Optimum plant population density for chickpea and dry pea in a semiarid environment

Transcription

1 Optimum plant population density for chickpea and dry pea in a semiarid environment Y. T. Gan 1, P. R. Miller 2, B. G. McConkey 2, R. P. Zentner 1, P. H. Liu 3, and C. L. McDonald 1 1 Semiarid Prairie Agricultural Research Centre, Agriculture and Agri-Food Canada, Swift Current, Saskatchewan, Canada S9H 3X2 ( gan@em.agr.ca); 2 Department of Land Resources and Environmental Sciences, Montana State University, Bozeman, MT 59717, USA; 3 Department of Water Resources Engineering, Gansu Agricultural University, Lanzhou, Gansu, , The People s Republic of China. Received 7 February 2002, accepted 5 July Gan, Y. T., Miller, P. R., McConkey, B. G., Zentner, R. P., Liu, P. H. and McDonald, C. L Optimum plant population density for chickpea and dry pea in a semiarid environment. Can. J. Plant Sci. 83: 1 9. Chickpea (Cicer arietinum L.), an annual grain legume, is being broadly included in cereal-based cropping systems throughout the semiarid Canadian prairies, but information on optimum plant population density (PPD) has not been developed for this region. This study, which was conducted from 1998 to 2000 in southwestern Saskatchewan, determined the effect of PPD on field emergence, seed yield and quality, and harvestability of kabuli and desi chickpea compared with dry pea (Pisum sativum L.). Seed yields of all legumes increased with increasing PPD when the crops were grown on conventional summerfallow. The PPD that produced the highest seed yields ranged from 40 to 45 plants m 2 for kabuli chickpea, from 45 to 50 plants m 2 for desi chickpea, and from 75 to 80 plants m 2 for dry pea. When the legumes were grown on wheat stubble, the PPD that gained optimum seed yield ranged from 35 to 40 plants m 2 for kabuli chickpea, from 40 to 45 plants m 2 for desi chickpea, and from 65 to 70 plants m 2 for dry pea. The proportion of large-sized (>9-mm diameter) seed in the harvested seed was >70% when the kabuli chickpea was grown on summerfallow regardless of PPD, whereas the large-seed proportion decreased with increasing PPD when the crop was grown on wheat stubble. Increases in PPD advanced plant maturity by 1.5 to 3.0 d and increased the height of the lowest pods from the soil surface by 1.4 to 2.0 cm (or 5 to 10%), with desi chickpea receiving the greatest benefits from increased PPD. The percentage of plants established from viable seeds per unit area decreased substantially as PPD increased, with kabuli chickpea emergence decreasing from 90% at PPD = 20 plants m 2 to 72% at PPD = 50 plants m 2, from 81 to 69% for desi type, and from 83 to 59% for dry pea. The reason for the low field emergence with increased PPD is unknown, but methods which lead to improved field emergence represent a great opportunity to increase seed yield and reduce production costs for both chickpea and dry pea in this semiarid region. Key words: seed size, Cicer arietinum, Pisum sativum, seeding rate, summerfallow Gan, Y. T., Miller, P. R., McConkey, B. G., Zentner, R. P., Liu, P. H. et McDonald, C. L Densité de peuplement optimale pour la culture du pois chiche et du pois en milieu semi-aride. Can. J. Plant Sci. 83: 1 9. Partout dans la partie semi-aride des Prairies canadiennes, on ajoute le pois chiche (Cicer arietinum L.) aux assolements de céréales, mais on ignore la densité de peuplement optimale (DPO) de cette culture dans la région. L étude, effectuée dans le sud-ouest de la Saskatchewan de 1998 à 2000, devait établir l incidence de la DPO sur la levée au champ, le rendement grainier, la qualité des semences et la récolte du pois chiche kabuli et du pois chiche desi, comparativement à ceux du pois de grande culture (Pisum sativum L.). Le rendement grainier des légumineuses augmente avec la DPO quand on les cultive sur une jachère d été ordinaire. La DPO donnant le meilleur rendement grainier se situe entre 40 et 45 plants au m 2 pour le pois chiche kabuli, entre 45 et 50 plants au m 2 pour le pois chiche desi et entre 75 et 80 plants au m 2 pour le pois de grande culture. Quand la culture se fait sur chaume de blé, la DPO aboutissant au meilleur rendement grainier s établit entre 35 et 40 plants au m 2 pour le pois chiche kabuli, entre 40 et 45 plants au m 2 pour le pois chiche desi et entre 65 et 70 plants au m 2 pour le pois de grande culture. Quand on cultive le pois chiche kabuli sur une jachère d été, la proportion de grosses semences (> 9 mm de diamètre) dépasse 70 %, peu importe la DPO, mais un relèvement de cette dernière diminue la proportion de grosses semences quand la culture pousse sur du chaume de blé. Une hausse de la DPO permet aux plants de parvenir à maturité de 1,5 à 3,0 jours plus tôt et les gousses les plus basses dépassent le sol de 1,4 à 2,0 cm de plus (soit 5 à 10 %). C est le pois chiche desi qui bénéficie le plus d un relèvement de la DPO. La proportion de plants issus de semences viables par unité de surface diminue sensiblement avec l augmentation de la DPO. Ainsi, le pourcentage de levée du pois chiche kabuli tombe de 90 à 72 % quand la DPO passe de 20 à 50 plants au m 2, alors qu il tombe de 81 à 69 % pour le pois chiche desi et de 83 à 59 % pour le pois de grande culture. On ignore pourquoi une plus grande DPO réduit la levée, mais les techniques susceptibles d améliorer ce facteur contribueraient à accroître le rendement grainier et à réduire le coût de production du pois chiche et du pois de grande culture dans cette région semi-aride. Mots clés: Calibre des semences, Pisum sativum, Cicer arietinum, densité de semis, jachère d été, maturité Chickpea has been grown in semiarid regions of the world for hundreds of years (Siddique and Sykes 1997; Kumar and Abbo 2001). This annual legume was recently introduced to the Canadian prairies and is now being rapidly adopted by producers (Gan and Noble 2000). Chickpea is a deep rooting crop with a strong ability to take up water from deep soil 1 layers (Gan et al. 2000), but it requires stress during the later part of its life cycle to hasten maturity, reflecting its indeterminate growth habit. These characteristics make the crop well suited to the drier regions of the Canadian prairies Abbreviations: PPD, plant population density

2 2 CANADIAN JOURNAL OF PLANT SCIENCE Table 1. Agronomic information for chickpea and dry pea grown at Swift Current and Stewart Valley, Saskatchewan Seed weight Seed germination Harvest date Year Cultivar (mg seed 1 ) (%) Swift Current Stewart Valley 1998 Carrera Aug. Myles Aug. Amit Aug. Dwelley Aug. Sanford Aug Carrera Aug. 23 Aug. Myles Sept. 15 Sept. CDC Chico Sept. 15 Sept. CDC Yuma Sept. 15 Sept. CDC Xena Sept. 15 Sept Carrera Aug. 12 Aug. Myles Sept. 8 Sept. CDC Chico Sept. 8 Sept. CDC Yuma Sept. 8 Sept. CDC Xena Sept. 8 Sept. where failure of the crop due to late maturity is relatively low. In recent years, over 90% of the Canadian chickpea production has been centred in the Brown and Dark Brown soil zones of southwestern Saskatchewan and southeastern Alberta (Gan and Noble 2000). However, little is known about the growth and development responses of chickpea to changes in PPD. Producers urgently need this information, and a choice of chickpea types for optimal seed yield and quality in this region. Three market classes of chickpea (namely large-seeded kabuli, small-seeded kabuli, and desi) are currently grown in the semiarid Canadian prairies. Large-seeded kabuli has an average seed weight of between 440 and 550 mg seed 1, while small-seeded kabuli has an average seed weight of 200 to 300 mg seed 1 (Anonymous 2002). The seed coat of kabuli (both large- and small-seeded) chickpea is thin with a creamy colour. Desi chickpea seed size varies from 170 to 320 mg seed 1, depending on cultivar (Anonymous 2002). The seed coat of desi chickpea is thick with a tan to brown colour. In commercial production of chickpea, seed costs are a major input expense, often exceeding CAN$140 to $180 ha 1. If a lower seed rate could be used without adverse effects on seed yield and quality, then production costs could be reduced substantially. Conversely, chickpea is a poor competitor with weeds, due to its slow growth during early growth stages, its short plant stature (< 60 cm in height), and open canopy. We observed that chickpea often suffers from competition with weeds such as pigweed (Amaranthus retroflexus L.) and kochia (Kochia scoparia L.). In addition, the choice of herbicides is limited, with only two herbicides registered in Canada for post-emergent application on chickpea. Previous research indicates that a larger plant population suppresses weed growth and thereby increases seed yields in annual legumes (Ball et al. 1997; Siddique et al. 1998). For example, Ball et al. (1997) reported that weed dry matter was substantially lower in lentil (Lens culinaris Medik.) fields planted with a high seeding rate. The suppression of weeds with a higher crop population density is more pronounced in situations where good soil moisture favours plant emergence and growth. Harvest conditions are particularly important to chickpea production in the Canadian prairies. The average frost-free (0 C basis) period in the chickpea production region ranges from 92 to 120 d (Cutforth et al. 1993), which is critically close to the minimum period required for chickpea to reach maturity. Delays in maturity may subject chickpea to frost damage, which detrimentally influences seed coat colour and grades, thus reducing its market value. Canopy height and the lowest pod height from the soil surface are also crucial for minimizing harvest losses of chickpea. Setting harvest equipment close to the ground to maximize the harvesting of lower pods also increases the risk of equipment damage from contacting or ingesting rocks and soil. Furthermore, grain contamination with soil picked up during harvest can cause chickpea seed discolouration and downgrading. The objectives of this study were: (i) to determine optimal PPD for kabuli and desi chickpea and dry pea grown in the semiarid Canadian prairie; (ii) to evaluate the impact of PPD on seed yield, proportion of large-sized (9-mm diameter) seeds, and harvestability of chickpea; and (iii) to examine the interaction of seeding rate and field emergence rate for chickpea and dry pea. MATERIALS AND METHODS Sites The experiment was conducted from 1998 to 2000 at two sites in southwestern Saskatchewan. One site was on a Swinton silt loam, an Orthic Brown Chernozem (Ayres et al. 1985), located at the Agriculture and Agri-Food Canada Research Centre near Swift Current, Canada (50.2 N, W). The second site was on a Sceptre heavy clay, a Rego Brown Chernozem (Ayres et al. 1985) near Stewart Valley approximately 30 km north of Swift Current (50.6 N, W). Experiment Design The treatments consisted of three factors: (i) chickpea market class, (ii) plant population density, and (iii) field phase (i.e., conventional summerfallow and wheat stubble). Three

3 GAN ET AL. OPTIMUM PLANT DENSITY FOR CHICKPEA AND DRY PEA 3 Table 2. Pesticide applications for chickpea and dry pea grown at Swift Current and Stewart Valley, Saskatchewan Herbicide application Fungicide application Site-year Previous fall Pre-seeding In-crop In-crop Swift Current Sept May 18 June 6 July Glyphosate, 1.34 kg ha 1 + Trifluralin, 1.1 kg ha 1 Sethoxydim, 0.14 kg ha 1 Chlorothalonil, 1.98 kg ha 1 2,4-D 600 amine, 0.74 kg ha 1 8 May Glyphosate, 0.89 kg ha Sept April 14 June 20 July Glyphosate, 0.45 kg ha 1 + Trifluralin, 1.1 kg ha 1 Imazamox, kg ha 1 + Chlorothalonil, 1.48 kg ha 1 2,4-D 600 amine, 0.74 kg ha 1 30 April Imazethapyr, kg ha 1 5 Aug., Glyphosate, 0.89 kg ha 1 Chlorothalonil, 1.98 kg ha Oct April 26 May and 19 July 29 June, 12 July, 21 July 2,4-D 700 ester, 0.6 kg ha 1 Trifluralin, 1.1 kg ha 1 Sethoxydim, 0.21 kg ha 1 Chlorothalonil, 1.73 kg ha 1 1 May 2 June, metribuzin, 0.16 kg ha 1 Glyphosate, 0.44 kg ha 1 6 Sept., diquat, 0.40 kg ha 1 Stewart Valley Sept April 15 June 20 July Glyphosate, 0.45 kg ha 1 + Trifluralin, 1.1 kg ha 1 Sethoxydim, 0.14 kg ha 1 Chlorothalonil, 1.48 kg ha 1 2,4-D 700 ester, 0.87 kg ha 1 Glyphosate, 0.45 kg ha 1 7 Sept., diquat, 0.40 kg ha 1 9 Aug. Chlorothalonil, 1.98 kg ha Oct April 25 May, metribuzin, 0.16 kg ha 1 30 June, and 13 July 2,4-D 700 ester, 0.69 kg ha 1 Trifluralin, 1.1 kg ha 1 29 May, sethoxydim, 0.21 kg ha 1 Chlorothalonil, 1.73 kg ha 1 24 April 31 Aug., diquat, 0.40 kg ha 1 Glyphosate, 0.45 kg ha 1 types of chickpea and one type of dry pea were included: (a) large-seeded kabuli chickpea (Dwelley and Sanford in 1998, CDC Xena and CDC Yuma in 1999 and 2000); (b) smallseeded kabuli chickpea (Amit in 1998, CDC Chico in 1999 and 2000); (c) desi chickpea (Myles in all site-years); and (d) dry pea (Carrera, a semi-leafless cultivar with yellow cotyledons). Dwelley, Sanford, and Amit have been the most popular cultivars grown in the Canadian prairies, whereas CDC Xena, CDC Yuma, and CDC Chico are newly released cultivars. We switched to the newly released cultivars in the 2nd and 3rd years of the study because they are agronomically superior and have begun to replace the earlier cultivars (Warkentin, personal communication, 2002). Myles is a standard desi-type chickpea, accounting for over 95% of the total desi chickpea production in western Canada. Four seeding rates were used to obtain the target PPD of 20, 30, 40, and 50 plants m 2 for chickpea and 35, 50, 65, and 80 plants m 2 for the dry pea. Actual seed rates were based on seed size, pre-seed germination, and an estimated field emergence rate of 75% (Table 1). At each siteyear, crop types and PPD were arranged in a factorial, randomized complete block design with four replicates. All treatments were tested at two field phases (on conventional summerfallow and wheat stubble). The experiments on the two field phases were established separately (but adjacent in the field) for ease of field operations. Crop Management and Data Collection Seed was treated with 600 g a.i. each of carbathiin and thiabendazole, and 16 g a.i. of metalaxyl 100 kg 1 seed (Hwang et al. 2000). Plots were seeded on 16 May in 1998, 7 May at Swift Current and 20 May at Stewart Valley in 1999, and 2 May at Swift Current and 5 May at Stewart Valley in At the time of seeding, the noon soil temperature at a 10-cm depth was between 9 and 13 C. The various seeding rates were accomplished with a 2-m-wide hoe press drill equipped with a spinner seed metre. Each plot was 7.5 m long and consisted of 10 rows with a 20-cm row spacing. Phosphorus fertilizer was applied (as monoammonium phosphate) with the seed at a rate of 7.5 kg P ha 1. All plots received 5.5 kg ha 1 of Nitragin, an appropriate soil implant Rhizobium inoculant (a granular form) for symbiotic N fixation (Lipha Tech Inc. Saskatoon, Canada). The Rhizobium inoculant was applied in the seed rows. Weeds were controlled with a comprehensive management program (Table 2) to maintain the overall weed pressure at a minimum level. Some of the products used for weed control caused a certain degree of plant injury, but treatment responses were not affected. Ascochyta blight, a foliar disease caused by Ascochyta rabiei, is the most prevalent disease in chickpea. In this study, the disease symptoms occurred in late June to early July when chickpea plants were in the initial flowering stage. All plots were sprayed with chlorothalonil (Syngenta, Canada) at the recommended rate once symptoms were visually detectable on leaves and/or stems. Repeated applications of chlorothalonil were used as needed. Actual PPD were determined by counting plants in two randomly selected 0.5-m 2 quadrats per plot. The plant counts were conducted after seedling emergence was complete. The field emergence rate was calculated the number of emerged plants divided by the number of viable seeds planted per unit area. Plant maturity was recorded when >90% of the pods in

4 4 CANADIAN JOURNAL OF PLANT SCIENCE a plot turned a brownish tan colour and the seed moisture content averaged 300 to 350 g kg 1. Days to maturity were presented as days after seeding. The centre eight rows of each plot (9.2 m 2 ) were harvested with a plot combine when the crop had dried sufficiently for satisfactory threshing (Table 1). The seed samples were air-dried, cleaned, weighed, and seed yields were calculated and presented on a dry weight basis. The proportion of seeds greater than 9 mm in diameter was determined by passing the harvested seed through a series of sieves. The determination of seed size fractions was conducted only for large-seeded kabuli chickpea, because only this market class of chickpea receives price premiums for seed greater than 9 mm. Meteorological data were collected at the experimental sites. Data Analysis Data were analysed using the PROC MIXED procedure of the SAS Institute, Inc. (Littel et al. 1996), with blocks and sites as random effects, and PPD and crop type as fixed effects. Treatment differences were declared significant at P < Since year and year-by-field phase interactions were significant, the analysis was performed separately for each year at each field phase. In 1998, the experiment was conducted at Swift Current only. In each of 1999 and 2000, there was no significant difference between the sites and thus the data for Swift Current and Stewart Valley were pooled. Regression analysis was conducted to estimate linear and quadratic effects of PPD when results of the analysis of variance indicated these effects were significant at P < RESULTS AND DISCUSSION Growing season precipitation was normal to above normal for all of the study years (Table 3), but in 1998, nearly half of this precipitation was concentrated in June. Soil water in the spring of 1998 was substantially lower than the 34-yr average. Mean growing season air temperature for 1998 was 1.3 C (or 8%) higher than the 40-yr mean, with considerable drought stress in late July to early August. In contrast, in 1999, cool and wet conditions occurred during the entire growing season, with total precipitation being 15% higher than normal and air temperature being 0.8 C (or 6%) lower than normal. The favourable moisture conditions in 1999 promoted long vegetative growth in chickpea, but the low temperatures limited seed yield potential. In 2000, available water was similar to that of 1999, but air temperatures in late July to early August were higher than normal, promoting pod set and seed fill in chickpea. Field Emergence Actual PPD were close to the target PPD for all three market classes of chickpea and for dry pea in all years (Table 4). Field emergence rates (i.e., the proportion of plants emerged from number of viable seeds planted per unit area) decreased substantially as PPD increased (Table 5). For example, the mean field emergence rate of kabuli chickpea decreased from 90% at a PPD of 20 plants m 2 to 72% at a PPD of 50 plants m 2. Similarly, the field emergence rates for desi chickpea decreased from 81% at low PPD to 69% at high PPD, and decreased from 83 to 59% for dry pea. This Table 3. Growing season precipitation and temperatures, and spring soil water at different soil depths, at Swift Current, Saskatchewan Year May June July August Total/mean Precipitation (mm) yr mean z Mean air temperature (C) yr mean z Spring soil water y Soil depth (cm) Year (mm) yr mean x 69 ± ± ± 34 z From 1961 to y Spring soil water was measured in wheat stubble plots of a long-term crop rotation study at Swift Current. x From 1967 to phenomenon was consistent across years. The reasons for the lower field emergence rates with increasing PPD are unknown. However, we observed in the field that a portion of the seed planted under the high PPD treatments were unevenly distributed in the seed rows, which may have caused poor seed-soil contact. The chances of having more than one seed clumped together in the seed rows was higher with the high PPD treatments than when a lower seed rate was used. Furthermore, if one of the seeds became diseased in the soil (despite seed treatment with fungicides), there was a higher potential for the disease to spread to neighbouring seeds as the spacing of seeds along a row decreased. Beech and Leach (1989) observed in Australia that increasing PPD increases the rate of plant mortality. In one of the two experiments, the authors observed a 20% plant mortality in the crop community having highest PPD. In these studies, the higher rate of plant mortality with higher plant density was probably due to intraspecific competition for resources during the early growth stages. However, these authors did not determine the rate of seedling emergence. Jettner et al. (1999) reported that in southern Australia an average of 54% of the chickpea seeds sown emerged from the soil and developed into viable plants. They reasoned that the low field establishment of chickpea was likely due to physical damage that occurred during the handling of the seed, particularly damage caused by planting with air-type seeders. In our study, the mean field emergence rates were higher than those obtained by Jettner et al (1999), possibly reflecting the lack of mechanical impact damage by our hoepress drill. Nevertheless, owing to the high seed cost in chickpea production, research is warranted to determine appropriate handling and planting methods to ensure a high proportion of the planted chickpea successfully emerge.

5 GAN ET AL. OPTIMUM PLANT DENSITY FOR CHICKPEA AND DRY PEA 5 Table 4. The target and actual plant population densities of chickpea and dry pea cultivars grown in southwestern Saskatchewan, Chickpea: target plant Crop type population density (plants m 2 ) Year /cultivar Chickpea: actual plant population density (plants m 2 ) z 1998 Amit 24 (1.3) y 37 (1.6) 47 (2.5) 55 (2.7) Dwelley 23 (2.2) 34 (1.8) 42 (2.7) 59 (2.6) Sanford 26 (1.2) 37 (2.4) 46 (1.5) 53 (3.2) Myles 23 (0.8) 36 (1.8) 43 (1.9) 52 (3.8) 1999 CDC Chico 21 (0.9) 29 (1.4) 35 (1.3) 44 (1.9) CDC Xena 18 (1.0) 32 (1.2) 37 (1.4) 48 (2.2) CDC Yuma 20 (1.2) 32 (1.2) 39 (1.3) 48 (2.2) Myles 17 (1.3) 30 (1.7) 38 (1.9) 46 (1.5) 2000 CDC Chico 27 (3.3) 34 (1.5) 43 (1.9) 53 (2.7) CDC Xena 26 (5.0) 35 (4.0) 44 (2.1) 50 (2.6) CDC Yuma 26 (2.2) 34 (1.3) 40 (1.4) 51 (2.7) Myles 22 (2.7) 32 (2.1) 40 (2.0) 48 (3.2) Dry pea: target plant population density (plants m 2 ) Dry pea: actual plant population density (plants m 2 ) z 1998 Carrera 39 (2.0) 57 (2.3) 76 (4.7) 88 (4.9) 1999 Carrera 29 (2.0) 48 (1.0) 59 (2.2) 75 (1.9) 2000 Carrera 35 (2.3) 47 (1.7) 62 (2.7) 73 (4.7) z Means of Swift Current and Stewart Valley as there was no significant difference between the two sites. y Data in parentheses are SEs. Seed Yield Seed yields of desi and kabuli chickpea increased with increases in PPD from 20 to 50 plants m 2 when the crops were grown on summerfallow (Fig. 1). Desi chickpea had a greater increase in seed yield with increased PPD than kabuli chickpea. The seed yield of dry pea increased with increasing PPD from 30 to 80 plants m 2 in 2 of 3 yr. However, when the legumes were grown on wheat stubble, seed yield responses varied among years. In 1999 and 2000, the yield of the legumes increased with increasing PPD, with patterns that were similar to those when grown on summerfallow. However, in 1998, seed yields increased with PPD for desi chickpea, decreased for kabuli chickpea, and were unaffected for dry pea. Spring soil water and growing season precipitation in 1998 were lowest among the 3 study years (Table 3), causing depressed yields for all legumes (Fig. 1). With such dry conditions, kabuli chickpea grown on wheat stubble suffered severe water stress and produced half as much yield as the crop was grown on summerfallow. We observed that on wheat stubble, the legumes grown at the high PPD treatment reduced the duration of green leaf and number of fertile pods per plant compared with those grown at lower PPD (data not shown). In contrast, the better moisture conditions in 1999 and 2000 allowed longer green leaf duration, enabling most of the pods to fill regardless of plant density. Overall, seed yield responses to PPD were positive for both chickpea and dry pea (Fig. 1). These responses followed a Table 5. Field emergence rates (i.e., number of plants emerged per 100 viable seeds planted) for chickpea and dry pea grown at different target plant population densities, in southwestern Saskatchewan, Plant population density (plants m 2 ) ANOVA Year (P level) Percent field emergence z Large kabuli chickpea < Mean <0.01 Small kabuli chickpea < Mean <0.01 Desi chickpea < Mean Dry pea Plant population density (plants m 2 ) ANOVA Year (P level) Percent field emergence z < <0.01 Mean <0.01 z Means of Swift Current and Stewart Valley, since there was no significant difference between the two sites. classical asymptotic curve with a linear and quadratic model accounting for >60% of the variation in seed yields. Plants at high PPD generally accumulated a greater amount of biomass per unit area measured at anthesis than plants grown at lower PPD (data not shown). Leach and Beech (1988) reported that annual pulse crops at high PPD had a more rapid canopy cover in a short period of time, allowing the plants to intercept more light early in the season than crops grown at low PPD. The increased radiation interception with high PPD critically increased the efficiency of photosynthesis, provided other factors such as soil water did not limit crop growth. Studies conducted elsewhere with annual legumes have reported responses similar to those found in our study. Saini and Faroda (1998) measured yield increases of kabuli chickpea of up to 36% with PPD increases from 23 to 35 plants m 2 in northern India. Doubling kabuli chickpea plant population resulted in a 52% yield increase in a Mediterraneantype environment in Jordan (Kostrinski 1974). Similarly, the seed yield of desi chickpea increased with an increase in PPD from 33 to 50 plants m 2 in northern Syria (Singh and Saxena 1996). In a subtropical region of Australia, Beech and Leach (1989) showed that a PPD of at least 40 plants m 2 was required to obtain maximum chickpea yields. In southwestern Australia, Jettner et al. (1999) found that a

6 6 CANADIAN JOURNAL OF PLANT SCIENCE Fig. 1. Seed yields of three market classes of chickpea and dry pea grown on conventional summerfallow and on wheat stubble, with different plant population densities in southwestern Saskatchewan. (Equations with *,** are those whose slopes were significant at P < 0.05 and P < 0.01from zero, respectively; ns means the slopes were not significant from zero.) PPD of 50 plants m 2 was desirable for desi chickpea grown in low-yielding (about 1.0 t ha 1 ) areas, and a plant density of 70 plants m 2 was needed in high-yielding (>1.5 t ha 1 ) areas to maximize economic returns. However, in a low rainfall Mediterranean-type environment in southern Australia, Siddique et al. (1984) reported no seed yield response to PPD though biomass yields tended to increase as PPD increased. The authors reasoned that the lack of yield response to PPD in that particular environment was mainly due to chickpea being grown on sandy soil in which low available water limited seed yield potential. Seed Size The price that producers receive for kabuli chickpea increases as the proportion of seeds greater than 9 mm in diameter increases. For example, kabuli chickpea buyers

7 GAN ET AL. OPTIMUM PLANT DENSITY FOR CHICKPEA AND DRY PEA 7 Table 6. Height of the lowest pods from the soil surface for chickpea grown under different plant population densities, in southwestern Saskatchewan in 1999 and 2000 Plant population density (plants m 2 ) ANOVA Year (P level) (cm) Large kabuli chickpea <0.01 Mean Fig. 2. Relationship between plant population density and the proportion of large-sized (>9 mm) seed for kabuli chickpea grown on (a) conventional summerfallow, and (b) on wheat stubble in southwestern Saskatchewan. (Equations with *,** are those whose slopes were significant at P < 0.05 and P < 0.01from zero, respectively; ns means the slopes were not significant from zero.) have offered a premium of $0.12 kg 1 (averaged for 1999, 2000, and 2001) for kabuli seeds greater than 9 mm in diameter compared to those 8 mm or less in size. In our study, the seed size fractions varied significantly for kabuli chickpea grown under the different conditions (Fig. 2). For chickpea grown on conventional summerfallow, the proportion of seed that was >9 mm in size was >75% in 1998, >60% in 1999, and >70% in 2000, which were significantly higher than those obtained for the crop grown on wheat stubble in the same years. In general, PPD had a marginal impact on the proportion of 9-mm seeds when the crop was grown on summerfallow. However, there was a significant, linear reduction in 9-mm seed proportion due to increased PPD when the chickpea was grown on wheat stubble, except in 1999 when the effect was not statistically significant. Harvestability Days to maturity of the legumes varied widely among years. In 1998, chickpea required 81 to 85 d to mature, whereas Small kabuli chickpea Mean Desi chickpea < <0.01 Mean Note: Data in the table are means of Swift Current and Stewart Valley, since there was no significant difference between the two sites within a given year. they required 99 to 113 d in 1999, and 103 to 120 d in Maturity of dry pea followed similar yearly patterns as chickpea, but the dry pea matured 12 to 17 d earlier than chickpea. Higher than normal temperatures in 1998 (Table 3) permitted rapid growth and development, resulting in the earliest maturity for all legumes. In a given year, the legumes grown at Stewart Valley (a heavy clay soil) required 3 to 5 more days to mature than when grown at Swift Current (a silt loam soil), possibly due to differences in soil water release in the later part of the growing season. Desi chickpea required similar days to mature as smallseeded kabuli cultivars; both maturing 2 to 3 d earlier than large-seeded kabuli chickpea. Plant population density had a significant influence on plant maturity (Fig. 3). On average, as PPD increased from 20 to 50 plants m 2, crop maturity accelerated by 2.1 d for large-seeded kabuli, 3.0 d for both small-seeded kabuli and desi chickpeas, and 1.5 d for dry pea. The positive relationship between PPD and crop maturity was consistent across the 3 study years, although the magnitude of this relationship varied from year to year. Advanced maturity with high PPD is beneficial for pulses to reduce risks of an early frost. Earlier maturity and thereby earlier harvest can result in a significant gain in seed quality and net returns due to a more desirable seed coat colour and improved grades. The short stature of annual pulses, particularly in years when crop growth is limited by water, is a common problem in the semiarid Canadian prairies. We found that the average height of the lowest pods from the soil surface was >25 cm for large-seeded kabuli chickpea, >22 cm for small-seeded kabuli chickpea, and >20 cm for desi chickpea. PPD had a small but significant influence on the lowest pod height (Table 6). Lowest pods were further from the soil surface for plants grown at higher PPD than for plants grown at lower PPD. For example, as PPD increased from 20 to 50 plants m -2, the lowest pod height increased 1.4 cm in large kabuli

8 8 CANADIAN JOURNAL OF PLANT SCIENCE Fig. 3. Days to maturity for three market classes of chickpea and dry pea grown from different plant population densities over 3 yr in southwestern Saskatchewan. (Equations with *,** are those whose slopes were significant at P < 0.05 and P < 0.01from zero, respectively; ns means the slopes were not significant from zero.) chickpea, 1.7 cm in small kabuli chickpea, and 2.0 cm in desi chickpea. The lowest pod height for desi chickpea was 5 cm lower than for kabuli chickpea, but the response to increasing PPD was greater (10%) than for kabuli chickpea. Positive influences of PPD on lowest pod height in desi chickpea have also been observed in low rainfall areas of Australia (Jettner et al. 1999). Optimum PPD The economically optimum PPD can be obtained when the margin between the value of grain produced and the cost of production is maximized. Increasing PPD hastens crop maturity, reduces the yield and grade losses from early-fall frosts, and increases the height of the lowest pod that can potentially reduce both harvest losses and repair costs for harvesting equipment. In addition, higher PPD increases crop competitiveness with weeds, which reduces yield losses and requirements for herbicide applications. However, increased PPD increases the production costs directly through increased seed cost, seed treatments and handling. These costs increase at a greater rate than the PPD increases because the seeding rates need to be proportionally higher than the PPD due to the fewer plants that emerged as the target PPD increases. Additionally, increased PPD decreased the proportion of >9-mm seed in some cases, which reduces the value of the seed. Because of the multiple effects associated with PPD changes, coupled with the large variations in seed cost, grain prices, seed size premiums, and the values of early maturity and of the increased pod height, we could not clearly identify economically optimum PPD. Based on the highest seed yields of the several site-years within this paper, we have tentatively set the optimum PPD for maximizing seed yields as 40 to 45 plants m 2 for desi chickpea, 35 to 40 plants m 2 for both large- and small-seeded kabuli chickpea, and 60 to 70 plants m 2 for dry pea. Beyond these PPD levels, seed yields will either level off or decrease depending on growing conditions. However, there are three exceptions to these recommendations: (1) For desi chickpea and dry pea grown on summerfallow, grain yield increased linearly over the range of PPD used in the study. Further research involving higher PPD is needed to refine the optimal PPD for these two crops grown on summerfallow. (2) For large-seeded kabuli chickpea, the optimum PPD does not correspond to the highest grain yield, but appears to be a reasonable target given the trend towards decreasing the harvested seed size in three of the six growing environments (Fig. 2a, b) and increasing seed cost for obtaining a PPD higher than 45 plants m 2.

9 GAN ET AL. OPTIMUM PLANT DENSITY FOR CHICKPEA AND DRY PEA 9 (3) For small-seeded kabuli chickpea grown on summerfallow, the optimum PPD appears to be 40 to 45 plants m 2, or 10% higher than those grown on wheat stubble. The increase in seed yields with high PPD was greater than the additional seeds that were needed to achieve the high PPD. Nevertheless, in a growing season with more drought stress occurring, there is risk of diminishing seed yields at high PPD for all crops. Hence, for production of these annual pulses on wheat stubble, we suggest a target seed rate being 10% lower than that needed for production on summerfallow in the same year. Our results clearly indicated that seed yields increased with increases in PPD for both desi- and kabuli-type chickpea when the crops were grown on summerfallow. When chickpea was grown on wheat stubble, seed yields increased as PPD increased to a certain level, then no further benefits were detected. Lack of water in stubble fields limited plant growth at high PPD where competition among individual plants became a significant factor, particularly in the semiarid Canadian prairies. Available soil water in the spring of 1998 was 23% lower than the 34-yr average, and the growing season rainfall was the lowest among 3 study years, yet it was near to the long-term average (Table 3). In that year, plants grown on wheat stubble at high PPD suffered severely from water stress. If plant density is increased beyond the above recommendations, we expect that limited water supply will significantly reduce biomass yields, and consequently the seed yields will plateau or decline in a normal to dry years. ACKNOWLEDGEMENTS We acknowledge the excellent technical assistance of Greg Ford, Ray Leshures, Lee Poppy, Marty Peru, and Barry Blomert, and the funding from Saskatchewan Pulse Growers, the Agricultural Development Fund of Saskatchewan Agriculture and Food, and the Matching Investment Initiative of Agriculture and Agri-Food Canada. We also gratefully acknowledge Drs. Rosalind Ball and Manjula Bandara for reviewing the manuscript. Anonymous Varieties of grain crops In 2002 Saskatchewan seed guide. Saskatchewan Agriculture and Food, Regina, SK. Ayres, K. W., Acton, D. F. and Ellis, J. G The soils of the Swift Current Map Area 72J Saskatchewan. Sask. Inst. Pedol. Publ. 86. Extension Division, University of Saskatchewan, Saskatoon, SK Extension Publ Ball, D. A., Ogg Jr., A. G. and Chevalier, P. M The influence of seeding rate on weed control in small red lentil (Lens culinaris). Weed Sci. 45: Beech, D.F. and Leach, G. J Effect of plant density and row spacing on the yield of chickpea (cv. Tyson) grown on the Darling Downs, south-eastern Queensland. Aust. J. Exp. Agric. 29: Cutforth, H. W., Jones, K. and Lang, T. A Agro-climate of the Brown soil zone of southwestern Saskatchewan. Research Branch, Agriculture Canada, Ottawa, ON. Publ. 379MOO88. Gan, Y., McConkey, B. G. and Zentner, R. P Soil water conservation for pea, lentil, and chickpea in the Brown soil zone. Pages 9 11 in Proc. Third Pulse Crop Research Workshop. Winnipeg, MB November Gan, Y. and Noble, G Chickpeas in Southwestern Saskatchewan Potential and risks. Pages 8 9 in Proc. Third Pulse Crop Research Workshop, Winnipeg, MB November Hwang, S. F., Chang, K. F., Gossen, B. D., Turnbull, G. D. and Howard, R. J Effects of chemical seed treatment, soil temperature and seed damage on rhizoctonia seedling blight and seed yield of chickpea. Page 24 in Proc. Third Pulse Crop Research Workshop. Winnipeg, MB November Jettner, R. J., Siddique, K. H. M., Loss, S. P. and French, R. J Optimum plant density of desi chickpea (Cicer arietinum L.) increases with increasing yield potential in south-western Australia. Aust. J. Agric. Res. 50: Kostrinski, J Chickpea population and seed yield. Agric. Research Organization, Volcani Centre, Israel. Special Publ. No. 34, p. 54 Kumar, J. and Abbo, S Genetics of flowering time in chickpea and its bearing on productivity in semiarid environments. Adv. Agron. 72: Leach, G. J. and Beech, D. F Response of chickpea accessions to row spacing and plant density on a vertisol on the Darling Downs, south-eastern Queensland. 2. Radiation interception and water use. Aust. J. Exp. Agric. 28: Littel, R. C., Milliken, G. A., Stroup, W. W. and Wolfinger, R. D SAS system for mixed models. SAS Institute, Inc., Cary, NC. Saini, S. S. and Faroda, A. S Response of chickpea (Cicer arietinum) genotype H to seeding rates and fertility levels. Indian J. Agron. 43: Siddique, K. H. M. and Sykes, J Pulse production in Australia: past, present and future. Aust. J. Exp. Agric. 37: Siddique, K. H. M., Sedgley, R. H. and Marshall, C Effect of plant density on growth and harvest index of branches in chickpea (Cicer arietinum L.). Field Crops Res. 9: Siddique, K. H. M., Loss, S. P., Regan, K. R. and Pritchard, D. L Adaptation of lentil (Lens culinaris Medik) to short season Mediterranean-type environments: response to sowing rates. Aust. J. Exp. Agric. 49: Singh, K. B. and Saxena, M. C Winter chickpea in Mediterranea-type environments. A technical bulletin. International Centre for Agricultural Research in Dry Areas, Aleppo, Syria.

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