S. Grains Council, 2002).
The TopCross grain production system is used to produce corn with >65 g kg–1 of oil on a dry matter basis (U.S. Grains Council, 2002). Corn seed is planted as a physical mixture (TC Blend) containing 91% high-yielding, cytoplasmic male-sterile (cms) hybrid (grain parent) seed and 9% pollinator seed (Edge, 1997). The resulting male-sterile grain parent plants contain agronomic traits necessary for high grain yields while pollinator plants produce essentially all of the pollen to fertilize all corn plants within the field. Through the biological process of xenia, the dominant or semi dominant genetic traits for oil in the pollen increase the oil concentration of grain produced on the cms grain parent plants (Strachan and Kaplan, 2001). In contrast, populations of NOC conventional hybrids, hybrids that produce grain with approximately 30 to 40 g kg–1 oil on a dry weight basis, contain 50 to 100% of the plants capable of producing pollen. The use of blends containing a high proportion of cms seed relative to pollinator seed is also being studied as a method for enhancing yield potential and stability (Weingartner et al., 2002).
Evidence exists that the pollination of a HOC TC Blend is more sensitive than a NOC hybrid to environmental stresses. In Ohio, Thomison et al. (2002) observed poor pollination, reduced kernel set, and subsequent yield losses in HOC because of substantial silk clipping and root lodging caused by western corn rootworm (Diabrotica virgifera Le Conte). Silk clipping and root lodging were much greater in HOC TC Blend fields following corn than in those following soybean or wheat (Triticum aestivum L.). Strachan and Kaplan (2001) reported that HOC TC Blends with 9% pollinator plants may be more sensitive to injury from rootworm feeding on silks than NOC with 50 to 100% male-fertile plants and concluded that economic thresholds for silk-feeding pests for HOC may be more restrictive than those currently accepted for NOC.
Defoliation or leaf damage, such as that associated with hail, frost, wind, crop protection chemicals, and insects, can impact pollination and subsequent grain production. Early-season defoliation can delay anthesis and silking (Dungan and Gausman, 1951; Cloninger et al., 1974; Singh and Nair, 1975; Vasilas and Seif, 1985a), shorten the duration of pollen shed (Vasilas and Seif, 1985a, 1985b), and reduce the total amount of pollen produced (Dungan and Gausman, 1951). Vasilas and Seif (1985a) found that defoliation at the 14-leaf stage (14L) had a differential effect on anthesis and pollen shed by inbred lines and a hybrid. Johnson (1978) defoliated nine hybrids at the five-leaf stage and observed that defoliation delayed pollen shed and silking but did not change the pollen shed-to-silking interval. A shortage of pollen is usually not a problem in NOC since much more pollen is produced than is normally needed and these populations contain high percentages of male-fertile plants. However, in HOC, only 9% of the plants produce pollen.
Although the agronomic performance of blends (mixtures of seed of different corn genotypes) have been evaluated in many studies (Hoekstra et al., 1985), no information is available on how the components of blends respond to early-season leaf destruction by various biotic and abiotic factors. Thomison and Nafziger (2003) reported that early-season defoliation effects on HOC TC Blends were similar to those observed for normal corn hybrids. However, in this study, the impact of defoliation on pollen production could not be assessed because outcrossing among plots was not limited. The study did not compare defoliation effects on HOC TC Blends or their components with NOC during early vegetative growth.
Little is known concerning defoliation effects on grain quality, especially as it relates to the chemical composition of corn grain. Unlike commodity grain production, profitability in HOC is based on both grain yield and oil concentration. Grain with higher oil concentration commands higher premiums, and if oil concentration falls below a specified concentration (often in the range of 60–65 g kg–1 on a dry matter basis), no premium is offered. A better understanding of the response of HOC to defoliation will help growers make informed management decisions when their corn fields have been damaged by hail, frost, or some other factor.
The objectives of this investigation are to compare effects of early-season defoliation on the agronomic performance and grain quality of HOC with its NOC hybrid counterpart and to determine if the two components of a HOC TC Blend, the grain parent and the pollinator, differ in their responses to defoliation. The goal of this research is to determine whether guidelines for assessing early-season leaf injury in conventional corn hybrids are suitable for HOC TC Blends or whether new guidelines specific to HOC TC Blends must be developed.
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MATERIALS AND METHODS
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Field experiments were conducted in 1999 and 2000 at four locations in Ohio (Table 1). Plots were established at The Ohio State University (OSU) Ohio Agricultural Research and Development Center (OARDC) near Wooster and the OSU Farm Science Review near London in 1999; and at the OARDC-Western Branch near South Charleston and the OSU Waterman Farm in Columbus in 2000.
Two HOC TC Blends, Pioneer brands 34K79 (in 1999) and 34B25 (in 2000), were compared with their NOC hybrid counterparts, 34K77 (in 1999) and 34B23 (in 2000). The relative maturity and growing degree day (GDD) ratings (from VE to R6; Ritchie et al., 1992) of the NOC hybrid counterparts (hereafter referred to as NOC hybrids) were 108 d and 1480 GDDs for 34K77 and 109 d and 1500 GDDs for 34B23. Both HOC TC Blends contained the same pollinator. Defoliation treatments were no defoliation, 100% leaf removal at V5, 50% leaf removal at V13, and 100% leaf removal at V13. Treatments for 50% defoliation were imposed by cutting off the distal half of each leaf for all leaves on all corn plants. Treatments for 100% defoliation involved cutting all fully developed leaves at their leaf collars on all plants. Defoliation treatments were imposed by hand as described by Hicks et al. (1977). The leaf collar staging system (Ritchie et al., 1992) used to characterize vegetative development in these experiments is based on the numbers of leaves with visible collars and is generally about one to two stages behind that used by hail adjusters, which is based on an indicator leaf that is mostly exposed but that does not yet have its collar visible (National Crop Insurance Assoc., 1984).
Cultural practices and soil types associated with each experiment are shown in Table 1. Nutrient, insect, and weed management strategies for minimizing crop stress were followed at each location. The previous crop at each location was soybean. High-oil corn TC Blends were planted in 15.2- by 15.2-m plots (twenty 0.76-m rows), and check plots were 3.1 by 15.2 m (four 0.76-m rows). All plots were separated by at least 6.1 m of buffer (eight 0.76-m rows). High-oil corn and NOC were separated by 30.5 m (forty 0.76-m rows) of corn buffer. Larger plot sizes were used in HOC plots to better simulate populations of corn in TC Blend fields. Buffer rows among plots were planted to cms seed to limit outcrossing of pollen from adjacent plots.
Measurements were taken from the center four rows of the HOC TC Blend plots and the center two rows of the conventional NOC plots. Plant populations were recorded before the 100% defoliation treatment at V5 and at physiological maturity. Notes on pollen shed and silk emergence were recorded every 2 to 3 d during flowering at London in 1999 and Columbus in 2000. High-oil corn TC Blend pollinator plants were identified by pollen shed and flagged. Pollinator and cms grain parent plants expressed differences in tassel and silk color, which helped to distinguish between plant types. Plant heights at silking were recorded for five representative plants from each plot. For the HOC TC Blend plots, plant heights of pollinators and cms grain parents were recorded. The number of plants exhibiting stalk lodging (stalk broken below the ear node), root lodging (stalk leaning more than 30% from vertical), stalk rot, and plants with barren or poorly developed ears (barrenness) at physiological maturity was recorded. Stalk rot was determined by pinching stalks between the first and second aboveground nodes (Ohio State Univ. Ext., 1995). Ears from the center two rows (6.1-m sections) of each plot were hand-harvested. For the HOC TC Blend plots, weights of ears from the pollinator and grain parent plants were combined to estimate total yield, but the ears were kept separate for later measurements of grain moisture and various ear and kernel components. Five ears representative of each corn type were collected from each plot, shelled, and the resulting grain measured for moisture concentration. Grain yields were adjusted to reflect yield at 15.5 g kg–1 moisture. A separate sample of 10 ears each of NOC, HOC pollinator, and cms grain parent was selected from each plot for determination of ear yield components for all treatments. The number of kernels per ear was estimated by multiplying the number of kernel rows by the number of kernels per row, ear weight was measured, and incomplete kernel development at the tip of the ear ("blank tip") scored. A visual rating was used to estimate the percentage of ear filled to account for missing kernels ("percentage ear filled"). Ears were shelled to determine 300-kernel weight and grain composition analysis. Percentage oil, protein, and starch were determined by near infrared transmittance (Itnyre, 1992).
Two studies were conducted to address the objectives of this investigation. In Study 1, responses of the HOC TC Blend and its NOC hybrid counterpart to defoliation were compared. In Study 2, responses of the HOC pollinator and the cms grain parent to defoliation were compared. A split-plot design with three replications in a randomized complete block arrangement was used for both studies. In Study 1, the main plot was hybrid (HOC TC Blend vs. NOC hybrid), and subplots were the defoliation treatments. In Study 2, the main plots were the defoliation treatments, and the subplots were the two types of corn comprising the TC Blend (the grain parent and the pollinator).
Data were analyzed using a two-factor randomized complete block design with split plot combined over locations each year. A mixed model was used with locations as random effects and corn hybrids, types, and defoliation treatments as fixed effects according to the method described by McIntosh (1983). Least significant differences at probability level 0.05 (LSD 0.05) were calculated using the results of the combined analysis each year.
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RESULTS AND DISCUSSION
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Climatic conditions varied considerably during the course of the study with higher air temperatures and less rainfall in 1999 compared with 2000 (Table 2). Precipitation during the 1999 growing season was 152.4 and 119.6 mm below average at Wooster and London, respectively, whereas in 2000, it was 81.3 and 83.8 mm above average at South Charleston and Columbus, respectively (Table 2). Monthly air temperatures in 1999 were 1.17 and 0.80°C above average at Wooster and London, respectively, but near normal in 2000 at South Charleston and Columbus (Table 2). Plant responses are therefore probably representative of growth under more drought-stressed conditions in 1999 and representative of growth under normal to slightly wetter environmental conditions in 2000.
Defoliation Effects on the High-Oil Corn TC Blend and Normal-Oil Corn Hybrid
Defoliation significantly affected yields in 1999 and 2000 (P < 0.05) (Table 3). Complete defoliation at V5 reduced yields by 19% in 2000, but effects were not significant in 1999 (Table 4). Fifty percent defoliation at V13 did not significantly affect yield in 1999 but reduced yield 12% in 2000 (Tables 3 and 4). Complete defoliation at V13, averaged across locations and hybrids, resulted in similar decreases in grain yield each year (28% in 1999 and 30% in 2000) (Table 4). Although HOC TC Blends generally yield less than their NOC hybrid counterparts (Thomison et al., 2002), no differences in yield between HOC and NOC were evident in this study (Tables 3 and 4). The absence of significant hybrid x defoliation interactions in 1999 and 2000 suggests that effects of defoliation on the yields of HOC TC Blends and NOC hybrids are similar (Table 3).
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Table 3. Mean squares from analysis of variance for the effect of defoliating a high-oil corn TC Blend and its normal-oil corn conventional hybrid counterpart on yield, ear yield components, and grain oil concentration in 1999 and 2000.
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Table 4. Yield, ear yield components, and grain oil concentration of a high-oil corn (HOC) TC Blend and its normal-oil corn (NOC) conventional hybrid counterpart, averaged across locations, subjected to varying levels of defoliation in 1999 and 2000.
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Although National Crop Insurance Association (NCIA, 1984) guidelines for assessing hail damage estimate a 10% yield loss from 100% defoliation at the seven-leaf stage (7L, approx. V5), results of past research have been mixed with reported yield reductions of 26% (Johnson, 1978) and increases of 48% (Hicks et al., 1977) for similar early-defoliation treatments. Yield losses observed in this study for 50% defoliation at V13 (Table 4) were also less than the 15% yield loss estimated by NCIA guidelines. However, Vasilas and Seif (1985b) showed that complete defoliation at 14L resulted in yield reductions of approximately 90% for an inbred and 40% for a hybrid. Inconsistent responses to 100% defoliation at V5 and 50% defoliation at V13 may be attributed to the drier, less favorable growing conditions in 1999 compared with 2000 (Table 2). Crookston and Hicks (1988) suggested that an early reduction in vegetative growth of corn might help reduce yield losses from late-season drought stress. In a study comparing inbreds and their single-cross hybrid, Vasilas and Seif (1985b) found that partial defoliation at 14L could reduce or increase grain yields of hybrids and inbreds. Under drought conditions, Crookston and Hicks (1988) and Vasilas and Seif (1985b) attributed increases in grain yield associated with early-season defoliation to reduced transpiration, thus promoting more efficient water use by corn plants.
Defoliation did not affect final plant populations in either year (data not shown) or affect kernels per ear in 1999 but did significantly affect kernels per ear in 2000 (Tables 3 and 4). Past research has indicated that kernels per ear is the yield component affected most by defoliation during early-season vegetative growth (Johnson, 1978; Vasilas and Seif, 1985b). When data were combined across years, this study also shows that grain yield is closely associated with kernels per ear for each type of corn produced (Fig. 1). Yield responses in relation to kernels per ear were similar for the cms grain parent in the HOC TC Blend and for NOC. Comparisons of kernels per ear of the cms grain parent in the HOC TC Blend and NOC by defoliation treatment show very similar responses to defoliation (Table 4). Genetically, the male-sterile HOC grain parent differs from the NOC hybrid only in the ability to produce pollen, so ear and yield responses between the HOC grain parent and the hybrid might be expected to be similar. In addition, kernels per ear on the cms grain parent of the HOC TC Blend and on the NOC hybrid were similar across all defoliation treatments, indicating that pollinator plants in the HOC TC Blends supplied sufficient pollen for maximum kernel set on cms grain parents across a range of defoliation stress environments (Table 4). Defoliation did not alter oil concentration in either the HOC TC Blend or in NOC (Tables 3 and 4). When averaged across years, oil concentration averaged 74.5 g kg–1 for cms grain parents in the HOC TC Blends and 34.5 g kg–1 for the NOC hybrids. These results indicate that economic losses resulting from defoliation, such as that caused by hail damage, are related only to loss in total grain yield and not to any changes in oil concentration of harvested grain.
Hybrid and defoliation effects on agronomic traits and other ear components were not consistent in 1999 and 2000 (Tables 3 and 5). Defoliation significantly altered (i) 300-kernel weight in 2000 but not in 1999, (ii) percentage ear fill in 1999 but not in 2000, (iii) grain moisture at harvest in 2000 but not in 1999, (iv) test weight in 1999 but not in 2000, (v) protein concentration in 2000 but not in 1999, (vi) starch concentration in 1999 but not in 2000, (vii) stalk lodging in 1999 but not in 2000, (viii) plant height in 2000 but not in 1999, and (ix) stalk rot in 2000 but not in 1999. Although a significant hybrid x defoliation interaction for barrenness was present in 2000, barrenness was negligible (<5%) and slightly higher in the NOC hybrid as a result of 100% defoliation (data not shown). Grain moisture at harvest was higher for the 100% defoliation treatments at V5. Hybrid x defoliation interactions were not significant. Hicks et al. (1977) also observed that defoliation before anthesis delayed maturity and resulted in slightly higher grain moisture at harvest. Thomison et al. (2002) reported higher grain moisture concentration in HOC TC Blends compared with NOC hybrid counterparts, but hybrid effects were not significant in this study. Lower test weights have previously been observed in HOC corn (Thomison et al., 2002). Strachan and Kaplan (2001) attributed part of the lower test weight in TopCross HOC grain to lower-density oil replacing higher-density starch. Lower test weights were associated only with the HOC TC Blend and 100% defoliation treatments in 1999 (data not shown). Previous research has not reported lower test weights in corn defoliated before anthesis (Hicks et al., 1977). Protein concentration was reduced only when corn plants were completely defoliated at V13 in 2000. The differences in oil, protein, and starch concentrations between HOC and NOC in this research are similar to those previously published (Strachan and Kaplan, 2001; Thomison et al., 2003). Consistent with this research, Thomison and Nafziger (2003) reported that defoliation of HOC before V15/16 generally had a negligible effect on grain oil and protein concentration. Johnson (1978) found that NOC hybrids completely defoliated at V4 contained similar grain oil concentration as untreated controls but reduced protein concentration. Stalk lodging averaged 11%, for plants completely defoliated at V13 in 1999, the only treatment to show an effect vs. an average of 3% lodging for plants not defoliated. Only complete defoliation at V5 reduced plant height in 2000. Hybrid by defoliation effects were not significant for plant height during either year of testing. In 2000, complete defoliation at V13 increased stalk rot with the effect more pronounced on the NOC hybrid (data not shown).
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Table 5. Effects of hybrid, defoliation, and location on statistical significance of select agronomic and grain quality traits for a TC Blend cytoplasmic male-sterile grain parent and its conventional normal-oil corn counterpart in 1999 and 2000.
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Only the complete defoliation treatment at V5 delayed flowering in the HOC TC Blends and in the NOC hybrids (data not shown). When averaged across years, pollen shed of the high-oil pollinator and silk emergence of the cms grain parents in the HOC TC Blends were delayed approximately 4 to 5 d. Although flowering was delayed, both components of the high-oil blend were delayed the same amount. Both pollen shed and silk emergence of the NOC hybrids were delayed approximately 5 to 7 d when these hybrid plants were completely defoliated at V5. Vasilas and Seif (1985a) described similar delays in flowering of inbreds caused by defoliation at 7L (approximately V5–V6) and 14L (approximately V12–13) with some inbreds showing greater differences than others in flowering responses.
Defoliation Effects on the High-Oil Corn TC Blend Grain Parent and Pollinator
Defoliation significantly altered kernels per ear on male-sterile grain parents and pollinators in both years of the study (Table 6). Complete defoliation at V13 reduced kernels per ear about 16% each year (Table 7). In 1999, the difference in kernels per ear was greater at London than at Wooster, accounting for a location x corn type interaction. Corn type, averaged across locations and defoliation treatments, significantly affected kernels produced per ear in only 2000. In 2000, kernels per ear, averaged across defoliation treatments, was 23% less for the pollinator than the grain parent. When expressed on a percentage basis, pollinator response to defoliation was similar to the male-sterile grain parent in 1999 but slightly more severe in 2000 (Fig. 2). Part of this difference may be because pollinator plants do not have the same agronomic characteristics or hybrid vigor as male-sterile grain parents.
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Table 6. Mean squares from analysis of variance for the effect of defoliating a high-oil corn TC Blend cytoplasmic male-sterile grain parent and pollinator on ear weight, ear yield components, and grain oil concentration in 1999 and 2000.
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Table 7. Ear weights, ear yield components, and grain oil concentration of a high-oil corn TC Blend cytoplasmic male-sterile (cms) grain parent and pollinator, averaged across locations, subjected to varying levels of defoliation in 1999 and 2000.
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Fig. 2. Effects of defoliation treatment on kernels per ear for grain parents (GP) and pollinators (Poll) in high-oil corn TC Blends. UTC, untreated check.
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Defoliation significantly altered harvested ear weight in only 1999 (Table 6). In 1999, 100% defoliation at V13, averaged across locations and corn types, reduced ear weights by 25% compared with those of the untreated checks (Table 7). Differences between the pollinator and grain parent and defoliation x corn type interactions for ear weight were not significant either year. A location x corn type interaction in 1999 indicated that ear weights of the pollinator and grain parent varied considerably depending on environment. Previous research indicates that hybrids and inbreds can differ in response to defoliation. These differences have been related to maturity (Hanway, 1969; Hicks et al., 1977) and total leaf number (Vasilas and Seif, 1985b). Vasilas and Seif (1985c) concluded that genotypic differences in the yield response to complete defoliation coincided with genotypic differences in final leaf number and the effect of defoliation on increasing the interval between anthesis and silking.
Defoliation did not significantly affect 300-kernel weight of either the male-sterile grain parent or the pollinator in either year of testing (Table 6). Three-hundred-kernel weights were 15 and 21% lower in 1999 and 2000, respectively, for the pollinator when compared with the male-sterile grain parent, averaged across all defoliation treatments (Table 7). The lower kernel weight of the pollinator may be attributed, in part, to the higher oil concentration. No defoliation x corn type interactions for 300-kernel weight were detected in either year of testing.
Defoliation and defoliation x corn type interactions did not significantly alter grain oil concentration in 1999 or 2000 (Table 6). Grain oil concentration of the pollinator, averaged across years, locations, and defoliation treatments, was approximately 87% greater than the oil concentration (76 g kg –1) of grain harvested from the grain parent (Table 7). A location x corn type interaction existed for oil concentration in 1999 (Table 6) because of the difference in magnitude of oil concentration between corn types at the two sites.
Protein and starch concentrations of harvested grain varied considerably depending on location and year of testing (data not shown). When averaged across locations and years, protein concentration of grain harvested from the pollinator was greater than protein concentration of grain harvested from the grain parent (132 vs. 84 g kg–1). A defoliation x corn type interaction for protein in 1999 was related to differences in the degree of response to defoliation between corn types in 1999. Defoliation and defoliation x corn type interactions did not significantly alter starch concentration either year. Grain starch concentration was significantly different for corn type in 1999 and 2000 (data not shown). Starch concentration, averaged across years, locations, and defoliation treatments, was greater in the grain parent (657 g kg–1) than the pollinator (537 g kg–1). The higher concentration of starch in the grain parent is consistent with the lower oil concentration of the grain parent compared with that of the pollinator (Table 7).
Defoliation did not significantly affect percentage ear fill or occurrences of blank ear tips during either year of testing (Table 8) nor did it affect the amount of barrenness, root lodging, stalk rot, or test weight (data not shown). Location x corn type interactions for percentage ear fill, plant height, and blank tip in 1999 and for blank tip in 2000 resulted from corn type effects that varied at the two sites. Significant defoliation x corn type interactions existed for stalk lodging in 2000 (Table 8) (not measured in 1999) with the pollinator exhibiting greater stalk lodging compared with the grain parent for most of defoliation treatments (data not shown). A location x type interaction was due to the greater lodging of the pollinator compared with the grain parent at Columbus (26 vs. 7%) than at South Charleston (22 vs. 17%) (data not shown). In 1999, the grain parent and pollinator differed slightly in their responses to defoliation for plant height (Table 8), with the 100% defoliation treatment at V13 causing slightly more height reduction in the grain parent. In 1999, complete defoliation at V13 resulted in average plant height of 164 and 185 cm for the grain parent and pollinator plants, respectively. Nondefoliated plants were 178 and 195 cm for the grain parent and pollinator, respectively. Whether under complete defoliation or nondefoliation, the grain parent remained approximately 90% of the pollinator. High-oil pollinator plants have sometimes been characterized as somewhat less vigorous, with lower yield potential and greater susceptibility to stalk and root lodging compared with grain parent hybrids (Edge, 1997). This characterization would suggest that growth and reproductive development of the pollinator following defoliation might be slower and at a disadvantage when compared with growth of the grain parent. However, plant height measurements of this study indicate that pollinator growth was comparable to that of the grain parent and that the height advantage of the pollinator over the grain parent was not adversely impacted. Taller pollinators may allow wind and gravity to be more effective in dispersing pollen to neighboring cms grain parents, thus increasing the probability of maximum kernel set across an entire field.
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Table 8. Effects of defoliation, corn type, and location on statistical significance of select agronomic traits for a TC Blend cytoplasmic male-sterile grain parent and pollinator in 1999 and 2000.
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A better understanding of the pollinator may help explain grain yields of HOC when compared with corresponding NOC hybrids. Although not significant in either year of testing, there is a trend for grain yield of HOC to be slightly less than grain yield of the corresponding NOC hybrid (Table 4). The primary purpose of the pollinator is to supply ample pollen to support maximum kernel set for the entire HOC field. The HOC pollinator produced fewer kernels per ear with subsequent reduced harvestable grain weight per ear when compared to the cms grain parent on a per-plant basis (Fig. 3). The pollinator partially compensated for this reduced kernel number per ear by producing kernels with greater kernel weight although the kernel weights were less than kernel weights for the grain parent (Fig. 4). In addition, the cms grain parent occasionally produced slightly more kernels per ear when compared with the corresponding NOC hybrid (Table 4), indicating that the pollinator supplied ample pollen for kernel production.
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Fig. 3. Grain weight per ear as a function of kernels per ear of cytoplasmic male sterility grain parents and pollinators in high-oil corn TC Blends.
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Fig. 4. Three-hundred-kernel weight as a function of kernels per ear of cytoplasmic male sterility grain parents and pollinators in high-oil corn TC Blends.
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SUMMARY
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Defoliation effects on yield and agronomic traits influencing hybrid performance were similar for HOC TC Blends and NOC hybrids across a range of environmental conditions. Defoliation did not affect oil concentration of harvested grain but did affect grain yield. The lack of a differential response to defoliation between HOC TC blends and NOC hybrids suggests that guidelines for assessing early-season leaf injury in conventional corn may be useful in assessing defoliation injury in HOC TC Blends.
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ACKNOWLEDGMENTS
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We thank DuPont Specialty Grains and Pioneer Hi-Bred, a DuPont Company, for their support of this study, and the numerous grain quality analyses provided.
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NOTES
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Salaries and research support provided in part by State and Federal funds appropriated to The Ohio State Univ. Ohio Agric. Res. and Dev. Cent. Ohio State Univ. Manuscript no. HCS05-04.
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ABSTRACT
INTRODUCTION
