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Published in Agron. J. 97:446-451 (2005).
© American Society of Agronomy
677 S. Segoe Rd., Madison, WI 53711 USA

Production Paper

Responses of Short-Season Corn Hybrids to a Humid Subtropical Environment

H. Arnold Bruns* and H. K. Abbas

Crop Genetics and Prod. Res. Unit, Box 345, Stoneville, MS 38776

* Corresponding author (abruns{at}ars.usda.gov)

Received for publication August 30, 2004.

   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 NOTES
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Corn (Zea mays L.) hybrids commonly grown in the lower Mississippi River valley often encounter heat and drought stress during reproductive growth, which impairs yield and increases preharvest mycotoxin contamination. Four short-season hybrids developed for production at ≥ 40° N latitude and two hybrids adapted to the Midsouth USA were grown at Stoneville, MS (33°26' N, 90°55' W), in 2002 and 2003 using N fertility treatments of 112 kg N ha–1 preplant, 224 kg N ha–1 preplant, or 112 kg N ha–1 preplant + 112 kg N ha–1 side-dress at growth stage V6 (six leaves). Growing degree units at 10°C base temperature required for growth stages R1 (silking) and R6 (physiological maturity) of the four short-season hybrids were 50 and 100 units greater, respectively, than when grown in their adapted environments. Yields of two of the short-season hybrids compared well with the adapted hybrids. Kernel weights and grain bulk density differed among hybrids but were not below levels subject to dockage. Aflatoxin and fumonisin levels were higher in 2002 than 2003. Three of the short-season hybrids did have aflatoxin levels in 2002 at least three times greater than the other three hybrids. Plots receiving 224 kg N ha–1 preplant yielded more than the other N fertility treatments. Kernel weights were 10 mg greater for the higher N fertility treatments. Nitrogen fertility had no effect on mycotoxins. Short-season hybrids need to be individually evaluated for production potential in the lower Mississippi River valley.

Abbreviations: GDU 10, growing degree unit at 10°C base temperature


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 NOTES
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
CORN HAS A WIDE RANGE of adaptability, growing continuously in the Western Hemisphere from 40° S latitude to 58° N latitude and altitudes ranging from below sea level to 3950 m mean sea level (Martin et al., 1975, p. 326). Martin et al. (1975)(p. 326) also states that the architecture of the species ranges from plants approximately 0.5 m tall, with eight to nine leaves and maturing grain within 50 d of planting, to plants over 6.0 m tall, having 40 to 42 leaves and requiring nearly a year to mature grain. Most modern corn hybrids grown in North America range from 2.0 to 3.0 m in stature and produce 18 to 24 leaves. They will require between 1166 growing degree units at 10°C base temperature (GDU 10's) (approximately 82 d) to 1775 GDU 10's (approximately 120 d) to obtain physiological maturity (growth stage R6 as defined by Ritchie et al., 1997) (Shaw, 1988). The GDU 10 requirement for corn hybrids tends to decrease as their area of adaptability moves farther away from the equator (Neild and Newman, 2003).

Corn was not a major crop enterprise in the lower Mississippi River valley until the mid-1990s (USDA-NASS, 2001). In the Mississippi Delta states of Arkansas, Louisiana, and Mississippi, corn production has increased from about 161000 ha grown in 1990 to 382 500 ha in 2000. Grain yields for the region have increased from a mean of about 6.0 to 8.0 Mg ha–1 during the same era. Corn fits well in rotation with cotton (Gossypium hirsutum L.), and the demand for locally grown grain by commercial poultry and channel catfish (Ictalurus punctatus Rafinesque) growers has spurred interest in producing the crop (Martin et al., 2002). Changes in government farm programs have also aided the increase in corn production for the lower Mississippi River valley.

The lower Mississippi River valley is a humid subtropical environment characterized by long periods of hot weather from late spring to early autumn (Wax and Brown, 2003). Precipitation events in the region usually begin to decline in number and intensity during June and reach a minimum during August. Average monthly rainfall totals at Stoneville, MS, for June, July, and August based on data from 1964 to 1993 were 94, 93, and 57 mm, respectively (Boykin et al., 1995). Frequent periods of drought are common for this area, necessitating irrigation to limit drought stress and the resulting loss in crop yields.

A frequent assumption regarding corn production in the lower Mississippi River valley is that because the region has a long period between the last killing frost of spring and the first frost in autumn (>220 d), corn hybrids requiring 1500 GDU 10's or more would be well suited for the area. Generally, corn hybrids that utilize the greatest amount of an area's growing season will yield more than those requiring fewer GDU 10's to mature (Poehlman, 1959, p. 263; Larson, 2002). However, in the lower Mississippi River valley, March typically has frequent, heavy rains with very few days suitable for soil tillage. A large hectarage of corn in the region is therefore not planted until April. Days with maximum mean temperatures at or above 32°C occur regularly in the area by 15 June (Boykin et al., 1995). In 2000 to 2003 at Stoneville, MS, the total GDU 10's did not reach 1500 until around 20 July based on a 1 April starting date (DREC, 2004). Corn hybrids requiring 1500 GDU 10's to acquire growth stage R6, planted on or after 1 April, would have likely experienced some degree of heat stress during reproductive growth in 2002 and 2003. Temperatures above optimum can increase respiration rates, decrease photosynthesis rates, and denature enzymes in leaves necessary for phloem loading (Hale and Orcutt, 1987).

Maximum economic grain yields of corn can only be obtained when there are sufficient levels of soil moisture and plant nutrients, especially N. Drought in combination with heat stress and inadequate levels of plant nutrients is known to increase the incidence of ear-rotting fungi, particularly Aspergillus flavus and Fusarium verticilliodes (Syn. F. moniliforme J. Sheld). These fungi produce the carcinogens aflatoxin and fumonisin, respectively, making grain unsafe for food and feed (Anderson et al., 1975; Payne, 1999; Bruns, 2003; CAST, 2003). A conservative estimate of yearly losses combined with expenditures on research related to mycotoxins in crops in the USA is between $0.5 and $1.5 billion (Robens and Cardwell, 2003).

In the lower Mississippi River valley, N fertilizer applications for corn are recommended to be split with approximately 112 kg N ha–1 applied preplant broadcast and the remainder as a sidedress application at growth stage V6. This is to avoid possible leaching losses that can occur with heavy spring rains, particularly on sandy soils. In Mississippi, the current recommendation for N fertilization of corn is 23.2 kg N ha–1 for each megagram per hectare of yield goal up to 6.3 Mg ha–1 and then 30.2 kg N ha–1 for each additional megagram per hectare yield goal (Larson and Oldham, 2003). Wet weather or other factors occasionally prevent the second N fertilizer application. A preplant application of the crop's entire N requirement may be an alternative to the risk of not being able to apply all of the needed N fertilizer during the growing season. Deficiencies of N impair not only grain yield, but increase the plant's vulnerability to aflatoxin contamination (Jones and Duncan, 1981).

This research was done to evaluate the production potential of corn hybrids that require fewer GDU 10's to achieve growth stage R6 than those commonly recommended for the lower Mississippi River valley. Specific objectives were to determine if short-season corn hybrids yield as well as or better than hybrids requiring more GDU 10's and if they have less or at least no more aflatoxin and fumonisin contamination than commonly grown hybrids for the region. Nitrogen fertility rates were varied in the study to evaluate the effects of missing the second N fertilizer application vs. applying the required N in a split application as recommended or applying all of the N as a preplant application of these types of hybrid corns.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
NOTES
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
The research was done at the Mississippi State University's Delta Branch Experiment Station at Stoneville, MS (33°26' N, 90°55' W), in 2002 and 2003. Soil at the site was a Dundee silty clay (fine-silty, mixed, thermic Aeric Ochraqualfs). The experimental design was a randomized complete block replicated five times with the treatments arranged in a 6 x 3 factorial. Treatments were six hybrids, four short-season and two later maturing, and three N fertility treatments. The hybrids used in the experiment were Hoegemeyer1 brand cultivar 2593, Garst-AgriPro1 brand cultivar 9185Bt, DeKalb1 brand cultivar DK C42-22Bt, Syngenta Seed1 brand cultivar N79-L3Bt, and Pioneer1 brand cultivars 3897 and 3394. The GDU 10 units required to achieve growth stages R1 and R6 by each of these hybrids are listed in Table 1 and were either supplied by the seed company or estimated from information acquired from sales literature. The hybrids 2593, 9185Bt, DK C42-22Bt, and 3897, hereafter referred to as being short season, are adapted for production in areas above 40° N latitude. The N fertility treatments were 112 kg N ha–1 preplant, 224 kg N ha–1 preplant, or 112 kg N ha–1 preplant plus 112 kg N ha–1 at growth stage V6. Individual plots were four 76-cm-wide rows, 9 m long, and consisted of one hybrid and one N fertility treatment.


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  		Table 1. Stated and observed growing degree units at 10°C base temperature (GDU 10's) to achieve growth stages R1 (silking) and R6 (physiological maturity) for six corn hybrids grown for 2 yr at Stoneville, MS, with different N fertility treatments.

 
The experiment was planted 19 Apr. 2002 and 16 Apr. 2003 at a seeding rate of 100 400 kernels ha–1 with an expected final stand of 85 400 plants ha–1. The previous crop at the site each year was soybean [Glycine max (L.) Merr.]. Seedbed preparation consisted of forming 40-cm-high ridges, spaced 76 cm apart for planting. Soil samples were taken before seedbed preparation, analyzed for macronutrient content, and P and K fertilizer applied preplant to rates for a yield goal of 12.0 Mg ha–1 grain.

The GDU 10's for each year were calculated using temperature data acquired from the weather station located at the experimental site (DREC, 2004) and the formula described by Shaw (1988). Precipitation events ≥ 10.0 mm and irrigation events were also noted. Dates that individual hybrids acquired growth stage R1 and growth stage R6 were recorded each year.

Weed control was accomplished by a pre-emergence application of Permit {[3-cholro-5-(4,6-dimethoxypyrimidin-2-ylcarbamoylsulfamoyl)-1-methylpyrazole-4-carboxylic acid], Monsanto,1 St Louis, MO} at the rate of 37.0 g a.i. ha–1 and cultivation at growth stage V6. Plots were furrow-irrigated twice each season at growth stages VT (tasseling) and R1 in 2002 and at growth stages R2 (blister) and R5 (dent) in 2003. Irrigations were done when shared irrigation equipment became available and on a schedule previously described (Bruns et al., 2003) at a rate of approximately 25 mm per irrigation. Plots were inoculated with A. flavus strain F3W4 by spreading autoclaved wheat (Triticum aestivum L.) seed colonized by the fungus between the two center rows of each plot at the rate of 15 g m–2 (Olanya et al., 1997; Abbas et al., 2003, 2004).

Plant populations were determined during the later part of grain filling by counting plants within the two center rows of each plot. Immediately before harvest, the same rows were inspected and counts taken on lodged plants and dropped ears. Plots were machine-harvested approximately 40 d after growth stage R6. Grain from each plot was weighed and a sample of approximately 0.5 kg collected during harvest for determination of moisture content, grain bulk density, kernel weight, and analyses for aflatoxin and fumonisin contamination. Grain moisture content and bulk density were determined using a Seedburo1 Model GMA 128 Grain Moisture Analyzer (Seedburo Equipment Co., Chicago, IL). Grain yields were adjusted and reported at a moisture level of 155 mg g–1. Grain samples were then oven-dried at 30°C for 18 to 24 h. Kernel weights were determined by hand counting and weighing 100 sound kernels from each plot. Aflatoxin and fumonisin contamination levels were determined using Veratox1–Aflatoxin Kits and Veratox–Fumonisin Kits (Neogen Co., Lansing, MI). Procedures used for their evaluation have been previously reported (Abouzied et al., 1995; Abbas et al., 1998). Data were statistically analyzed according to procedures outlined by McIntosh (1983) for combining data over years, treating years as fixed effects, and using PROC MIXED (SAS Inst., 2001). Confidence limits were calculated for GDU 10 data and Pearson's correlations determined where appropriate.


   RESULTS AND DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 NOTES
 RESULTS AND DISCUSSION
CONCLUSIONS
 REFERENCES
 
The GDU 10's required to achieve both growth stage R1 and growth stage R6 by the four short-season hybrids in this experiment were greater than what is stated by their suppliers when grown in their adapted environments (Table 1). The GDU 10's required by 3394 to achieve anthesis was greater for this experiment than what is stated by its provider. The hybrid 2593, which according to its supplier requires 1365 GDU 10's to achieve growth stage R6, required about 1480 GDU 10's in the environment at Stoneville, MS. This GDU 10 requirement was greater than that of N79-L3Bt and 3394, which both have a higher stated GDU 10 requirement for growth stage R6 and have been regularly grown in the lower Mississippi Valley. The hybrids 9185Bt, DK C42-22Bt, and 3897 also demonstrated greater GDU 10 requirements to achieve growth stage R6 in this experiment than what is stated in sales literature when grown in their adapted environments. The GDU 10's required for DK C42-22Bt and 3897 to achieve growth stage R1 were lower (P ≤ 0.01) in this experiment than for three of the other hybrids (Table 1).

Mean plant populations in the experiment were greater (P ≤ 0.01) in 2003 (89000 plants ha–1) than in 2002 (81500 plants ha–1). No other differences in plant population were observed in this experiment. Stalk lodging, root lodging, and dropped ears virtually did not occur in this experiment and therefore are not reported.

Grain yields differed (P ≤ 0.01) among hybrids both years of the experiment but were not found to be correlated with the GDU 10's required to achieve either growth stage R1 or R6 (Table 2). Yields for hybrids 2593 and N79-L3Bt were greater (P ≤ 0.01) in 2003 than 2002 while no such differences were observed among the other hybrids. The hybrid 3897 produced less (P ≤ 0.01) grain than all other cultivars both years of the experiment. This hybrid had the least stated GDU 10 requirement (1188 GDU 10's) to achieve growth stage R6 of any in the experiment. It was developed and marketed as a dual-purpose corn (grain and silage) for production in New England, the Great Lakes region of the USA, and the corn-producing areas of Canada (www.pioneer.com/products/prdguide/eb/010/pdfs/eb3897g.pdf; verified 27 Dec. 2004). These data demonstrate that it was unable to acclimate to the environment of the lower Mississippi River valley.


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  		Table 2. Grain yield, kernel weight, grain bulk density, and grain moisture at harvest for six corn hybrids of differing maturity classifications, produced under furrow irrigation with different N fertility treatments grown at Stoneville, MS.{dagger}

 
The hybrid DK C42-22Bt, which was also developed for production in Canada and the northern USA, was the highest-yielding hybrid in the experiment in 2002 and among the higher-yielding ones in 2003 (Table 2). The other very early maturing hybrid, 9185Bt, in 2002 had comparable yields to the two hybrids adapted to the Midsouth (3394 and N79-L3Bt) and 2593. In 2003, its yield was comparable to 3394 but less than DK C42-22Bt, N79-L3Bt, and 2593.

The hybrid 2593 yielded well in the humid subtropical environment of Stoneville, MS. Its grain yield in 2002 was comparable to the adapted hybrids 3394 and N79-L3Bt and less than (P ≤ 0.01) only DK C42-22Bt (Table 2). In 2003, it was exceeded in yield only by N79-L3Bt and outyielded 3394. It and DK C42-22Bt produced similar quantities of grain in 2003.

Greater differences in kernel weight among the hybrids were observed in 2002 than 2003 (Table 2). However, no consistent patterns between the 2 yr of the experiment were evident. Differences in observed kernel weight did not correlate with yield.

Grain bulk density was greater (P ≤ 0.01) for the two adapted hybrids (3394 and N79-L3Bt) than the short-season hybrids (Table 2). However, even the lowest grain bulk density observed in this experiment (707.9 kg m–3 for 3897) exceeded the minimum U.S. standard (695 kg m–3) for U.S. no. 2 yellow corn, which is the most common grade traded on the world market (USDA Grain Inspection, Packers, and Stockyards Administration, 1996).

Grain moisture content at harvest was greatest (P ≤ 0.01) for N79-L3Bt than for all other hybrids and greater for 3394 than the short-season hybrids (Table 2). However, the grain moisture content at harvest for all hybrids was lower than necessary. Corn grain needs to be at a moisture content of 155 g kg–1 to be safely handled and stored. Grain harvested at levels above 155 g kg–1 moisture is subject to destruction by grain storage molds and heat generated by respiring kernels (Olson and Sander, 1988). However, corn grain at 120 g kg–1 moisture content is subject to mechanical damage during harvest and handling, which can lower yields, decrease grain bulk density, and increase susceptibility to fungal attack, which increases the possibility of postharvest mycotoxin contamination, all of which lowers income (Hall and Hill, 1974; Olson and Sander, 1988; Watson, 1988). Bruns and Abbas (2004) concluded that mature corn that is left to field-dry should be closely monitored for changes in preharvest grain moisture content to avoid such potential losses.

In 2002, both aflatoxin and fumonisin contamination levels were above the acceptable maximum limits (20.0 mg Mg–1 and 2.0 mg kg–1 respectively) as set forth by the U.S. Food and Drug Administration (USFDA) for interstate commerce (Table 3) (USFDA, 2000). Though no differences in mycotoxin contamination were observed among hybrids in 2003, aflatoxin levels of all hybrids, except N79-L3Bt, exceeded the 20.0 mg Mg–1 maximum limit for aflatoxin and the 2.0 mg kg–1 limit for fumonisin. The observed levels in 2003, though, would have been acceptable for most livestock feed (USFDA, 2000).


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  		Table 3. Aflatoxin and fumonisin concentrations in corn hybrids of differing maturity produced under furrow irrigation with different levels of N fertility.{dagger}

 
After the accumulation of 720 GDU 10's, which marked the beginning of reproductive growth in most of the short-season hybrids, less water in both number of events and total accumulated was acquired in 2002 than 2003 (four events for 175.7 mm vs. seven events for 242.6 mm, respectively) (Table 4). The number of days with maximum air temperatures ≥ 35°C during this same time period was greater in 2002 than 2003 (12 vs. 5 d) (DREC, 2004). The combination of less soil moisture and more days of maximum air temperatures ≥ 35°C in 2002 likely explains the higher mycotoxin levels observed that year (Table 4).


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  		Table 4. Irrigation and precipitation events greater than 10 mm, plus adjusted accumulative growing degree units at 10°C base temperature (GDU 10's) acquired between 19 Apr. and 3 Aug. 2002 and 16 Apr. and 31 July 2003 on corn research plots grown at Stoneville, MS.{dagger}

 
In 2002, 3394 had less (P ≤ 0.01) aflatoxin contamination than all other hybrids in the experiment (Table 3). Both N79-L3Bt and 2593 had similar aflatoxin contamination levels, which were less (P ≤ 0.01) than the three remaining hybrids. Fumonisin contamination levels did not differ significantly among hybrids in 2003 (Table 4). However, in 2002, both 2593 and 3897 were greater (P ≤ 0.01) in fumonisin contamination than all other hybrids while fumonisin contamination was greater in 3394 than N79-L3Bt.

Grain yields and kernel weights were the only dependent variables to be significantly affected by N fertility (Table 5). Applying 224 kg N ha–1 preplant produced greater (P ≤ 0.01) yields than the 112 kg N ha–1 rate or splitting the application by applying 112 kg N ha–1 preplant and another 112 kg N ha–1 at growth stage V6. No significant difference in grain yield was observed between the latter two treatments. Greater (P ≤ 0.01) kernel weights were acquired by both of the higher N fertility rates. No other dependent variables were observed to be significantly different among the N fertility treatments of this experiment, nor were any significant interactions involving N fertility observed.


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  		Table 5. Grain yields and kernel weights of corn grown with furrow irrigation and different N fertility treatments at Stoneville, MS.{dagger}

 

   CONCLUSIONS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 NOTES
 RESULTS AND DISCUSSION
 CONCLUSIONS
REFERENCES
 
The observed GDU 10's required to achieve growth stages R1 and R6 at Stoneville, MS, were greater for all the short-season hybrids in the experiment than that stated by their suppliers. Higher daily minimum temperatures in the lower Mississippi River valley contribute to a more rapid gain in GDU 10's than in more northern latitudes. However, more northern latitudes experience greater solar duration near the summer solstice, and some hybrids adapted to such an environment may be exhibiting a photoperiod response when grown nearer the equator that has yet to be documented.

Grain yields for 2593 and DK C42-22Bt compared well to those of 3394 and N79-L3Bt, and both exceeded the area average yield of 8.1 Mg ha–1 both years (USDA-NASS, 2001). Some short-season corn hybrids can likely be profitably produced in the lower Mississippi River valley. However, not all such hybrids appear to acclimate to the humid subtropical environment of the area. The below-average grain yield of 3897 indicates this. Agronomic evaluations of short-season corn hybrids are necessary to determine which ones perform well in such an environment and which ones should be avoided before their commercial production is recommended. Further research specific to managing the production of these hybrids in the lower Mississippi River valley is also needed.

The observed grain moisture levels at harvest indicate that the experiment was ready to be harvested sooner than it was. However, physical constraints due to the experiment's location relative to other studies in the field prevented harvesting sooner. In a production situation, monitoring grain moisture in the field is critical to avoiding either harvesting corn too high in moisture and risking spoilage during storage or too dry and suffering mechanical damage during harvest and handling.

Though the grain characteristics of kernel weight and bulk density differed among hybrids, none were below values that would have resulted in dockage on the market. Aflatoxin levels of three of the early maturing hybrids (9185Bt, DK C42-22Bt, and 3897) were considerably higher than the other three hybrids in 2002. However, ample soil moisture through both rainfall and irrigation and planting earlier than what was possible in this experiment to encounter fewer days of ≥35°C air temperatures may help reduce the incidence of both aflatoxin and fumonisin contamination.

Following the current recommendation of splitting the N fertilizer application for corn did not show an advantage for yield or grain quality, nor did it reduce the level of mycotoxins when compared with applying the season's worth of N fertilizer as a preplant treatment. In fact, grain yields in this experiment were lower (P ≤ 0.01) with the split application vs. the 224 kg N ha–1 treatment. There is also a reduction in yield and kernel weight if the sidedress application of N fertilizer is missed due to weather or other factors. Previous research showing increases in aflatoxin with less N fertility cannot be ignored even though such observations were not made in this experiment (Jones and Duncan, 1981).


   ACKNOWLEDGMENTS
 
Appreciation is expressed to Mr. Roderick Patterson, Mr. Roosevelt Johnson, and Ms. Bobbie Johnson for their technical support in conducting the research.


   NOTES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 NOTES
RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
1 Trade names are used in this publication solely for the purpose of providing specific information. Mention of a trade name, proprietary product, or specific equipment does not constitute a guarantee or warranty by the USDA-ARS and does not imply approval of the named product to the exclusion of other similar products. Back


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




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