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© 2006 Plant Management Network. Inter- and Intraplant Competition in Corn Emerson D. Nafziger, Professor of Crop Production Extension, Department of Crop Sciences, 1102 South Goodwin, University of Illinois, Urbana 61801 Corresponding author: Emerson D. Nafziger. ednaf@uiuc.edu Nafziger, E. D. 2006. Inter- and intraplant competition in corn. Online. Crop Management doi:10.1094/CM-2006-0227-05-RV. Abstract While competition for resources within and among plants is understood to affect yield per plant and thus per unit area, direct effects of crop management on plant competition are not always well understood. I narrowly define interplant competition here as a decrease in the amount of light intercepted per plant due to proximity to adjoining plants; such competition may or may not affect corn yield per unit area. Examples of management factors that influence interplant competition include uniformity of plant spacing, timing of plant stand loss, and plant population density. Intraplant competition effects are defined here as factors that affect internal allocation of photosynthate within the plant. I will restrict the discussion to plant damage effects that result in yield loss, and will relate this to reductions in light interception. I conclude that the goal of crop management is to have the corn plant set an adequate number of kernels, then to maximize the interception of light energy during the grainfilling stage in order to maximize yield. Introduction The word "competition" as it relates to the interaction between plants of the same crop species growing next to one another in a field is rather nebulous. On one hand, it is logical that competition must be harmful, since it implies a restriction on crop growth and yield compared to what plants growing without competition can produce. On the other hand, the implication that plants are "strengthened" by their ability to maintain productivity in the face of interference from nearby organisms suggests that competition might be a useful phenomenon, to be exploited for greater crop productivity. This confusion can be decreased if one considers separately the interplant competition having to do with individual plants, and that having to do with plants in a crop plant community. Competition almost always negatively affects the yield of individual plants, by decreasing the amount of light, water, and nutrients to which an individual plant has access. Competition with other plants decreases the photosynthetic capacity of individual plants, and such decreases increase as plant population density increases per unit of land area. For plants in a crop community, decreased productivity of individual plants as interplant competition increases with increasing plant numbers per unit of ground area is accompanied by productivity increases of the community, and these two competing trends are at equilibrium at the optimal plant population density. This phenomenon is governed through intraplant competition, by which the allocation of photosynthates within each plant changes as plant population density increases, to the point where allocation to grain (the product of interest here) by each plant and by the plant community is optimized. Of interest in the current discussion is how plant injury and the manipulation of interplant competition affect yields of corn plants in a crop community. The purpose of this paper is to report on several aspects of inter- and intraplant competition, as measured by yields of individual corn plants and of plants within a crop community. The goal is better understanding of how crop management practices and crop injury affect crop yields. Interplant Competition One of the more obvious determinants of interplant competition is the distance to adjoining plants, both within the row and across rows. Plant population density determines the average distance between plants and so is a primary determinant of interplant competition. But the uniformity of spacing between adjoining plants also determines the degree of competition within a given plant population density. Nielsen (12) reported a loss of about 2.5 bu/acre for each 1-inch increase in the standard deviation of interplant spacing, while Doerge et al. (1) reported an increase of 3.4 bu/acre per 1-inch improvement in standard deviation. However, Lauer and Rankin (3) and Liu et al. (4) reported no yield loss due to plant spacing variability up to standard deviation of about 5 inches. The latter authors subsequently reported, however, that faster planting speed tended to increase plant spacing variability and to decrease yield (5). It is not clear why different investigators have found such diverse responses to plant spacing variability. Part of the reason may be that such variability may not be adequately described by a measure like standard deviation. I showed that missing and doubled plants, which have similar effects on standard deviation of plant spacing, have opposite effects on yield, with doubles often increasing yields and skips usually decreasing yields, through their effects on plant population density (10). Furthermore, skips at low plant population density have a different effect than the same proportion of missing plants when planted at high density. More recently, we attempted to assess the effect of the amount of space each plant occupies in the row with the yield of that individual plant. Grain weight from individual plants is highly variable (Fig. 1), and though there was a significant correlation between space in the row and yield of individual plants at the higher population (30,000 plants per acre), the amount of space each plant occupied in the row explained only about 4 percent of the large amount of variation in grain yield among plants. The incidence of plants with grain yield less than 50 g was 1 and 6 percent at 25,000 and 35,000 plants per acre, respectively, and such plants occurred not only at close plant spacing, but over a wide range of plant spacing. On an area basis, the grain yield was 194 and 203 bu/acre for the lower and higher populations, respectively.
Interplant competition is influenced to a considerable extent by the uniformity of size between neighboring plants down the row. In a study involving hand-planting of corn at different times and in different patterns down the row, we showed that uneven plant size always reduced yield, and that the reduction was related to age (size) difference between competing plants, and to a lesser extent to the percentage of adjoining plants that differed in size; there was slightly more yield loss when alternating plants differed in age than when plants were in alternating groups of three plants of the same size down the row (11). We also showed that plants competed across (30-inch) rows, such that rows bordered with younger (later-planted) rows yielded more than those with border rows the same age. Work in Ontario also showed a large effect of delayed plants growing next to older and larger plants, and the authors concluded that size variability causes considerably larger yield losses in corn than does spacing variability (6). I have conducted several other studies to examine the effect of uneven plant size on corn grain yield. Intellicoat polymer (Landec Ag, Inc., Monticello, IN) is a commercial treatment that involves coating seed with a polymer that allows water to enter the seed only above a certain trigger temperature. I used corn seed coated with more than the normal quantity of polymer (in order to delay water uptake longer), both by itself and and mixed with uncoated seed, in order to influence the range of emergence times and hence plant uniformity. Uncoated, coated, and a 1:1 mixture of coated and uncoated seed in this study produced stands that ranged from 7 to 20 days to 90 percent emergence, and that ranged from 2 to 17 days in the duration of emergence. While all produced the same final stand, coating half or all of the seed decreased grain yield by 11 and 15 percent, respectively (Table 1). While some of this yield decrease was likely related to the delay in the start of plant development due to delayed emergence, previous planting date work (9) would predict a yield loss of less than half this percentage for similar delays. Thus this yield loss is greater than we would have expected from an emergence delay, and so some of the loss was likely due to unevenness of plant size. Table 1. Effect of polymer seed coating on emergence and corn yield. Data are from an unpublished study at Urbana, Illinois, planted on May 7, 1999.
We have also recently completed a study in which some plants were damaged at early growth stages, leaving neighboring plants undamaged, in order to assess effects of uneven plant damage on yield. This was done to simulate hail or insect injury that might affect some plants but not others. Damage was inflicted on plants in the V3, V5, or V7 stage, either on all plants in 4-row plots or on every fourth plant down the row. Two types of damage – cutting off the leaf area and "clubbing" plants from above with a wooden board – were imposed at each stage. Using one site-year (DeKalb, Illinois in 2003) as an example, the largest yield loss came from plants losing all of their leaf area, especially at V7, and from plants being uniformly clubbed at R5 (Fig. 2a). Yield losses were negligible when one-fourth of the plants were damaged, because in this environment, undamaged plants compensated for damaged ones, in one instance (when one-fourth of plants were cut at V7) providing full compensation for damaged plants that produced almost no yield (Fig. 2b). Full compensation also occurred when one-fourth of the plants were simply removed, with no loss in overall yield (Fig. 2a). Thus, while injury to one-fourth of the plants did not decrease overall yield, injured plants contributed little to yield, indicating that such injury can render plants unproductive.
The response of corn to changes in interplant competition due to uneven plant size or injury depends on the ability of plants to retain sufficient developmental plasticity to respond to such changes. Decreasing interplant competition by thinning at different growth stages is one way to test the changes in plasticity. In one such study in Illinois, we thinned plants from 40,000 by removing up to 50% of the stand at different stages. While the ability of remaining plants to compensate decreased as thinning was delayed, removing alternating plants caused yield of remaining plants to increase by 80 percent when thinning was done at stage V4, and by almost 50 percent even when thinning was delayed to V16 (Fig. 3). At this level of stand loss, the ability of plants to compensate decreased at a rate of about 3 percent per (leaf) growth stage. As expected, removing less of the stand resulted in less compensation, but also less "need" to compensate; removal of 12.5 percent of the stand was almost fully compensated by remaining plants, regardless of stage of plant removal.
Intraplant Competition Intraplant competition might be thought of as the set of complex internal interactions by which plant dry matter is allocated to different plant parts. The most interesting part of this, for our purposes, is how dry matter is apportioned to grain or to the rest of the plant. This involves interplant competition as well, as shown in the example of thinning time discussed above. Changing plant population is another way to affect the interaction between interplant and intraplant competition. In a plant population study at Urbana, Illinois in 2004, increasing the population from 15,000 to 40,000 plants per acre caused linear decreases in both kernels per ear and in kernel weight, while yield increased in a curvilinear manner to reach a maximum at about 35,000 plants per acre (Fig. 4). This was in a highly productive environment, and it is not certain that responses of yield components in other environments will be precisely the same, but data from other studies shows very similar responses of yield to plant population.
Defoliation is another means of manipulating photosynthate supply, and perhaps to affect the ability of the plant to allocate dry weight to grain. In a study at Urbana, Illinois conducted over three years (2003-2005), removing one-third, two-thirds, and all of the leaf area decreased grain yield by 3, 32, and 100 percent when done at R1 (silking), and by 7, 28, and 67 percent when done at R3 (kernel milk stage). In an earlier study, Joos found (2) that damaging leaf area using a variety of methods or removing leaf area directly had similar effects on light interception during grainfill (Figs. 5 and 6), and, more importantly, that yield loss was directly proportional to decreases in light interception in two different production environments (Fig. 7).
One question about photosynthate supply is whether or not corn plants might have a mechanism to increase or optimize the amount of light interception. One study in Argentina showed that corn plants of at least some hybrids possess the ability to reorient their leaves during early vegetative development in response to neighboring plants, with the response mediated by red-far red light ratios (7). This reorientation resulted in greater sunlight interception, and was accompanied by reduced tillering and taller plants, all of which might be expected to increase grain yield. Changes in canopy structure were found to be associated with plant population and row spacing in some hybrids (8), though at higher plant populations these changes did not affect light interception by the canopy. In general, though higher light interception and photosynthetic rates should favorably increase plant size during vegetative development, the plant has little ability to "store up" photosynthate to use during pollination and grain fill, hence the advantage of slightly faster development of photosynthetic capacity (leaf area) in young plants may not routinely translate into higher grain yields. In a row spacing study in 2004, light interception was higher in narrow and twin rows at high populations at growth stage V10, but grain yields were more closely associated with light interception at R2 stage (complete canopy) than during vegetative growth (Table 2). Table 2. Light interception at V10 and R2 and grain yield of corn grown at several plant population densities in 30-inch, 15-inch, and twin 22-8-inch rows. Data are from an unpublished study at Urbana, Illinois in 2004.
I see the goal of corn production management as attempting to assure that adequate kernel numbers are set per ear, and then of managing to assure that the corn crop canopy is compete, with at least 97 percent light interception during grainfill, and that this canopy remains intact as long as possible to complete grainfill. Within normal ranges of plant population, the increase in yield from increasing plant population is directly related to increases in light interception (Fig. 8). This implies that intercepted light is in fact used in photosynthesis, an assumption that may not always hold if leaves are not fully functional, as might be the case with, for example, foliar diseases. If this assumption holds, however, then managing row spacing, plant population density, soil nutrient supply, and protection of the canopy should result in maximum light interception, particularly during grainfill, and so should work to maximize yields.
Literature Cited 2. Joos, D. K. 2001. Effects of leaf area removal and damage on canopy light interception and corn grain yield. MS thesis, Univ. of Ill., Urbana. 3. Lauer, J., and Rankin, M. 2004. Corn response to within row plant spacing variation. Agron. J. 96:1464-1468. 4. Liu, W., Tollenaar, M., Stewart, G., and Deen, W. 2004. Within-row plant spacing variability does not affect corn yield. Agron. J. 96:275-280. 5. Liu, W., Tollenaar, M., Stewart, G., and Deen, W. 2004. Impact of planter type, planting speed, and tillage on stand uniformity and yield of corn. Agron. J. 96:1668-1672. 6. Liu, W., Tollenaar, M., Stewart, G., and Deen, W. 2004. Response of corn grain yield to spatial and temporal variability in emergence. Crop Sci. 44:847-854. 7. Maddonni, G. A., Otegui, M. E., Andrieu, B., Challe, M., and Casal, J. J. 2002. Maize leaves turn away from neighbors. Plant Physiol. 130:1181-1189. 8. Maddonni, G. A., Otegui, M. E., and Cirilo, A. G. 2001. Plant population density, row spacing and hybrid effects on maize canopy architecture and light attenuation. Field Crops Res. 71:183-193. 9. Nafziger, E. D. 1994. Corn planting date and plant population. J. Prod. Agric. 7:59-62. 10. Nafziger, E. D. 1996. Effects of missing and two-plant hills on corn grain yield. J. Prod. Agric. 9:238-240. 11. Nafziger, E. D., Carter, P. R., and Graham, E. E. 1991. Response of corn to uneven emergence. Crop Sci. 31:811-815. |
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