Responses of faba bean (Vicia faba L.) to sowing rate in south-western Australia I. Seed yield and economic optimum plant density

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1 Aust. J. Agric. Res., 1998, 49, Responses of faba bean (Vicia faba L.) to sowing rate in south-western Australia I. Seed yield and economic optimum plant density S. P. Loss ABE, K. H. M. Siddique AB, R. Jettner C, and L. D. Martin D A Agriculture Western Australia, Locked Bag No. 4, Bentley Delivery Centre, WA 6983, Australia. B Centre for Legumes in Mediterranean Agriculture, University of Western Australia, Nedlands, WA 69, Australia. C Agriculture Western Australia, Clive St, Katanning, WA 6317, Australia. D Muresk Institute of Agriculture, Curtin University of Technology, Northam, WA 641, Australia. E Corresponding author; sloss@aidpo.agric.wa.gov.au Abstract. Sowing rate influences plant establishment, growth, seed yield, and the profitability of a crop. However, there is limited published information on the optimum sowing rate and plant density for faba bean in Australia. The response of the growth and seed yield of faba bean (cv. Fiord) to sowing rate (7 27 kg/ha) was examined in 19 field experiments conducted over 3 years in south-western Australia. The economic optimum plant density was estimated at each site by fitting an asymptotic model to the data and calculating the point where the cost of extra seed equalled the return from additional seed yield, allowing a 1% opportunity cost for the extra investment. On average across all sites and seasons, only 71% of the seeds sown emerged. Increasing sowing rate resulted in more dry matter production at first flower and at maturity, and at about half of the sites there was a small trend of reduced harvest index. In general, the mean number of seeds per pod ( ) and mean seed weight (32 45 g/1 seeds) were unaffected by sowing rate. As sowing rate increased, the number of pods per plant (5 35) generally decreased, but this was compensated by the large plant population and more pods per unit area. The asymptotic models fitted to the seed yield data accounted for 15 81% of the variance. In 8 experiments, the models indicated that yield was continuing to increase substantially as sowing rate increased at the largest sowing rate treatment. The estimated optimum plant densities in these experiments were beyond the range of the data or had large standard errors and, hence, were excluded from any further consideration. Among the remaining 11 experiments, the estimated optimum plant densities varied from 31 to 63 plants/m 2, with a mean of 45 plants/m 2. This study demonstrates that targeting sowing rates greater than the current commercial practice for faba bean in southern Australia of 15 3 plants/m 2 results in more yield and profit. Additional experiments are required with sowing rates in excess of 27 kg/ha to estimate accurately the optimum plant density for faba bean. Fungal diseases were either absent or controlled with fungicides in these experiments but the interactions between disease, time of sowing, and sowing rates also deserve further attention. Additional keywords: plant population, Mediterranean climate, seeding rate, dry matter production. CSIRO /98/6989$5.

2 99 S. P. Loss et al. Introduction Pulse production in Australia is dominated by the cool-season legumes, particularly narrow-leafed lupin (Lupinus angustifolius L.), which is cultivated mostly in south-western Australia (Siddique and Sykes 1997). A range of grain legume species has been identified that are adapted to neutral to alkaline, fine-textured or duplex soils which are unsuitable for narrow-leafed lupin (Siddique et al. 1993; Thomson et al. 1997). Among these species, faba bean (Vicia faba L.) shows good adaptation and can produce seed yields up to 4 t/ha, even in areas considered to be low rainfall (i.e. annual average <35 mm). When sown early, the faba bean cultivar Fiord escapes drought in this environment by reaching first flower in late winter when rainfall is reliable, and filling seed in the first-formed pods before the onset of high temperatures and moisture stress in spring (Loss and Siddique 1997; Loss et al. 1997a, 1997b; Thomson et al. 1997). The choice of sowing rate is an important agronomic practice influencing plant density and crop establishment. Plant density can affect canopy development, radiation interception, dry matter production, evaporation of water from the soil under the crop, weed competition, the development of fungal and viral diseases, podding and harvesting height, seed yield, and, ultimately, the profitability of a crop in the farming system. Pulse seed is expensive relative to other crops, and with recommended sowing rates above 1 kg/ha in many cases, seed can make up a significant proportion of the total cost of production. There is limited information published on the optimum sowing rate and plant density for faba bean in Australia. In the subtropical environment of northern New South Wales, Marcellos and Constable (1986) concluded that 2 plants/m 2 is optimal for the smallseeded cultivar Fiord in high-yielding situations, but that 3 35 plants/m 2 is more appropriate for less favourable environments. Depending on the mean seed size and germination rate of the seed, these latter densities could require sowing rates of up to 2 kg/ha, which is costly and difficult to achieve with most conventional types of sowing machinery. In the Mediterranean-type environments of southern Australia, a plant density of 3 plants/m 2 is generally targeted by farmers, but in medium to low rainfall areas or situations where the risk of fungal diseases is large, densities of plants/m 2 are often used (Lamb and Poddar 1992; Siddique et al. 1998b). Even lower densities are targeted where large-seeded varieties are grown. At a high rainfall site (>6 mm/year) in South Australia, Adisarwanto and Knight (1997) recently demonstrated an interaction between sowing time and plant density with Fiord faba bean. As plant density was increased above 2 plants/m 2, seed yields decreased when sown in April, but with later sowings, yields increased. In the Mediterranean environment of northern Syria, Saxena et al. (1991) demonstrated that 2 26 plants/m 2 was optimum for the local largeseeded faba bean when grown under supplementary irrigation, whereas higher densities (44 plants/m 2 ) were recommended under dryland conditions. In Spain, 16 plants/m 2 was found to be optimum under irrigation (Aguilera-Diaz and Recalde-Manrique ). In this present study, the effects of sowing rate on seed yield and crop profitability were investigated at 19 sites over 3 seasons to determine the economic optimum plant density for faba bean in the Mediterranean-type environments of south-western Australia. The effects of sowing rate on canopy development, radiation absorption, and dry matter production and partitioning at one site over 3 years are presented in the second part of this study (Loss et al. 1998). Materials and methods Sites, experimental design, and management The experiments were conducted at 3 sites in, at 8 sites in, and at 8 sites in, representing a range of agro-ecological zones in the cropping region of south-western Australia (Table 1). Most experiments were conducted in farmers fields. The experiments were laid out in a completely randomised block design with 4 replications including 6 sowing rate treatments ranging from 7 to 27 kg/ha. The highest rate was chosen as it represents about twice the commonly used sowing rate in southern Australia. Good quality faba bean seed (cv. Fiord, 4 g/1 seeds) was used with germination rates of 85 9% in all 3 years. In most cases, the experiments were sown soon after the first autumn rains in May; however, at some sites in, this did not occur until mid June (Table 1). The experiments were sown using a plot cone-seeder with the sowing depth of 5 8 cm. The plots were 1 44 m (8 rows) by 2 m long. Rates of fertiliser are listed in Table 1. Weeds were controlled before sowing with 1 5 L/ha of Sprayseed (25 g/l paraquat diaquat) or 1 L/ha Roundup (45 g/l of glyphosate), applied 1 or 2 weeks before sowing. Immediately before sowing, 2 L/ha of Bladex (5 g/l cyanazine) or simazine (5 g/l) was applied. In some cases, diuron (9 g/l) was also applied after sowing but before the crop had emerged. Grass weeds that emerged after the faba bean seedlings were controlled with up to 5 ml/ha of Fusilade (212 g/l of fluiazifop-p-butyl), Sertin (186 g/l of sethoxydim), Verdict (13 g/l of haloxyfop-r) or Targa (94 g/l of quizalopfop-pethyl), whereas broad-leafed weeds were minimal and removed by hand where required. Redlegged earth mite (Halotydeus destructor), lucerne flea (Sminthurus viridis), aphids (Aphis craccivora), and pod borer (Helicoverpa sp.) were controlled with insecticides when required. In and, serious infections of Ascochyta fabae and Botrytis fabae were noted in several experiments and 2 5 kg/ha of Dithane M45 (8 g/kg of mancozeb) was applied. Daily maximum and minimum temperatures and rainfall were collected from nearby weather stations or farmer records.

3 Sowing rate and seed yield of faba bean 991 Table 1. Locations, soil type, sowing dates, fertiliser (kg/ha) and May October (M O) and long-term mean rainfall (mm) at the experimental sites Location Soil type A Sowing dates Fertiliser B Rainfall M O Mean M O Bencubbin Reddish sandy clay loam (6 4) over 3.v DAP Farmer s field site a silty clay loam at 3 cm (7 8) (3 49 S, E) Boyup Brook Grey-black gravelly sand (5 2) over 31.v.95 1 DAP Farmers s field site a sandy loam at 3 cm (5 9) (33 5 S, E) Deep uniform cracking grey-brown 27.v DAS Farmer s field site clay loam (8 ) 24.v DAP 39 (29 17 S, E) 4.vi DAP 48 Frankland Loamy sand (4 7) over gravelly 3.vi.95 1 DAP Farmer s field site loam at 25 cm S, E Loamy sand (4 8) over sandy clay at 21.vi DAP cm (4 9) Gnowangerup Grey-brown sandy loam (4 6) over a 24.v.95 1 DAP Farmer s field site grey loamy clay (8 9) at 3 cm (33 57 S, E) Merredin Reddish brown sandy loam (7 ) 24.v DS Agriculture WA Research over a silty loamy clay (9 ) at 29.v.95 8 DAP 29 Station (33 29 S, E) 3 cm 14.vi DAP 255 Northam Reddish brown loam (5 ) over a 6.vi SS Muresk Inst. of Agriculture yellowish brown clayey sand (6 ) 14.v SS 47 (31 43 S, E) at 4 cm 4.vi SS+AS 441 Nyabing Hard setting grey loamy clay (6 ) 23.vi DAP Farmer s field site over a grey clay at 1 cm (7 2) (33 31 S, E) Pingaring Dark greyish brown sandy loam 25.v.95 1 DAP Farmer s field site (5 6) over yellowish red medium (32 45 S, E) clayat4cm(7 7) Scaddan Grey brown sandy loam (7 2) over a 21.v DAP Farmer s field site heavy grey clay (8 1) at 15 cm (33 6 S, E) Three Springs Reddish brown coarse sandy loam 16.v DAP Farmer s field site over sandy clay loam ( vi DAP 369 (29 34 S, E) increasing to 6 6 at 2 3 cm) A Values in parentheses are ph (1 : 5 CaCl 2). B SS, single superphosphate; DS, double superphosphate; DAP, diammonium phosphate; AS, ammonium sulfate Plant density and dry matter at flowering In each experiment, plant density was measured at about 6 weeks after sowing using 5 quadrats ( 5 m 2 each) per plot. To estimate plant mortality after this initial measurement, plant densities were also estimated in the 2 quadrats ( 5 m 2 each) per plot taken for yield and yield components at maturity (see below) at Northam and Merredin in only. Within a week of 5% first flower (i.e. 5% of plants with at least one fully opened flower with the corolla visible), the above-ground dry matter production was estimated by cutting the shoots at ground level from 2 representative quadrats ( 5 m 2 each) per plot at most sites. These samples were dried in an oven at 7 C for 48 h and weighed. Seed yield and components Apart from and Boyup Brook in, and Three Springs, Frankland, and Scaddan in, seed yield and total dry matter production were estimated at maturity from 2 representative quadrats ( 5 m 2 each) per plot at all sites. The quadrat seed yield was only used to calculate harvest index, the ratio of seed weight to the weight of above-ground dry matter at final maturity. Ten representative plants were also sampled from each plot at maturity, and the numbers of pods and seeds, and seed weight, were recorded. Seed yield was also determined by a mechanical plot harvester which removed the inner 6 rows of each plot to minimise any edge effects. All subsequent seed yield references are machine-harvested yields. Economic optimum plant density The economic optimum plant density for each field experiment was calculated using a similar method to that of French et al. (). The following model, which describes an asymptotic increase in machine-harvested seed yield (y, t/ha) as plant density (x, plants/m 2 ) is increased, was fitted to the data on an individual plot basis (see Fig. 2): y = ax/(1 + bx) At all sites, this asymptotic curve was found to be more appropriate than the 3-parameter quadratic model also investigated by French et al. (), which describes a curve where yield rises to a maximum and then decreases as plant density is increased further.

4 992 S. P. Loss et al. Economic optimum densities were estimated as the point on the response curve where the cost of sowing additional seed equals the return from the additional grain produced, allowing a 1% opportunity cost for investing the additional money required for the extra seed. The slope at this point on the response curve was calculated according the method of French et al. () assuming a faba bean seed cost of $A34/t, a return from the harvested grain of $24/t, a mean seed weight of 4 g/1 seeds, and an establishment proportion of 71% (see results). The slope at this point was 8 t/m 2 ha plant. A second set of analyses was also performed assuming a 5% opportunity cost. Maximum yield potential of a site was defined as the asymptotic value of y or a/b and standard errors were calculated by the equation described by French et al. (). Statistical analysis All other data were analysed using a standard analysis of variance using GENSTAT for Windows 3 2 (Lawes Agricultural Trust, Rothamsted Experimental Station). Each experiment was analysed separately, because of differences in soil type, season, sowing date, and location. Least significant differences (l.s.d.) were calculated at P = 5. The relationship between economic optimum density and maximum yield potential of all sites was examined using a linear regression. Results Seasonal conditions The season was one of the driest in decades. Only 168 mm of rain was recorded for the growing season at Merredin compared with an average May October rainfall of 231 mm (Table 1). Plants at Merredin relied heavily on the small amounts of rainfall throughout the season and were visibly water-stressed for several days in early August just before first flower. Rainfall was also below average at and Northam. By contrast to the previous year, the winter was wetter, particularly in July. Growing season rainfall was above average at Merredin, Northam, Pingaring, and Three Springs, and within 25 mm of the average at Boyup Brook and. At these sites, plants showed no obvious signs of moisture stress until close to maturity. Rainfall was below average at the southern sites of Gnowangerup and Frankland, but nevertheless, a fungicide application was applied to control Ascochyta fabae and Botrytis fabae at these sites and Boyup Brook. Apart from the late arrival of autumn rains and subsequent delay in sowing relative to the previous seasons, the growing season was quite favourable with good rains in winter and spring at most sites. The May October rainfall was above average at Northam,, and especially Three Springs. Rainfall was about average at all other sites expect Scaddan, which experienced below average conditions. No serious disease infections were noted at any of the sites except at Nyabing where a fungicide application was required to control Ascochyta fabae. Density (plants/m 2 ) 6 (a) 4 2 (b) 4 2 (c) 4 2 Northam Merredin Northam Merredin Bencubbin Three Springs Nyabing Scaddan Northam Merredin Three Springs Pingaring Gnowangerup Boyup Bk Frankland Sowing rate (kg/ha) Fig. 1. The plant densities established from a range of sowing rate experiments at various sites in south-western Australia in (a), (b), and (c). Plant establishment Some sites, such as Three Springs in, had good plant establishment rate, whereas others, such as Merredin in, had much lower rates of establishment (Fig. 1). At most sites, plant densities were slightly above average in and slightly below average in. Across all sites and seasons there was a significant linear regression between plant density (PD, plants/m 2 ) and sowing rate (SR, kg/ha) according to the following equation (r 2 = 743, P < 1): PD=SR 178

5 Sowing rate and seed yield of faba bean 993 On average across all sites and seasons, plant densities varied from 12 plants/m 2 when sown at 7 kg/ha to 48 plants/m 2 when sown at 27 kg/ha. Assuming a mean seed weight of 4 g/seed, then only 71% of seeds sown emerged on average. At Northam and Merredin in, plant densities at maturity ranged from 88% to 95% of the plant densities estimated at 6 weeks after sowing, with a small trend of greater mortality in the high sowing rate treatments (data not presented). Dry matter at flowering Dry matter production at 5% first flower ranged from 63 to 88 g/m 2 (Table 2) and there was a significant trend of greater dry matter as sowing rate was increased at all sites (P < 5). Table 2. Mean dry matter (g/m 2 ) at 5% first flower for the sowing rate treatments at various sites in three years Location Sowing rate (kg/ha) l.s.d (P = 5) Northam Merredin Three Springs Northam Merredin Pingaring Gnowangerup Frankland Northam Merredin Bencubbin Nyabing Seed yield and yield components There was a significant increase in seed yield (P < 5) as sowing rates were increased at all sites (Table 3). Harvest indices were large (34 62%) and were generally >4%. At half of the sites where it was measured, there was a significant trend of slightly reduced harvest index as sowing rate was increased (P < 5). The mean number of pods per plant (5 35) decreased significantly with increased sowing rate at all sites (P < 5, Table 4). The number of pods per m 2 was quite variable (12 674), but nonetheless there was a significant trend of more pods with high sowing rates at most sites (Table 4). The number of pods per m 2 was correlated with seed yield (r 2 = 431, P < 1). In contrast, the mean number of seeds per pod ( ) and mean seed weight (32 45 g/1 seeds) were generally unaffected by sowing rate (data not presented), except at Merredin in and at Northam in, where mean seed weight was reduced significantly with increased sowing rate (P < 5). Table 3. Mean machine-harvested seed yields (kg/ha) and harvest indices (HI, %) for the sowing rate treatments at various sites in three years Location Sowing rate (kg/ha) l.s.d (P = 5) Seed yield Northam Merredin Three Springs Northam Merredin Pingaring Gnowangerup Boyup Brook Frankland Three Springs Bencubbin Merredin Northam Nyabing Frankland Scaddan HI n.s. Northam Merredin Three Springs n.s. Northam Merredin n.s. Pingaring n.s. Gnowangerup Frankland n.s Northam n.s. Merredin n.s. Bencubbin Nyabing n.s., not significant. Economic optimum density Across all experiments, the asymptotic model accounted for 15 81% of the variation in the data and

6 994 S. P. Loss et al. Table 4. Mean number of pods per plant and pods per m 2 for the sowing rate treatments at various sites in three years Location Sowing rate (kg/ha) l.s.d (P = 5) Pods per plant Northam Merredin Three Springs Northam Merredin Gnowangerup Frankland Northam Merredin Bencubbin Nyabing Pods per m Northam Merredin Three Springs Northam Merredin Gnowangerup Frankland Northam Merredin Bencubbin Nyabing was particularly poor at Three Springs in and Northam, Scaddan, and Frankland in (Table 5). At 6 sites (Merredin in, Boyup Brook in, and Northam and Frankland in both and ), the economic optimum densities assuming a 1% opportunity cost or yield potentials estimated by the models had standard errors exceeding 4 plants/m 2 and 8 kg/ha, respectively (Table 5). These were considered excessive. In addition, the estimated optimum densities were well beyond the range of the data at Northam in and Merredin in. Hence, these sites are excluded from any further analysis and discussion, leaving 11 valid experiments. The plant density/seed yield response curves and predicted optimum densities for the valid experiments over 3 years are illustrated in Fig. 2. Among the valid experiments and assuming a 1% opportunity cost, the estimated optimum plant densities varied from 31 to 63 plants/m 2 at and Merredin respectively, both in, with a mean of 45 plants/m 2 (Table 5). When assuming a 5% opportunity cost, the predicted optimum densities were reduced by about 2% (data not presented). Estimated yield potentials at these sites ranged from 2267 to 551 kg/ha at Scaddan and Merredin, respectively, both in (Table 5). Among the valid experiments there was no significant relationship between estimated yield potential and optimum density (r 2 = 114). Economic optimum density was also poorly related to growing season rainfall and sowing date (r 2 = 138 and 1, respectively). Discussion As far as we are aware, this is the first published study to examine the effect of sowing rate on the profitability of faba bean production in Australia. Although the estimated optimum plant densities varied considerably (31 63 plants/m 2 assuming a 1% opportunity cost), the present investigation demonstrates that the seed yields and profits of most faba bean crops in southwestern Australia can be increased by using sowing rates and plant densities above those currently targeted by farmers in southern Australia. Similar results have been observed in low rainfall areas of South Australia (G. McDonald, pers. comm.). Many new faba bean producers are reluctant to invest an extra $1 in seed fora$1 1 return in extra yield because faba bean is considered a risky crop when compared with cereals. However, the predicted optimum densities were still above the current recommendation when the assumed opportunity cost for the extra investment in seed was increased from 1% to 5%. In most cases, targeting the mean optimum determined in this study of 45 plants/m 2 is likely to produce greater profit than the current recommendation of 3 plants/m 2 for cv. Fiord. For example, when 3 plants/m 2 were established at Bencubbin, the mean seed yield was 1989 kg/ha compared with a yield of 223 kg/ha at 45 plants/m 2 (Fig. 2). Assuming a fixed cost of production excluding seed expenses of $14/ha and other assumptions stated in the Materials and methods, then a simple gross margin analysis shows that the additional seed cost to achieve 45 plants/m 2 of $29/ha produces extra income worth $6/ha. In other words, profit was increased by $31/ha by increasing the plant density from 3 to 45 plants/m 2 at this site. The causes of the range in optimum plant densities estimated in this study are unknown. A good relationship between estimated optimum plant density and yield potential was reported in narrow-leafed lupin

7 Sowing rate and seed yield of faba bean 995 Table 5. Fitted model parameters for the yield response to plant density curves, variance accounted for by the models (%), economic optimum density (plants/m 2, ±s.e.) and estimated yield potential (kg/ha, ±s.e.) for faba bean sowing rate trials over three seasons Location Curve parameters Variance Economic Yield potential a b accounted optimum density ± ±285 Merredin ± ±861 Northam ± ± ± ±177 Three Springs ± ±255 Merredin ± ± 233 Northam ± ±1468 Pingaring ± ±132 Gnowangerup ± ±192 Boyup Brook ± ±917 Frankland ± ± ± ±122 Three Springs ± ±265 Bencubbin ±13 294±426 Merredin ± ±536 Northam ± ±4427 Nyabing ± ±38 Frankland ± ±851 Scaddan ± ±56 when different growing conditions were created by cultivating lupin on nearby paired sites with suitable or less favourable soil types (French et al. ). They concluded that large densities are optimal in high yielding situations for narrow-leafed lupin. Similarly, a low density (15 2 plants/m 2 ) is sometimes targeted for faba bean in areas of South Australia where yields are limited by low rainfall because of the increased risk of moisture stress in spring at high density (Lamb and Poddar 1992). In contrast, others have suggested high plant densities for faba bean in low-yielding situations caused by late sowing (Marcellos and Constable 1986; Adisarwanto and Knight 1997). There was no significant relationship between optimum density and yield potential in the present study and similar results were found with lentil (Siddique et al. 1998a) when grown in a similar range of environments. Yield potential is affected by many factors and it is not surprising that no relationship between yield potential and optimum plant density is observed in many studies. However, there were some suggestions of a negative relationship between optimum sowing rates and yield potential in this investigation. For example, the region is one of the most fertile environments for faba bean production in Western Australia and it consistently produced some of the highest yields in this and other studies (Loss and Siddique 1997). Optimum densities at were among the lowest estimated in this study (<4 plants/m 2 ), except in when yield potential was reduced by the dry season (Fig. 2). In addition, greater yield was produced as plant density was increased to >5 plants/m 2 in the dry season at Merredin and other dry sites. As was discussed by Donald (1963), it appears that when growth is limited by dry conditions, delayed sowing, or other factors, there is little inter-plant competition and crops sown at a high density are able to set more pods per m 2 and produce more seed than those at low density. As plant density was increased in the present study, the number of pods per plant generally decreased, but this was more than compensated for by the large plant population resulting in more pods per m 2 and increased seed yields. Similar results were reported for Fiord when sown after mid May in South Australia (Adisarwanto and Knight 1997). Mean seed weight, an important quality parameter, was largely unaffected by sowing rate in these and other experiments (Loss et al. 1997a). Apart from in, the asymptotic models fitted to the data indicate how yields continued to increase as plant density was increased above the commonly recommended 3 plants/m 2. At several sites, yields continued to increase significantly at the

8 996 S. P. Loss et al. Seed yield (kg/ha) (b) (a) (c) Plants/m 2 Merredin Three Springs Pingaring Gnowangerup Three Springs Nyabing Bencubbin Scaddan Fig. 2. The fitted curves of seed yield response to plant density at various sites in (a), (b), and (c). Dotted lines indicate estimated economic optimum density. highest sowing rate of 27 kg/ha and the economic optimum densities predicted by the models were well beyond the range of the observed data. It would be unwise to place any confidence in the estimated optimum densities at these sites, other than to say that they were larger than the densities measured in these experiments. Given that sites with optimum densities beyond the range of the data were excluded, the mean optimum density determined from the valid experiments in this study (45 plants/m 2 ) is likely to be suboptimal at several sites. Clearly, the experiments at these extraneous sites required sowing rates exceeding 27 kg/ha to estimate optimum plant density more accurately. Having stated that the mean optimum density of 45 plants/m 2 determined in this study is likely to have been under-estimated, there are several reasons to moderate this claim on a commercial scale. Fungal diseases were either absent or controlled with the use of fungicides. Farmer experience in southern Australia suggests that diseases such as Ascochyta fabae and Botrytis fabae can limit the yield and quality of faba bean, especially with high sowing rates and early sowing. These practices cause the development of a large crop canopy and greater senescence of leaves at the bottom of the canopy, which are thought to increase the risk of Botrytis fabae (B. MacLeod, pers. comm.). Under the appropriate weather conditions, this disease can enter an aggressive phase where fungicide applications are of little benefit. This is why reduced sowing rates are suggested with early sowing in medium to high rainfall regions (>4 mm/year) of South Australia (Lamb and Poddar 1992; Siddique et al. 1998a). The interactions between sowing rate, sowing time, and diseases deserve further attention. On average, only 71% of seeds sown germinated and emerged from the soil in the present study. It is likely that the main cause of the poor plant establishment for faba bean is due to physical damage to the seed during the processes of harvest, cleaning, transport, or mechanical sowing. Germination rates for faba bean seed are generally high and no serious seedling diseases have been noted in southern Australia. Grower experience and recent experiments in south-western Australia indicate that once established, faba bean seedlings are relatively tolerant of low soil moisture, surviving up to 1 weeks of dry weather with virtually no mortality (R. French, pers. comm.). Given the low establishment rates of faba bean, sowing rates of about 25 kg/ha are required to achieve the mean optimum density of 45 plants/m 2. Most current conventional sowing machinery is unable to cope with sowing rates above 2 kg/ha, particularly for faba bean which is large-seeded compared with other crops. Large-scale sowing machinery is likely to be more damaging than the experimental machines used in this study and there is a need to develop commercial machinery that can handle large sowing rates with minimal damage to faba bean seed, particularly if cultivars with seed larger than Fiord are being sown. Faba bean cultivars, some with seed sizes 3 times that of Fiord (e.g. Aquadulce), are grown in Australia. One would expect different seed yield and plant density response curves for large-seeded cultivars that have more rapid canopy development than Fiord when grown at the same plant density (Mwanamwenge et al. 1998). Similarly, different plant densities have been suggested for faba bean cultivars with a determinate or conventional indeterminate growth habit in northern Syria (Silim and Saxena 1992). A more detailed examination of the effect of sowing rate on the growth of Fiord, its canopy development, radiation interception, and dry matter partitioning over 3 seasons at Northam is presented by Loss et al. (1998).

9 Sowing rate and seed yield of faba bean 997 In conclusion, faba bean yields and profits can be increased in many situations in south-western Australia by targeting plant densities above those currently used in southern Australia. It is likely that the optimum sowing rate of faba bean varies depending upon the growing conditions, but the mean optimum plant density determined in this investigation (45 plants/m 2 )is greater than that recommended in other high-yielding environments in Australia. This mean optimum plant density was probably under-estimated because experiments were excluded where the predicted optimum densities were much greater than the range of the data. On the other hand, fungal diseases were absent or controlled in this study and, hence, were not limiting growth and yield at high plant densities. There is a need for further research on the plant density of faba bean using sowing rates greater than those of this study (i.e. >27 kg/ha) and to investigate the effects of time of sowing and disease on optimum plant density. Commercial machinery capable of handling sowing rates greater than 2 kg/ha with minimal damage to faba bean seed is required, particularly if large-seeded cultivars are being sown. Acknowledgments We are grateful for the valuable technical assistance of Mike Baker, Chris Veitch, David Wilkinson, Leanne Young, David Collins, Angie Roe, and other Agriculture Western Australia staff who managed the experiments, and Tammi Compton, Alicia Andresen, and Trish Jones for their technical assistance at Northam. Jason Boland and Jane Speijers made a valuable contribution to the model fitting and economic analysis. Our thanks also go to the many farmers who permitted the field experiments to be conducted on their properties. 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