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In production agriculture, it is not uncommon for a crop to experience both intra- and interspecific competition during the normal course of development. Although the competition between crop plants (i.e., intraspecifc) is often considered independently of crop-weed competition (i.e., interspecific), the mechanisms through which yields are reduced may be common to both. The objective of this study was to use the experimental structure of a critical time for weed removal study to examine the timing and effect of intraspecific competition on maize (Zea mays L.) biomass accumulation and phenological development. A field trial was conducted in which maize stands were thinned from a higher to a lower density at six stages of development. Results indicated that intraspecific competition at densities of 8 and 16 plants m -2 did not affect maize biomass accumulation until the 14th and 12th leaf tip stages, respectively. Before these stages, maize seedling growth at 8 or 16 plants m -2 was not resource limited. Increases in leaf area index and specific leaf area at the onset of intraspecific competition, and the recovery of plants following the removal of competitors, suggest that reductions in the rate of crop growth and development may have been linked to competition for light quantity.

Mounting concerns over the cost and environmental impact of N fertilizer combined with progressively higher plant densities in maize (Zea mays L.) production systems make progress in maize N use efficiency (NUE) and N stress tolerance essential. The primary objectives of this 3-yr field study were to (i) evaluate the N responsiveness, NUE, and N stress tolerance of multiple modern maize genotypes using suboptimal, optimal, and supraoptimal plant densities (54,000,79,000, and 104,000 plants ha -1 , respectively) with three levels of side-dress N (0, 165, and 330 kg N ha -1 ), (ii) identify key morphophysiological responses to the simultaneous stresses of intense crowding and low N availability, and (iii) consider our results with extensive reference to literature on maize morphophysiological responses to plant crowding and N availability. At optimal and supraoptimal plant densities, maize receiving 165 kg ha -1 of side-dress N displayed strong N responsiveness, high NUE, pronounced crowding tolerance, and plant density independence. However, crowding tolerance was contingent on N application. Relative to less crowded, N-fertilized environments, the 104,000 plants ha -1 , 0 kg N ha-1 treatment combination exhibited (i) reduced pre- and postanthesis plant height (PHT), stem diameter (SD), and total biomass; (ii) greater preflowering leaf senescence and lower R1 leaf areas at individual-leaf, per-plant, and canopy levels; (iii) enhanced floral protandry; (iv) lower pre- and postanthesis leaf-chlorophyll content; (v) lower per-plant kernel number (KN P ), individual kernel weight (KW), grain yield per plant (GYp), andharvest index per plant (HI P ); and (vi) enhanced per-plant grain yield variability (GY CV ). Genetic efforts to improve high plant density tolerance should, therefore, simultaneously focus on enhancing NUE and N stress tolerance.

Conflicting results have been reported on the effects of spacing and emergence variability on grain yield in maize (Zea mays L.). Effects of spacing and emergence variability on maize grain yield are the net result of the responses of all plants within the stand. The objective of this study was to quantify effects of spacing and emergence variability on crop yield in terms of increased or decreased crowding stress on resource capture (i.e., dry matter accumulation) and resource utilization (i.e., dry matter partitioning) of the individual plants within the crop canopy. Results of previously reported studies were analyzed in terms of plant dry matter accumulation, leaf area, plant growth rate during the critical period for kernel set bracketed by silking (PGR s ), grain yield, and harvest index, that is, the proportion of dry matter partitioned to the grain at maturity. Results show that a moderate increase in plant-spacing variability does not influence maize grain yield at the canopy level because reductions in grain yield of plants that experience enhanced crowding stress is compensated, in part, by increased yield of plants that experience reduced crowding stress; crowding stress affected dry matter accumulation but did not affect harvest index. In contrast, plant-emergence variability reduced grain yield at the canopy level because the reduction in grain yield was attributable, in part, to a reduction in harvest index of plants with PGR s less than the threshold for kernel set. Hence, plants can compensate for factors that influence resource capture, but cannot compensate for a reduction in factors that influence resource utilization.

Accurate estimates of total leaf area and the vertical leaf area profile are important in process-based crop growth models. The ben-shaped function that quantifies the area-per-leaf profile of a maize (Zea mays L.) plant can be used to estimate the area-per-leaf profile. The objectives of this study were to quantify the effects of maize hybrid, soil N, plant density, and row spacing on the coefficients of the bell-shaped function. The coefficients of the bell-shaped function that quantify (i) the breadth of the area-per-leaf profile, (ii) the skewness of the area-per-leaf profile, and (iii) the position of the largest leaf were estimated using nonlinear regression in four datasets. Datasets consisted of the fully expanded leaf areas of all leaves on maize plants grown in studies performed in Ontario, Canada, between 1997 and 2001 that included combinations of maize hybrids, plant densities, N levels, and row spacing. Observations fitted well to the bell-shaped function (r 2 > 0.95). The breadth of the area-per-leaf profile decreased under high soil N level and high plant density, and was lower for a newer than an older hybrid, whereas the opposite occurred with the position of the largest leaf. In contrast, the degree of skewness was not significantly altered by any of the factors examined in this study. Because of the relatively small impact of the examined agronomic factors on the coefficients of the bell-shaped function, a general model using mean coefficient values was validated with independent datasets. Results showed that this general bell-shaped function is a robust predictor of the area-per-leaf profile in maize.

Planter type, maintenance, and operation play an important role in uniform stand establishment in corn (Zea mays L.). Research was conducted to determine if planter type affects corn yield by altering plant spacing and emergence variability and to determine if planting speed and tillage influence these effects. This experiment was performed at two locations in south-central Ontario during a 2-yr period. Treatments were established with conventional tillage (CT) and no-tillage (NT) as main plots, three planter types (vacuum meter, finger-pickup, and air seeder) with differing mechanisms including varied seed-singulating mechanisms as subplots, and two planting speeds of 7.2 and 11.3 km per hour (kph) as sub-subplots. Planter type affected stand uniformity with mean standard deviation (SD) of within-row plant spacing of 7.9,10.3, and 19.9 cm for vacuum meter, finger pickup, and air seeder, respectively. A higher SD was observed in NT for finger pickup and air seeder but remained the same for vacuum meter. For all planters, SD increased at faster planting speeds. The number of days required to achieve 50% emergence was similar between the vacuum meter and finger pickup but was greater for the air seeder, especially when planting speed was increased and NT was used. Final plant population was unaffected by planter and planting speed treatments. Overall, grain yields decreased 35.9 kg ha -1 for each centimeter increase in within-row plant spacing SD and 292.8 kg ha -1 per day of delay in emergence. Results suggest that grower's attention to corn planter mechanisms and maintenance is more critical under a NT system or when operating speeds are increased.

Nonuniform plant spacing within the row in corn (Zea mays L.) may reduce grain yield. To investigate the response of corn to plant spacing variability, experiments were conducted at two locations in south-central Ontario during 2000 and 2001. Six plant spacing treatments, 6.7 to 16.2 cm in standard deviations (SD), were established by planting Roundup Ready corn with increasing proportions of conventional corn seeds and then removing the conventional corn using glyphosate before three-leaf stage. Using SD as well as short gap, long gap, double, and cluster as an index of plant spacing variability, effects of plant spacing variability on corn growth and grain yield were investigated. Averaged across locations and years, grain yield was not significantly affected by plant spacing variability. Plant spacing variability also had no significant effect on leaf number, plant height, leaf area index, and harvest index. There were no correlations between plant spacing variability and stalk lodging and barren or double ears. The lack of strong correlations among plant growth, grain yield, and plant spacing variability indicates that spacing uniformity within the range used in this study is not a significant factor in determining grain yield under commercial conditions and common plant densities used in Ontario.

repeatedly demonstrated that knowledge of phenological development is critical in understanding crop growth, An understanding of the environmental variables influencing the yield potential, and for prediction of phenology (Grant, phenological development of weeds is essential for simulation model development. Temperature and photoperiod are important variables 1989; Miller et al., 1993). Similarly, it is essential to governing the phenological development of weeds. Growth cabinet understand weed phenology in order to develop prestudies were conducted to characterize the phenological development dictive models. of barnyardgrass [Echinochloa crus-galli (L.) Beauv.] in response to Barnyardgrass is a common and troublesome C4 weed variations in temperature and photoperiod and to determine the dura- capable of infesting a variety of crops such as rice (Oryza tion of the juvenile phase and the effect of temperature and photoperiod sativa L.), soybean [Glycine max (L.) Merr.], cotton on reproductive development. Barnyardgrass was adapted to a tempera- (Gossypium hirsutum L.), corn (Zea mays L.), wheat ture range of 6.5 to 528C. Phenological development of barnyardgrass (Triticum aestivum L.), vegetables, beans, root crops, was described in terms of thermal days (cumulative day degrees above forage crops, orchards, and pastures in most of the aga base temperature for leaf appearance, tiller appearance, and shoot

Grain yield improvement of maize (Zea mays L.) hybrids in Ontario has been associated with increased stress tolerance of newer hybrids. This study was conducted to assess the effect of low soil N, weed interference, and soil water deficit on dry matter accumulation and grain yield of an older and a more recent hybrid. Studies were carried out at the Elora Research Station in 1992, 1993, and 1994 on a London loam (Aquic Hapludalf) that had been tile drained. The experiment was arranged in a split-split-split-plot design with four replications. Treatments consisted of two soil-N levels (70 and 225 kg ha -1 ), weed-free all season and weedy from the 3- to 4-leaf stage of maize, irrigation and no irrigation, and the hybrids 'pioneer 3902' (new) and 'Pride 5', (old). Maize phenology, dry matter accumulation, grain yield, stomatal conductance, leaf chlorophyll fluorescence, and leaf chlorophyll content were measured. Irrigation generally did not influence the measured parameters. Silking date and stomatal conductance during the grain-filling period were affected less by low soil N and weed interference in Pioneer 3902 than in Pride 5. Aboveground dry matter at silking and maturity was reduced about 20% by single stresses of low soil N and weed interference and about 55% by the combination of both stresses. Hybrid x stress treatment interactions for dry matter were not significant. The reduction in grain yield due to weed interference was 21% greater in Pride 5 than in pioneer 3902. The hybrid x N interaction for grain yield was not significant, because a smaller reduction in harvest index due to N stress in the new than in the old hybrid was offset by a larger reduction in dry matter accumulation during the grain-filling period. The differential response of the two hybrids to stress was greatest during the silking period, as indicated by the greater reduction in harvest index and kernel number in the old than in the new hybrid due to stress.

Improvement of nitrogen use efficiency in maize production requires quantifying genotypic variability in response to soil N. The objective of this field study was to quantify the relationship between leaf N concentration and leaf photosynthetic rate from pre-anthesis to grain fill for six maize (Zea mays L.) hybrids. Maize hybrids were grown at the Central Experimental Farm at Ottawa in Ontario (45°23'N, 75°43'W) in 1991, 1992, and 1993 at three N fertilizer rates (0, 100, and 200 kg N ha -1 ). A method of analysis was developed to relate leaf photosynthesis measurements at comparable irradiance levels to leaf N concentration measurements. Photosynthetic response to leaf N was characterized by two fitted coefficients, N 0 , threshold leaf N concentration for photosynthesis, and P 3 , photosynthetic rate at high (30 g N kg -1 dry weight) leaf N concentration. There were no hybrid differences in N 0 (P > 0.05). There were differences in P 3 , notably a significantly lower P 3 value at anthesis for an old hybrid compared with recent commercial hybrids. In 1992, a recent commercial hybrid, Pioneer 3902, had the highest P 3 value, the second highest crop growth rate (CGR) and the highest grain yield, but hybrid differences in P 3 were not generally correlated with CGR or grain yield (P > 0.05). Growth stage had a significant effect on both N 0 and P 3 ; both declined at later growth stages (P ≤ 0.05). The decline in P 3 was greatest in Pride 5 in 1993. Photosynthate production during grain fill was potentially limited in all hybrids by a decline in P 3 as the season progressed, although the degree of decline varied with year and hybrid. This analysis quantified the relationships between leaf N concentration and leaf photosynthetic rate for different growth stages and hybrids for evaluation of hybrid N use efficiency.

In a mixture of crop and weeds, competition exists between plants for incident solar radiation, soil nutrients, and soil moisture. Integrated weed management manipulates cropping-system factors such that the competitive relationship favors growth of the crop at the expense of the weed; however, relatively few studies have been reported on the influence of cropping-system factors of crop-weed competition. The objectives of this study were to evaluate the impact of weed interference on the performance of four maize (Zea mays L.) hybrids and to examine the influence of soil N level on the competitiveness of the hybrids. Experiments were conducted during 1990, 1991, and 1992 at the Elora Research Station, Elora, ON, on a London loam (Aquic Hapludalf) soil that had been tile drained [...]

Development of an integrated weed management system requires detailed information on crop-weed interactions, including the impact of the relative competitive ability of the crop during serious phases of development on weed growth. The objective of this study was to quantify effects of maize (Zea mays L.) plant density on weed interference in maize throughout the growing season. Experiments were carried out during 1990, 1991, and 1992 at the Elora Research Station, Elora, ON, on a London loam (Aquic Hapludalf) soil that had been tile drained. Maize was grown at three plant densities (4, 7, and 10 plants m −2 ) under three weed pressures. Weed pressures were established by varying the weedfree period after maize planting: all season (weed free), planting to 5to 7-leaf stage of maize (medium weed pressure), and planting to 3- to 4-leaf stage of maize (high weed pressure) [...]

Rye (Secale cereale L.) cover crops often delay development and reduce yield of corn (Zea mays L.). A 3-yr study (1982-1984) was conducted to investigate the influence of flour rye cultivars and one wheat (Triticum aestivum L.) cultivar, rate of N application in the spring, and removal of above-ground cereal phytomass on growth and development of corn. Experiments were carried out at the Elora Research Station, Ontario, in a split-plot design, with two N levels (145 and 220 kg ha −1 ) and five cereal cultivars as main plots, and cereal phytomass removal or retention as sub-plots. Cereal cultivars were planted in September after corn harvest and corn was planted in the spring [...]

A rye (Secale cereale L.)-corn (Zea mays L.) double-cropping system has the potential to maximize annual phytomass production in Ontario. Field experiments were conducted at Elora and Woodstock, ON, from 1981 to 1984 to evaluate the impact of rye harvest and corn planting date combinations during the spring as well as tillage methods on annual above-ground phytomass production. Winter rye was planted in late September or early October after corn harvest and either chemically killed in early May or harvested as silage before corn planting. The rye/tillage treatments consisted of no-rye/rototill, rye/rototill, and rye/no-till. Corn phenology was monitored during the growing season and corn above-ground dry matter was harvested in late September []

Differences in dry matter accumulation among crop cultivars can be attributed to differences in either the absorptance of incident photosynthetically active radiation (PAR) and/or the conversion of absorbed PAR into dry matter. This study was conducted to quantity the contribution of radiation use efficiency (RUE, g dry matter MJ −1 ) to the difference in dry matter accumulation between an old and a new maize (Zea mays L.) hybrid. An old (Pride 5) and a new (Pioneer 3902) maize hybrid were grown during 1989 and 1990 at the Elora Research Station, Ontario, on a London loam (Aquic Hapludalf) soil that had been tile drained (...)

Studies in Ontario have shown that corn (Zea mays L.) yields are reduced when corn is seeded immediately after rye (Secale cereale L.) harvest or chemical kill of winter rye. A study was conducted in 1983 and 1984 on a Maryhill (Typic Hapludalf) loam soil to determine the effect of spring tillage systems and timing of rye chemical kill on the subsequent corn crop. The rye was seeded in early October after corn silage harvest (...)

The use of a winter rye (Secale cereale L.) corn (Zea mays L.) doable cropping sequence in combination with appropriate tillage practices could increase biomass production and reduce soil erosion potential in southern Ontario. A 3-yr study (1982–1984) was conducted at two locations to determine the potential of this sequence for double cropping, and to evaluate spring tillage systems and management of the rye residue on subsequent productivity of corn. Winter rye was planted in early October after corn silage harvest, and either chemically killed or harvested as silage in the spring before corn planting. Rye treatments consisted of no rye, rye harvested in the spring and rye residue left on the plots. Spring cultivation treatments were no-till, tandem discing, and mold board plowing followed by secondary tillage. The use of a winter rye cover crop delayed corn development and reduced corn biomass yield by 11% at the Elora location and by 17% at the Woodstock location. The adverse effect of the rye crop was more pronounced under no-till than where the soil was tilled. Removal or retention of the rye residue had no consistent effect on the subsequent corn crop. An allelopathic effect resulting from the rye crop may be one plausible explanation for the reduction in corn yield. Total biomass yield (rye + corn) was increased relative to corn alone, if the soil was cultivated. Therefore, a winter rye-corn sequence may still be of interest, despite a reduction in corn yield, especially if advantages such as total biomass production and the potential for decreased soil erosion during fall and winter are considered. Research supported by Agriculture Canada (ERDAF).

Crop dry matter production, in the absence of other limiting fac-tors, is determined by the absorption of photosynthetically activeradiation (PAR) by a crop canopy and the efficiency with whichabsorbed PAR is converted into dry matter. This study was con-ducted to investigate the effects of plant density and phase of de-velopment on PAR absorption and conversion efficiency of two maize (Zea mays L.) hybrids. The maize hybrids Pioneer 3851 and Pioneer3925 were grown in 1984 and 1985 at plant densities of 3.9, 6, and10 plants m 2 in irrigated field experiments near Elora, Ontario, ona London loam (Aquic Hapludalf) soil that had been systematicallytile drained. Conversion efficiency was estimated from rates ofaboveground dry matter accumulation and PAR absorption from ap-proximately 2 wk before to 6 wk after silking. PAR absorption wasestimated from continuous measurements of incident PAR, PAR re-flectance and PAR transmittance. Crop growth rates declined duringthe phase of development under study due to a decline in incidentPAR and a decline in conversion efficiency from 5.8 to 3.5%; PARabsorptance was approximately 89% and remained fairly stable. Leafarea index among density-hybrid treatments ranged from 2.0 to 4.8,and PAR absorption of the latter was 9% higher than that of theformer. Although crop growth rates of the plant densities were notdifferent during the grain-filling period, differences in PAR absorp-tion were too small to show a significant effect of plant density onconversion efficiency. Photosynthetic conversion efficiency appearsto be influenced by phase of development, but the effects of plantdensity on efficiency appear to be small.