D. W. Stewart

Inter-relationships of applied nitrogen, SPAD, and yield of leafy and non-leafy maize genotypes

The SPAD chlorophyll meter was found to be a reliable, quick, and non-destructive tool used for directly measuring leaf chlorophyll and indirectly assessing the proportional parameter of leaf, and by extension, plant nitrogen (N) status. The meter has been used successfully to assess leaf N in conventional maize crops, but it has not been used with new maize (Zea mays L.) genotypes containing leafy (L) and reduced stature (RS) traits. SPAD meter readings were collected on the uppermost fully developed leaves (before silking) and on the ear leaf (after silking) of field grown maize genotypes with and without the L and RS traits. The experiment was conducted during 1996 and 1997 at two sites in Eastern Canada (Ottawa and Montreal). At each site in each year, a split plot arrangement of two treatment factors was used in a randomized complete block design with four blocks. The main plot treatments were levels of N (0, 85, 170, and 255 kg ha−1), with six maize genotypes as subplot treatments. The hybrids included: (i) leafy reduced-stature, LRS, (ii) non-leafy normal stature, NLNS, (iii) leafy normal stature, (LNS), (iv) non-leafy reduced-stature, NLRS, (v) conventional commercial hybrids, Pioneer 3905 as the hybrid check for late maturity, and (vi) Pioneer 3979, a check for early maturity. The hybrids were chosen on the basis of their contrasting canopies and root architecture. The SPAD meter readings were collected on the same five plant genotypes over time (six times per site per year, except four times for the Ottawa site in 1997). All genotypes showed increasing meter reading values as plants aged until silking. In general, SPAD meter readings increased as N fertilization level increased at each measurement date for both sites and years. In general, LNS and P3905 hybrids showed greater SPAD meter readings than other hybrids at all sampling dates for both sites and growing seasons. Applied N rates were significantly correlated with the SPAD meter readings. More highly significant relationships were found for N fertilizer levels and SPAD meter readings for the hybrids in 1997 than for the hybrids in 1996. For the Montreal site in 1997, LRS, LNS and P3905 hybrids were among those showing the highest r values between N level and SPAD readings. The correlation coefficients between SPAD readings and grain yield were generally lower. However, the NLNS hybrid had a high SPAD-yield correlation at the Macdonald site in 1997.

Weed Biomass Production Response to Plant Spacing and Corn (Zea mays) Hybrids Differing in Canopy Architecture1

Field experiments were conducted in 1996, 1997, and 1998 at Ste. Anne de Bellevue, Quebec, Canada, and in 1996 at Ottawa, Ontario, Canada, to quantify the impact of corn hybrids, differing in canopy architecture and plant spacing (plant population density and row spacing), on biomass production by transplanted and naturally occurring weeds. The treatments consisted of a factorial combination of corn type (leafy reduced stature [LRS], late-maturing big leaf [LMBL], a conventional Pioneer 3979 [P3979], and, as a control, a corn-free condition [weed monoculture]), two weed levels (low density [transplanted weeds: common lambsquarters and redroot pigweed] and high density [weedy: plots with naturally occurring weeds]), two corn population densities (normal and high), and row spacings (38 and 76 cm). At all site-years under both weed levels, the decrease in biomass production by both transplanted and naturally occurring weeds was greater due to the narrow row spacing than due to the high plant population density. The combination of narrower rows and higher population densities increased corn canopy light interception by 3 to 5%. Biomass produced by both transplanted and naturally occurring weeds was five to eight times less under the corn canopy than in the weed monoculture treatment. Generally, weed biomass production was reduced more by early-maturing hybrids (LRS and P3979) than by LMBL. Thus, hybrid selection and plant spacing could be used as important components of integrated pest management (weed control) for sustainable agriculture. Nomenclature: Common lambsquarters, Chenopodium album L. #3 CHEAL; corn, Zea mays L.; redroot pigweed, Amaranthus retroflexus L. # AMARE. Additional index words: Competitivness, early maturity, weed management. Abbreviations: LAI, leaf area index; Lfy, leafy; LMBL, late-maturing big leaf; LRS, Leafy reduced stature; P3979, Pioneer 3979; rd1, reduced stature.

Leafy reduced-stature maize for short-season environments: morphological aspects of inbred lines

Development of maize (Zea mays L.) types that produce leaf area rapidly and finish vegetative development quickly would increase production of maize in mid- to short-season areas. The Leafy (Lfy1) and reduced-stature (rd1) traits each make contributions to this end. However, these two traits have not previously been combined. Our objective was to evaluate the morphological aspects of non-leafy normal-stature (NLNS), leafy reduced-stature (LRS), non-leafy reduced-stature (NLRS), and leafy normal-stature (LNS) maize inbreds. Two traits, Lfy1 and rd1, were incorporated into a series of inbreds, resulting in a range of canopy architectures. Twelve variables were recorded for each of 30 inbreds over three years. The 12 variables were: seed emergence, above-ear leaf number, below-ear leaf number, dead leaf number at tasselling, live leaf number at tasselling, total leaf number, above-ear leaf area, ear leaf length, ear leaf width, ear height, internode length, and plant height. Inbreds containing the Lfy1 trait had more above-ear leaf area, above-ear leaf number, dead leaf number at tasselling, total leaf number and number of live leaves at tasselling than non-leafy inbred lines. Below-ear leaf number was not different among LRS, LNS, and NLNS inbred lines. LRS and NLRS inbred lines were also not different for below-ear leaf number. Plant height, ear height, and ear leaf length and width were higher in normal-stature than reduced-stature plants. The proportion of the seeds which emerged was higher for LRS inbreds than the other trait groups.

Leafy reduced-stature maize for short-season environments: Yield and yield components of inbred lines

Development of maize (Zea mays L.) types that produce leaf area and mature quickly would increase production of maize in mid- to short-season areas. The leafy (Lfy1) and reduced-stature (rd1) traits both make contributions to this end. However, these two traits have not previously been combined. Our objective was to evaluate the yield and yield components of non-leafy normal-stature (NLNS), leafy reduced-stature (LRS), non-leafy reduced-stature (NLRS), and leafy normal-stature (LNS) maize inbred lines. The two genes, ‘Lfy1’ and ‘rd1’, were incorporated into a series of inbred lines resulting in a range of canopy architectures. Ten variables were recorded for each of 30 inbred lines over three years. The 10 variables were: corn heat unit requirement from planting to tasselling, corn heat unit requirement from planting to silking, days between tasselling and silking, grain moisture content, husk dry weight, cob dry weight, ear length, maximum ear circumference, grain yield and ratio of grain yield to moisture content. Reduced-stature inbred lines reached anthesis more quickly than normal-stature inbred lines. Grain moisture content was less in reduced-stature inbred lines than normal stature trait groups. Leafy-reduced stature plants had the highest ratio of grain to moisture content and the lowest grain moisture content at harvest. Inbred lines containing the rd1 trait matured more rapidly than other trait groups. The LRS trait group yielded more than the other groups, and showed great potential for use in mid- to short-season environments.

Sample size determination for chlorophyll meter readings on maize hybrids with a broad range of canopy types

Abstract The portable chlorophyll meter [Soil Plant Analysis Development (SPAD)] has been used successfully for measuring leaf‐nitrogen (N) of several crops. Determination of the appropriate sample size, in terms of number of plants to be sampled within each plot, has recently become a matter of concern. An insufficient sample size does not allow for the detection of small, but real differences between treatment means, whereas an excessively large sample size constitutes a waste of time and resources. In this study, SPAD meter data were collected at two sites. Each of these two field experiments was organized following a split‐plot design with three blocks and two treatment factors: four nitrogen levels (main plot factor) and six maize (Zea mays L.) hybrids (subplot factor), selected to represent a broad range of canopy types. The approach followed in collecting SPAD meter readings for the determination of an appropriate sample size consisted of sampling one leaf per plant and taking a single reading per leaf. Confidence intervals for the mean of SPAD meter readings and the associated required sample sizes for the variability observed were generated using a standard procedure. Taking a single reading per leaf near the midpoint of the leaf blade, a sample size of 15 to 20 plants provided a level of precision of 5% (about ± 2.8 SPAD meter units). The variability among and within hybrids was highest at the zero N fertilization level (kg ha−1), for which the leafy and non‐leafy reduced stature and leafy normal stature hybrids showed the largest required sample sizes at both sites. At the Ottawa site, where an N fertilization effect was observed, required sample sizes at the 0 N level were larger than at any other level, including the recommended 170 N level. In summary, a relationship between sample size and precision level is presented for maize researchers using the SPAD technology.