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© 2003 Plant Management Network. Corn Greensnap from Extreme Wind is Influenced by Several Factors Roger W. Elmore, Department of Agronomy and Horticulture, University of Nebraska, Lincoln 68583; George Hoffmeister, Jr., 30969 Road L, Clay Center, NE 68933; Ralph Klein, P.O. Box 66, University of Nebraska, South Central Agricultural Laboratory, Clay Center 68933; and David B. Marx, Department of Biometry, University of Nebraska, Lincoln 68583 Corresponding author: Roger W. Elmore. relmore1@unl.edu Elmore, R. W., Hoffmeister, G., Jr., Klein, R., Marx, D. B. 2003. Corn greensnap from extreme wind is influenced by several factors. Online. Crop Management doi:10.1094/CM-2003-1212-01-RS. Abstract Severe storms with downdrafts and straight-line wind speeds up to 100 miles per hour (mph) in Nebraska resulted in widespread mid-season corn (Zea mays) stalk breakage (greensnap), in July 1993 and 1994. These weather events reduced grain yields up to 90%. Nebraska was again affected by winds up to 70 mph associated with thunderstorms in July 1998 and June and July 2000. We evaluated plants from several hybrids in small plot trials as well as in producers' fields in order to determine if leaf orientations (azimuths) affect a corn plant's tolerance to greensnap. Although hybrids differ greatly in their susceptibility to greensnap, leaf orientations of hybrids were similar and thus do not appear to affect susceptibility to greensnap. However, for plants broken at the node, the orientations of leaves attached to broken nodes were most often parallel to the direction of the damaging wind and pointing leeward while for the majority of plants broken at the internode, orientations of leaves attached to the broken internode were oriented into the direction of the damaging wind (windward). Leaf orientations of standing plants were often in a somewhat symmetrical, bimodal pattern and more uniformly distributed than broken plants. Those who simulate greensnap would best attempt to imitate these breakage patterns with mechanical stalk breakage simulators. Rows perpendicular to extreme winds suffer the most damage. We have not often seen both east-west and north-south rows affected in the same area from the same storm. Since predicting extreme winds is difficult we suggest producers choose tolerant hybrids and alter row direction among fields to reduce the risk of an extreme wind from damaging several fields. A range of planting dates and hybrid maturities may also reduce risk. Introduction Agricultural production and profit are adversely affected by unique weather events including frost, hail, flood, and extreme winds. Research to clarify viable options for producers not only is difficult and non-traditional, but is also opportunistic because of the nature of the problem. It is not possible with limited funds to plan, design, and locate experiments affected by unique weather events with certainty. Yet research is necessary to determine factors that may reduce the impact of future unique weather events. Greensnap, or mid-season corn stalk breakage, is caused by a unique and unpredictable weather event. It has occurred several times in Nebraska since the devastating storms of July 1993 and 1994 which contained extreme winds and downdrafts with straight-line wind speeds of 100 and 80 miles per hour (mph), respectively. Corn yields in south-central Nebraska fields were reduced more than 90% (5,18). The storms contained widespread and fast-moving thunderstorms characterized by damaging straight-line winds. Corn hybrids differed dramatically in their susceptibility to breakage in the 1993 and 1994 events; yields were directly proportional to the amount of stalk breakage (5). Unfortunately, factors that accelerated early-season plant growth and normally enhance productivity actually increased the crop's susceptibility to breakage (5). Among these factors were increased N rates, pre-plant N instead of side-dressed N, and conventional tillage instead of no-tillage. Increased stalk breakage was also closely related to increased soil organic matter. Wilhelm et al. (18) observed that plant growth stage strongly affected stalk breakage in 1993 (r = 0.55) but not in 1994. Seed companies made considerable attempts to improve their genetic materials with greensnap simulators. Leaf orientations (azimuths or horizontal orientations), may provide insight on why some plants break and some do not. Leaf orientations are expressed as the angular distance between the direction of a fixed point and the direction of the leaf midrib at the point where the midrib first projects away from the stalk. Most photosynthetic models assume that leaves are randomly placed (10) and that total leaf area is considered a more important factor than leaf placement. Researchers on other crops, however, have shown that leaf placement is not random in grain sorghum (Sorghum bicolor (L.) Moench) (13), sunflower (Helianthus annuus L.) (15), tobacco (Nicotiana tabacum L.) (16), and wheat (Triticum aestivum L. emend. (Thell.)) (11). The degree of inter-plant competition affects leaf placement in most crops. Corn researchers early on found that successive leaves alternate on opposite sides of the stalks and that each leaf slightly overlaps the leaf below it. In the absence of plant-to-plant competition, leaves form a spiral on the stalk. In normal corn production fields, however, corn leaves above the ear tend to reorient perpendicular rather than parallel to the row (7,8,9). This leaf orientation shift reportedly begins at the sixth leaf stage and is the result of twisting of the internode (8). Girardin and Tollenaar (9) related the leaf orientation shift to a twisting of the soft tissues of sheaths and internodes that are not fully lignified. Drouet and Moulia (2,3) found that changes in leaf orientation continued to increase between leaves 11 and 14 (leaf tip method) (17). The leaf orientation shift is a shade tolerance mechanism triggered by reduced red:far red light ratios (1). Yet, based on simulations, Drouet et al. (4) concluded that leaf orientation had little effect on daily light absorption efficiency. Corn in the western U.S. Corn Belt is subjected to more storms with extreme winds than corn in more eastern areas. Late June to mid-July storms with surface winds above 60 mph have caused greensnap. Only nine storm systems had wind speeds that high in Illinois, Indiana, and Ohio combined from 1994 to 2002. During the same years there were 200 potentially damaging storms in Iowa, 95 in Kansas, and 107 in Nebraska (14). Crop damage estimates over the nine years in the latter three states were 27, 10, and 107 million dollars, respectively. Storms generally moved from northwest to southwest or from west to east. Most of the Nebraska and Kansas storms occurred in the late afternoon through 3 a.m. while the storms in IA occurred either in early afternoon and or around midnight (data not shown). No one has reported on the effects of storm timing on greensnap although our unpublished data suggest there is a relationship. Prevailing early summer winds are from the south to southeast in Nebraska. This too affects plant growth and may predispose a plant to damage from extreme winds (unpublished data). Several northern Nebraska locations were affected by extreme winds (up to 70 mph) associated with thunderstorms in July 1998 (14). In another incident, central and south-central Nebraska locations were affected by extreme winds (up to 71 mph) associated with thunderstorms in late June and early July 2000 (14). We evaluated corn in small plot trials as well as in producers' fields after these storms to determine if leaf orientations affected a corn plant's tolerance to greensnap. If so, we wanted to determine the implications to producers, to researchers simulating greensnap, and if leaf orientations of hybrids were similar. Data Collection Locations in 1998 Data were recorded at six locations in northern Nebraska in 1998 near Creighton, Page, and Osmond (Tables 1 and 2). All damage resulted from a 6 July 1998 storm system with 60 to 70 mph winds. Five of the locations were irrigated; one (Creighton rain dependent) was not. All Creighton locations were on producers' fields; the Osmond and Page trials were nurseries managed by Pioneer Hi-Bred International also on producer's fields. The trials at the Osmond and Page locations were part of a joint project of Pioneer Hi-Bred International and University of Nebraska South Central Research and Extension Center to ascertain the effects of extreme winds on diverse hybrids with a range of greensnap scores. Data were recorded only from plots with broken plants with the actual number of hybrids shown in Table 1. Table 1. Location characteristics and details of wind and stalk breakage, 6 July 1998. The storm hit Page at 3:30 p.m., Creighton at 3:40 p.m., and Osmond at 4:30 p.m. Wind speeds recorded at Page and Osmond were 60 mph and 70 mph at Creighton.
a 1 to 7, poor to good greensnap tolerance (Pioneer Hi-Bred Int'l rating); b n = number of plants in data set. c Rain dep. = rain dependent, i.e. not irrigated. All other locations were irrigated. Table 2. Broken stalk percentage, average broken node, and leaf orientation of broken and standing plants, 6 July 1998.
a Rain dep = rain dependent. b Node 7 was the first one above the soil surface. c Sig. = Significance. Probability that broken plant orientation is different than that of standing plants: ns = not significant; ** P < 0.01. All plots at Page and Osmond had two replicates, were two rows wide, and were surrounded by an entire field of corn. Plots were 15 ft long. At Creighton all plants were within contiguous 10-ft-long, single-row plots. A total of 44, 57, 87, and 62 plots were examined at the north, west, south, and rain-dependent trials, respectively. Hybrids grown at the Creighton locations were: Pioneer 34R06, 34R06, 35N05, and Curry 2152 for the North, West, South, and Rain dependent location respectively. Data Collection Locations in 2000 Data were recorded at five irrigated locations in central and south central Nebraska in (Tables 3 and 4). Damage at Clay Center resulted from a 25 June storm system with 60 mph winds; damage at Bartlett resulted from a 5 July storm system with 71 mph winds. At Clay Center-Southwest and all three fields at Bartlett we recorded data on all plants within 30 contiguous 10-ft-long, single-row plots. At Clay Center-East we recorded data on all plants within 3 sets of 30 contiguous 10-ft-long, single-row plots. In both years and at all locations rows were spaced 30 inches apart. Corn was within a few days of tasseling when the storms occurred. Table 3. Location characteristics and details of wind and stalk breakage, 25 June 2000 at Clay Center and 5 July 2000 at Bartlett. The storm hit Clay Center at 2:55 p.m. with wind speeds of 60 mph and Bartlett at 9:35 a.m. with wind speeds of 72 mph.
a CC = Clay Center (Southwest trial, East trial, etc.); Bart = Bartlett. The Clay Center-Southwest field was gravity-irrigated with gated pipe; all others were center-pivot irrigated. b DeKalb hybrids DK 589 and DK 611 were planted on the west and east sides of DK655, respectively. They each had less than 7% breakage. Stalk breakage tolerance for these two hybrids were 6 and 3 respectively. c DeKalb hybrids, 1 to 9 rating scale breakage tolerance where 1-2 = Excellent to 9 = poor; Pioneer hybrids, 1 to 7 = poor to good greensnap tolerance; na = not available. d Pioneer brand 31A13 (Pioneer hybrid score = 5) was planted on the south half of this pivot irrigated field. It sustained minimal breakage. e Number of plants per category in data set. Table 4. Broken stalk percentage, average broken node, and leaf orientation of broken and standing plants, 2000.
† CC = Clay Center (Southwest trial, East trial, etc.); Bart = Bartlett. ‡ Leaf orientation means within a location-trial followed by the same letter are not different, P > 0.05. Leaf Orientation Determination The direction in which the leaf attached to the broken node or internode was pointing is referred to as leaf orientation. It was difficult to determine this on leaves at the broken nodes since they were often damaged when the plant was broken. The orientation of the sheath placket on the next lower node was easily estimated (Fig. 1). This is the orientation of the uppermost point where the two edges of the sheath overlap. We estimated the orientation of the leaf sheath placket on leaves attached to the node below the break using a one-to-twelve scale (12 = north, 3 = east, 6 = south, etc.). Orientations were later converted to degrees for statistical analyses. We assumed that leaf sheath placket orientations on the node below the break were oriented in approximately the same direction as the leaf attached to the broken node. Leaf orientations of standing plants were recorded from sheath plackets on the same node as that of the average of the broken plants in the same plot.
Damaging wind directions were determined at each location by a combination of eye-witness reports and the direction utility poles, trees, plants, etc. were laying following breakage. Several downdrafts resulted in crop damage at the Clay Center locations in 2000. Wind speeds and timing of the thunderstorms were obtained from the National Climatic Data Center, 2003. Neither leaf area nor leaf angles were measured because data were collected up to 6 to 8 weeks following extreme winds which caused greensnap. Statistical Analyses and Data Presentation Leaf orientation data were statistically analyzed using the von Mises distribution of circular data (6). Data resulting from this analysis were subsequently analyzed using PROC Mixed (12). We used a randomized complete block experimental design. Locations were analyzed separately. Leaf orientation data were weighted by the number of broken and standing plants in each plot. Replicate and recording crew in 1998 and individual men recording data in 2000 were considered random effects in the analyses. Leaf orientation data from broken and standing plants were plotted on Figures 2 to 7 at a radial distance equal to the relative frequency of the data in that orientation range. These points were connected by a line. Thus, data for each plant category are from the 12 possible orientations and do not represent data between the individual rays of the diagrams. "Total plants" in these figures are the sums of broken and standing plants. General Field Observations Field observations following greensnap events have generally shown that corn in rows perpendicular to the extreme winds are most affected. We have not often seen rows parallel to extreme winds affected (5). That was also the case in our 1998 data where east-west rows were damaged more severely than north-south rows in the Creighton and Page areas. At Osmond, north-south rows were more damaged than east-west rows. We have seen two exceptions to this generalization. Both north-south and east-west rows were damaged in the Clay Center area in 2000. At least three downbursts and associated straight-line winds developed within that system. Eyewitness reporters state that the extreme winds began from the southwest, shifted with time to the west, and finally to the northwest. Breakage patterns at Clay Center-East reflect the effects of the multiple-down burst system whereas those of Clay Center-Southwest did not reflect such strong patterns. Another exception to these observations is when crops are subjected to swirling winds possibly from a tornado. In these situations stalks were often broken regardless of row orientation. Our data were recorded on east-west rows at Creighton, Page, Bartlett, and Clay Center-East, and in north-south rows at Osmond and Clay Center-Southwest. Plant densities varied considerably among locations in both years (Tables 1 and 3) and within locations especially in 1998. These differences are typical of Nebraska irrigated and rain-dependent corn. Plant densities across all plots at the Creighton rain-dependent trial ranged from 13.9 to 22.6 × 103 plants per acre and from 12.2 to 45.3 × 103 plants per acre at the Creighton irrigated trials. In contrast to results of Girardin and Tollenaar (10), leaf orientations were not affected by plant density in the Creighton trials (based on analyses of a leaf orientation with plant density as a covariate). However, our studies were not designed to precisely test for plant density effects on leaf orientations. The average node of break varied among locations (Table 2 and 4); however, it was always below the primary ear node. Yield losses are more severe when breakage occurs below the primary ear node (18). Node seven at all locations was at the soil surface. Broken-Standing Plant Comparisons Leaves attached to broken nodes of broken plants were oriented differently than those of standing plants in five of the six 1998 locations (Table 2) and in three of the four, 2000 locations with breaks at the node (Table 4). The Clay Center-Southwest location in 2000 had only internode breaks. We assume that seed orientation was random at planting; if so, leaf arrangement was random until red/far red (R/FR) light reflection differences received by the seedlings altered the leaf orientation. In theory, average leaf orientations should be near 180°N. At Page in 1998, where orientations of broken and standing plants were not different, damaging winds were nearly perpendicular to the row direction. Broken plant leaf orientations (178°N) in this situation were also near 180° and thus not different from that of standing plants (188°N) (Table 2). This was similar to the Bartlett-South situation in 2000, where orientations of node-broken and standing plants were not different (Table 4). The standing plant distribution was distinctively bimodal with lobes oriented to the northeast and southwest (Fig. 2). Thus, the average orientation of standing plants was southeast (147°) and not different than that of the node-broken plants (149°).
The Bartlett-South breakage pattern (Fig. 2) has some of the least complex orientation data observed. There were only five plants broken in the internode of the 1321 plants sampled. The roughly symmetrical, bimodal distribution of the standing plant leaf orientations is punctuated with a distinct plume representing node-broken plants. Node-broken plants were generally oriented parallel to and leeward of the damaging winds. The Bartlett-East leaf distribution patterns were similar to those of Bartlett-South except there were a higher proportion of node-broken plants relative to standing plants at Bartlett-East and there were more internode-broken plants (Table 3). The leaf distribution modes of standing plants at both of these sites were on opposite sides of a southeast-to-northwest plane (Fig. 2). In contrast to these patterns, standing plant leaf distributions at Page were largely oriented north-south with relatively more standing plants whose leaves were oriented to easterly and westerly directions. This results in figures that possess more of a diamond appearance (not shown) compared to the Bartlett-East and -South locations. In addition, about half of the plants were oriented to the northern directions and about half were pointing to the southerly directions with some symmetry around an east-west line (parallel with the row direction). All plants broken at Page were broken at the node (Table 2). The Osmond 1998 data is also characterized by a bimodal leaf orientation pattern but it is centered on a north-south line (Fig. 3). Rows ran north-south and the damaging wind came from the northwest, 290°. Broken plant leaf distributions formed a plume to the east, roughly parallel with the wind and pointing leeward. All broken plants were broken at the node.
In contrast to the 1998 data, all damaged plants were broken in the internode at the 2000 Clay Center-Southwest location (Fig. 4). This was the first time we found all damaged plants in a field broken in the internode. The break points were ¼ to ½ way up the internode from the node. The broken internodes were often two internodes below an internode that was either not expanding or just beginning to expand, and just above an internode that appeared fully elongated. In contrast to leaf distribution patterns of plants broken at the node, leaves of plants broken in the internode generally point in the direction from which the damaging wind came (windward). Two other DeKalb hybrids (DK 589 and DK 611, data not shown) were planted in the same field but sustained only 7% breakage in contrast to 21% breakage for DK 655; all the damaged plants were broken in the internode for all three hybrids. Also, leaf distribution and patterns of broken and standing plants were similar among all three hybrids at Clay Center-Southwest. We are not certain why leaf orientations of plants broken at the internode are opposite those of plants broken at the node.
Creighton data are presented in Tables 1 and 2 with all four sites well represented by the Creighton-South data (Fig. 5). All damaged plants were broken at the node and rows ran east-west. Total and standing plant leaf orientations distributions are multi-lobed in contrast to the bi-lobed distributions at other sites; larger portions of total plants were oriented to the east-northeast than to the other directions. Broken plant leaf orientations were again mostly aligned to the east and northeast and were parallel to and leeward of the damaging wind.
Data obtained from Bartlett-Southwest and Clay Center-East locations are the most complex we have collected to date. Rows at both sites were planted east-west; data are shown in Tables 3 and 4. The Bartlett-Southwest data show a slightly asymmetrical bimodal distribution of leaf orientations of standing plants (Fig. 6). Standing plant leaf-distribution modes were oriented to the northeast and southwest. Differences are obvious between the plumes of the plants broken in the internode (pointing windward) and those broken at the node (pointing leeward).
Standing plants at Clay Center-East (Fig. 7) also show a slightly asymmetrical bimodal distribution of leaf orientations of standing plants. The two leaf distribution modes for standing plants are again on opposite sides of a southeast-to-northwest plane. Plants broken at the node had average leaf orientations of 177°. The distinctly bimodal and slightly asymmetrical pattern of leaf orientations of plants broken at the internode are quite different from what we have seen at other locations. This supports reports from eyewitnesses who state that the extreme winds began from the southwest, then came from the west, and finally from the northwest. Internode breakage patterns at Clay Center-East reflect the effects of the multiple, down-burst system more than those of Clay Center-Southwest (Fig. 4).
Similarities Among Hybrids If leaf orientation differences exist among hybrids, they may predispose hybrids to stalk breakage. We had four location-trials where 6 to 64 hybrids with a wide range of greensnap tolerance/susceptibility were examined; two locations are reported here. Average leaf orientations of all plants (broken and standing) did not differ among hybrids in any of these trials (data not shown). In addition, leaf orientations of broken and standing plants were consistent among hybrids. Thus, although the proportion of broken plants varied considerably among hybrids, leaf orientation of all hybrids responded to interplant competition in a similar way. That means that although leaf orientation affects which plants will break, it does not appear to affect which hybrids will break. Conclusions and Implications Certainly a hybrid’s greensnap tolerance and maturity, wind direction, row direction, management, and location in a field are all factors that influence greensnap. We have learned that plants broken during extreme-wind events were consistently oriented differently than standing plants. Leaves attached to the broken node of plants broken at the node were usually oriented parallel to the direction of the damaging wind and pointing leeward. Leaves attached to the broken internode of plants broken at the internode were usually oriented into the direction of the damaging wind, windward. Standing plants were more uniformly oriented than broken plants but were often distributed in a somewhat symmetrical, bimodal pattern. These observations have important implications to researchers and corn breeders. Many corn breeding programs use mechanical stalk breakage simulators to screen breeding lines and hybrids. To simulate the effects of destructive winds most accurately, we suggest that simulators largely break plants whose leaves are oriented more or less perpendicular to the row. Specifically for nodal breaks, researchers should break plants whose leaves at the break node are oriented parallel to the direction of the force applied, and break plants in such a way that leaves attached to the broken nodes point in the same direction as the force of breakage. For internode breaks, break plants in such a way that leaves attached to broken internodes point in the opposite direction to the force of breakage. More work is necessary to determine the factors which influence some plants to break in the internode and others to break at the node. Figure 8 illustrates various potential leaf orientations and the plants that are likely to break at the node and in the internode given a specific direction of force applied.
Leaf orientations among hybrids did not differ in any of these trials. In addition, leaf orientations of broken and standing plants were consistent among hybrids. Thus, hybrid responses to red/far red light reflection were similar and apparently did not affect leaf orientation and subsequent responses to damaging winds. More work is needed in this area. There are several ways producers may avoid or reduce the risk associated with extreme winds. Leaves of most broken plants were oriented nearly perpendicular to the row direction and parallel to damaging winds. If producers could alter row directions, it may reduce the risk of an extreme wind event from damaging several fields of corn in a given area. We certainly realize that certain fields may be best planted in specific row orientation due to obvious restraints such as furrow irrigation, field layouts, etc. We also realize that producers cannot predict the direction of damaging winds. Nevertheless, varying row directions among fields will help reduce risk associated with extreme winds. In addition, planting greensnap tolerant hybrids, spreading planting dates, using hybrids of diverse maturities will likely reduce damage from extreme winds. Acknowledgments We appreciate the contributions of the following people: Drs. Leroy Svec and David Benson, Pioneer Hi-Bred International, York, NE for hybrid selection, plot planting, and maintenance at the Page and Osmond locations and for partial financial support; Terry Gompert, Extension Educator, for location selection in the Creighton area; producers Warren Wortman and David Sorenson, for allowing us to obtain data from their fields near Creighton; Steve Niemeyer Extension Educator, for location selection in the Bartlett area; producers Ern Erxleben, Rob Ita, and Scott Plugge for allowing us to obtain data from their fields near Bartlett; producers Ken Spray and Lyle and Scott VonSpreckelson for allowing us to obtain data from their fields near Clay Center; and for our part-time employees Jim Pavelka, Jena Fitzke, Kristie Hajny, and Andy Unterseher. We also thank Kim Peterson for her work on the figures and Sandy Sterkel for her skills in editing and formatting. Without the assistance of all these people, this work could not have been accomplished. (Univ. of Nebraska, Lincoln, Agric. Res. Div. J. Ser. No. 12999.) Literature Cited 1. Ballare, C. L., Scopel, A. L., Sanchez, R. A. 1990. Far-red radiation reflected from adjacent leaves: an early signal of competition in plant canopies. Science. 247:329-332. 2. Drouet, J. -L., and Moulia, B. 1996. Spatial re-orientation between successive leaves in maize. Aspects Appl. Biol. 46:135-138. 3. Drouet, J. -L., and Moulia, B. 1997. Spatial re-orientation of maize leaves affected by initial plant orientation and density. Agr. Forest Meteorol. 88:85-100. 4. Drouet, J. -L., Moulia, B., Bonhomme, R. 1999. Do changes in azimuthal distribution of maize leaves over time affect canopy light absorption? Agronomie 19:281-294. 5. Elmore, R. W., and Ferguson, R. B. 1999. Mid-season stalk breakage in corn: hybrid and environmental factors. 1999. J. Prod. Agric. 12:293-299. 6. Fisher, N. I. 1993. Statistical analysis of circular data. Cambridge Univ. Press, New York, NY. 7. Fortin, M. -C., and Pierce, F. J. 1996. Leaf azimuth in strip-intercropped corn. Agron. J. 88:6-9. 8. Girardin, P. 1992. Leaf azimuth in maize canopies. Eur. J. Agron. 1:91-97. 9. Girardin, P., and Tollenaar, M. 1992. Leaf azimuth in maize: origin and effects on canopy patterns. Eur. J. Agron. 1:227-233. 10. Girardin, P., and Tollenaar, M. 1994. Effects of intraspecific interference on maize leaf azimuth. Crop Sci. 34:151-155. 11. Ledent, J. F. 1975. Leaf azimuth in wheat canopies. Cereal Res. Commun. 3:279-287. 12. Littell, R. C., Milliken, G. A., Stroup, W. W., and Wolfinger, R. D. 1996. SAS System for mixed models. SAS Institute Inc., Cary, NC. 13. Lugg, D. G., Youngman, V. E., and Hinze, G. 1981. Leaf azimuthal orientation of sorghum in four row directions. Agron. J. 73:497-500. 15. Shell, G. S. G., and Lang, A. R. G. 1975. Description of leaf orientation and heliotropic response of sunflower using directional statistics. Agr. Meterorol. 12:33-48. 16. Shin, J. S. 1982. An ecological study on the tobacco population. Part VI. The effect of plant spacing and density on the leaf distribution pattern and some agronomic characteristics. Res. Rep. Tobacco Res. Instit. 9:53-62 [Korea]. 17. Tollenaar, M., Daynard, T. B., and Hunter, R. B. 1979. The effect of temperature on rate of leaf appearance and flowering date in maize. Crop Sci. 19:363-366. 18. Wilhelm, W. W., Liebig, M. A., Varvel, G. E., and Blackmer, T. M. 1999. Midseason stalk breakage in corn as affected by crop rotation, hybrid, and nitrogen fertilizer rate. Agron. J. 91:160-165. |
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