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PRODUCTION PAPER

Corn Hybrids for Late Planting in the Southeast

^a Dep. of Plant Pathol., North Florida Res. and Educ. Cent., Univ. of Florida, 155 Research Rd., Quincy, FL 32351
^b Dep. of Plant Entomol., North Florida Res. and Educ. Cent., Univ. of Florida, 155 Research Rd., Quincy, FL 32351

^* Corresponding author (pjwiatrak{at}mail.ifas.ufl.edu).

Received for publication October 7, 2003.

	ABSTRACT

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

 
Delayed planting dates in the U.S. Southeast often result in insect and disease problems on corn (Zea mays L.). The objectives of this study were to determine the effects of corn hybrid (type) and planting date on yields and the extent of injury resulting from insects and pathogens. Tropical, Bt (Bacillus thuringiensis var. kurstaki Berliner), and non-Bt corn hybrids were evaluated for grain and silage yields and insect damage at planting dates from March to August from 1998 to 2001. Averaged across years, corn silage yields were greatest at March and April planting dates (16.1 Mg ha^–1) and least from July (6.1 Mg ha^–1) and August (4.5 Mg ha^–1) planting dates. Silage yields of tropical corn were 26 and 17% higher than non-Bt and Bt hybrids, respectively, because of less yield reduction at later planting dates. Corn grain yields were greatest at the March planting date (9.6 Mg ha^–1) and least at July and August planting dates (0.9 and 0.4 Mg ha^–1, respectively). Grain yields of tropical corn were 15 and 10% higher than those of Bt and non-Bt, respectively. Fall armyworm (Spodoptera frugiperda J.E. Smith), corn earworm (Helicoverpa zea Boddie), and southern corn rust disease (Puccinia polysora Underw.) ratings had negative correlations with corn yields. Insect damage was below 6% for Bt hybrids and above 11% for non-Bt hybrids. Tropical corn had higher silage and grain yields and better silage quality and disease resistance than either Bt or non-Bt hybrids.

Abbreviations: ANOVA, analysis of variance • Bt, Bacillus thuringiensis var. kurstaki Berliner • IVOMD, in vitro organic matter digestion

	INTRODUCTION

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

 
MOST OF THE SOUTHEASTERN USA areas are grain deficit, and large quantities of grain are imported to meet the needs of the livestock industry. The southeastern USA has a long growing season that could allow production of large quantities of corn grain and silage. However, a number of constraints make it difficult to take advantage of the available growing season. Among these are infertile soils, nonuniform and inadequate rainfall during the cropping season, severe insect injury, and plant disease. In the southeastern USA, corn can be planted from late February until early July and/or double-cropped if corn hybrids are available that can withstand insect and disease pressure during the summer. Summer rainfall is often more optimal for corn planted from mid-May until mid-June. Planting is generally terminated mid-April to early May due to severe insect pressure. Corn hybrids, from Midwest germplasm, have been grown for many years in the Southeast; however, over the last few years, interest has developed over the potential of tropical corn hybrids from Mexico and Central America. These tropical corn hybrids are adapted to planting at times that take advantage of rainfall patterns in the subtropical areas of the Deep South (Wright et al., 1991); however, significant insect problems are often encountered (Anderson and Linker, 1991).

In the southeastern USA, the fall armyworm and the corn earworm are the most destructive insect pests of field corn, grown either for silage or for grain (Wiseman et al., 1974). The fall armyworm primarily feeds in the leaf whorl though it may often infest and severely damage the ears as well. Occasionally, other armyworm species may be found infesting corn in the whorl stage (4- to 12-leaf stage). The corn earworm primarily feeds on developing corn ears although it may occasionally infest the whorl as well (in our studies, in only very low numbers). Injury by either species can be devastating, either through loss of biomass for silage or reductions in grain yield and/or quality.

Traditional insect control methods, however, are often prohibitive because of the economics of returns relative to input costs. Insect resistance is considered to be a more sustainable alternative to chemical control. For many years, efforts have been toward the development of genetically based insect-resistant corn hybrids (Ng et al., 1985; Videla et al., 1992; Wiseman and Widstrom, 1992). More recently, new technology has enabled the development of transgenic corn hybrids that produce the insecticidal crystalline protein CryIA(b) derived from the naturally occurring soil bacterium referred to as Bt. Large-scale commercial use of genetically engineered or transgenic plants began in 1995. This new technology was originally developed with the objective of controlling the European corn borer, Ostrinia nubilalis Hubner, primarily a pest of corn in the eastern and midwestern USA. To date, most large-scale commercial plantings of Bt corn have occurred in production areas where the European corn borer is the primary insect constraint to production (Ostlie et al., 1997). However, there is very little published information that describes effects of Bt corn types on either corn earworm or fall armyworm (Williams et al., 1997, 1998; Buntin et al., 2000). Similarly, there is little published information on the effect of insect populations and resulting injury on tropical corn types (Gross et al., 1982; McMillian et al., 1987).

If insects are controlled, mature late plantings are still susceptible to plant pathogens, and Bt varieties will still experience low yields if pathogens are not adequately controlled. Major fungal pathogens include southern corn rust, common corn rust (Puccinia sorghi Schw.), southern corn leaf blight (Bipolaris maydis), and mycotoxin-producing organisms (Aspergillus flavus and A. parasiticus).

This study was designed to measure the effects of insect injury on corn grain and silage yields in the southeastern USA over a wide planting window and corn types and to determine both silage and grain yield and quality in genetically altered plants compared with tropical and traditional temperate corn hybrids. Our long-term goal is to provide relevant information to corn producers and dairymen that will allow them to develop stable cropping, livestock, and dairy operations. The objectives of this study were to determine the effects of corn hybrid (type) and planting date on grain and silage yield and quality and the extent of injury resulting from fall armyworm and corn earworm infestation.

	MATERIALS AND METHODS

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

 
Cultural Practices
Field trials were conducted from 1998 to 2001 on a Dothan sandy loam (fine, loamy siliceous, thermic Plinthic Kandiudults) at the University of Florida's North Florida Research and Education Center in Quincy, FL. Plantings were made strip-till with a Brown Ro-till implement (Brown Manufacturing Co., Ozark, AL) and cone row planter at 64250 kernels ha^–1 (plant stands were thinned to 59300 plants ha^–1 about 2 wk after planting). Just after planting, fertilizer was banded beside rows at 28 kg N ha^–1, 24 kg P ha^–1, and 70 kg K ha^–1. Plots were sidedressed with 171 kg N ha^–1 when corn was about 0.4 m tall. Other cultural practices, including weed control, irrigation, and harvest, were implemented according to standard production practices. Insecticides were not applied either at planting or during the season. The inadequate precipitation was compensated with irrigation on an as-needed basis using a sprinkler irrigation system.

Experimental Design
Corn was planted at approximately 30-d intervals beginning late March. Experimental design was a randomized complete block in a split plot arrangement where planting dates were the main plots and corn hybrids were the subplots (Table 1). Plots were 6 m long by 11 m wide (12 rows) and separated from adjacent plots by 1.5-m alleys. Plot Rows 1, 2, 11, and 12 were maintained as undisturbed border/buffer rows. Data were analyzed using the general linear models (SAS Inst., 1999). In some cases, data did not meet the assumption of normality, and transformations did not confer normality. In these cases, PROC NPAR1WAY (SAS Inst., 1999) was used to generate Mann–Whitney and Kruskal–Wallis tests, the nonparametric equivalents of two-sample t tests and one-way analysis of variance (ANOVA), respectively (Stokes et al., 2000). If the result of a nonparametric test agreed with the outcome of ANOVA, the ANOVA result was presented. Pearson correlation coefficients (r) were calculated between insect damage and yields of corn from 1998 to 2000 and between disease incidence and corn yields in 2000.

Disease
Southern corn rust disease ratings were taken visually 7 to 10 d before harvest corn in 2000 by examining the ear leaf of 20 randomly selected plants in each plot. The scale was from 0 to 10, with 0 = none, 1 = some to 10%, 2 = 11 to 20%, 3 = 21 to 30%, 4 = 31 to 40%, 5 = 41 to 50%, 6 = 51 to 60%, 7 = 61 to 70%, 8 = 71 to 80%, 9 = 81 to 90%, and 10 = 91 to 100% disease.

Crop Yield
Silage yield samples were collected by cutting Rows 4 and 5 with a modified two-row silage chopper at one-half kernel milk line stage. Silage dry matter weights for all hybrids were adjusted to a standard 350 g kg^–1 dry matter. The in vitro organic matter digestion (IVOMD) was performed by a modification of the two-stage technique (Moore and Mott, 1974). Grain yield samples were taken at harvest maturity from Rows 6 and 7 when grain moisture was below 250 g kg^–1, using a modified grain combine or hand harvesting and shelling. Grain yield was adjusted to moisture content of 155 g kg^–1.

	RESULTS AND DISCUSSION

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

 
The year x planting date x hybrid interaction was observed for fall armyworms, corn earworms, silage yields, silage IVOMD, and grain yields (Table 3), and the results varied across years (Fig. 1–6) .

View larger version (20K): [in this window] [in a new window]   Fig. 1. Influence of planting date on fall armyworm defoliation from 1998 to 2001. Vertical bars indicate the standard error of four replicates.

View larger version (24K): [in this window] [in a new window]   Fig. 6. Influence of corn hybrid on southern corn rust disease pressure. Vertical bar indicate the LSD (P = 0.05).

View larger version (21K): [in this window] [in a new window]   Fig. 2. Influence of planting date on corn earworm damage from 1998 to 2001. Damage for tropical corn was not recorded in 1998 and 1999. Vertical bars indicate the standard error of four replicates.

View larger version (23K): [in this window] [in a new window]   Fig. 3. Influence of planting date on corn silage yields from 1998 to 2001. Vertical bars indicate the standard error of four replicates.

View larger version (22K): [in this window] [in a new window]   Fig. 4. Influence of planting date on silage in vitro organic matter digestion (IVOMD) from 1998 to 2001. Vertical bars indicate the standard error of four replicates.

View larger version (23K): [in this window] [in a new window]   Fig. 5. Influence of planting date on corn grain yields from 1998 to 2001. Vertical bars indicate the standard error of four replicates.

Corn Earworms
Corn earworm damage, at approximately 3 wk after pollination, was relatively low for March and April planting dates (Fig. 2). Averaged across years, the corn earworm damage was lowest from March and April planting dates (5 and 8%, respectively) and greatest from June and July planting dates (16 and 18%, respectively). However, the corn earworm damage, averaged across years, was 67 and 49% lower from Bt than non-Bt or tropical corn, respectively. These results agree with Buntin et al. (2000), who noted that the larvae of fall armyworm and corn earworms developed more slowly and caused much less kernel damage on ears of resistant (Bt) than susceptible plants. Selection of Bt hybrids may reduce damage of corn ears in years with high corn earworm infestation.

Silage Yields
Silage yields of corn were mostly higher from April and March planting dates and relatively low from June, July, and August planting dates (Fig. 3). Averaged across years, the greatest corn silage yields were obtained from March and April planting dates (16.1 Mg ha^–1 for both) and lowest yields from July and August planting dates (6.1 and 4.5 Mg ha^–1, respectively). Although corn yields decreased with later planting dates, the smallest reduction in yield from later planting dates was observed from tropical corn and Bt hybrids. Silage yields, averaged across years, were 26 and 8% higher from tropical and Bt corn, respectively, compared with non-Bt corn. Observations by Sparks (1986) showed that heavy larvae feeding on leaves and other aboveground parts of the plant can cause devastating yield loses. However, using Bt hybrids would be sufficient to substantially reduce yield losses associated with fall armyworm damage (Williams et al., 1998). Based on the Pearson correlation coefficient, negative correlations of corn silage yields with fall armyworms and corn earworms were noted in 1999 (r = –0.40 and r = –0.46, respectively), 2000 (r = –0.34 and r = –0.25, respectively), and 2001 (r = –0.40 and r = –0.20, respectively) (Table 3). In 1998, no correlations of silage yields were noted with insect damage; therefore, higher corn silage yields were obtained from June than other planting dates. Some of the observed decline in silage yields with later planting dates can be attributed to severity of southern corn rust. Based on disease data recorded in 2000, corn silage yields were negatively correlated with southern corn rust (r = –0.45) (Table 3). The insect and disease pressure may decrease silage yields of corn; however, the yield loss may be reduced by selection of Bt or tropical corn.

In Vitro Organic Matter Digestion
The concentration of IVOMD in corn silage mostly decreased with later planting dates, with tropical corn having relatively stable concentration of IVOMD over planting dates (Fig. 4). Averaged across years, the concentration of IVOMD was greater than 63% from March and April planting dates and less than 50% from August and July planting dates. The concentration of IVOMD from tropical corn, averaged across years, was 4 and 2 concentration units higher than Bt and non-Bt corn, respectively. For late planting dates, tropical corn offers better digestibility than Bt and non-Bt corn.

Grain Yields
Highest grain yields of corn were obtained from March and April planting dates and declined thereafter (Fig. 5). Averaged across years, greatest grain yields (9.6 Mg ha^–1) were obtained from the March planting date and the least (less than 1 Mg ha^–1) from July and August planting dates. Lower grain yields, at later planting dates, were attributed to fall armyworms in 1998 (r = –0.43) and fall armyworms and corn earworms in 1999 (r = –0.39 and r = –0.50, respectively), 2000 (r = –0.28 and r = –0.24, respectively), and 2001 (r = –0.32 and r = –0.22, respectively) (Table 3). The grain yields of tropical corn, averaged across years, were 15 and 10% higher than Bt and non-Bt, respectively. Buntin et al. (2000) also noted that the fall armyworm and corn earworm infest ears, causing direct loss of grain. However, Bt resistance reduced ear infestations and larval number per ear, causing less kernel damage on resistant than susceptible plants and therefore being relatively effective in preventing significant yield losses of corn by fall armyworm and corn earworm (Buntin et al., 2000). Much of the observed decline in grain yields with the later planting dates can be attributed to increasing severity of southern corn rust during the 4-yr vegetation periods. A negative correlation of grain yields was noted with the southern corn rust disease (r = –0.64) (Table 3). However, disease pressure was lower on tropical corn than Bt and non-Bt corn (Fig. 6). The insect injury and disease at the later planting dates contributed to yield loses; however, smaller reduction in yields were noted from tropical corn and Bt hybrids compared with non-Bt corn.

	CONCLUSIONS

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

 
Fall armyworm defoliation and corn earworm damage varied among corn hybrids and planting dates. Generally, over the years, higher fall armyworm and corn earworm infestations occurred on non-Bt corn compared with Bt corn hybrids, especially at late planting dates. To reduce insect infestation, non-Bt corn can be generally planted until April, and the planting date of Bt corn can be extended into May. The IVOMD concentration generally decreased with later planting dates. This decrease of IVOMD is attributed to increased disease in Bt and non-Bt corn, whereas the tropical hybrids with better disease resistance and less leaf browning maintained better plant health and higher concentration of IVOMD. Silage and grain yields of corn were generally higher for March and April planting dates and declined afterwards in most years. Silage yields of Bt corn, averaged over years, were 8% higher than non-Bt corn hybrids; however, tropical corn yielded 17% higher than Bt corn. The grain yields of tropical corn, averaged across years, were 15 and 10% higher than Bt and non-Bt, respectively. Much of the observed decline in silage and grain yields with later planting dates can be attributed to increasing severity of southern corn rust. This study indicates that selection of higher-yielding hybrids from tropical locations (disease resistance seems to be inherently higher than for hybrids from Midwest germplasm) can result in the potential to grow corn over an expanded planting window with tolerable reductions in either grain or silage yield.

	ACKNOWLEDGMENTS

 
We thank B. Kidd and S. Grzes for helping us with the experiments and technical assistance.

	NOTES

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

 
This research was supported by the Florida Agricultural Experiment Station and approved for publication as Journal Series no. R-09817.

	REFERENCES

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

 

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This Article Abstract Figures Only Full Text (PDF) Free Alert me when this article is cited Alert me if a correction is posted Services Similar articles in this journal Similar articles in ISI Web of Science Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via HighWire Citing Articles via ISI Web of Science (5) Citing Articles via Google Scholar Google Scholar Articles by Wiatrak, P. J. Articles by Sprenkel, R. Search for Related Content PubMed Articles by Wiatrak, P. J. Articles by Sprenkel, R. Agricola Articles by Wiatrak, P. J. Articles by Sprenkel, R. Related Collections Best Management Practices Maize Maize Management