Effects of high plant populations on the growth and yield of winter oilseed rape (Brassica napus)

Journal of Agricultural Science, Cambridge (1999), 132, 173 180. 1999 Cambridge University Press Printed in the United Kingdom 173 Effects of high plant populations on the growth and yield of winter oilseed rape (Brassica napus) J. E. LEACH*, H. J. STEVENSON, A. J. RAINBOW AND L. A. MULLEN IACR-Rothamsted Experimental Station, Harpenden, Herts AL5 2JQ, UK (Revised MS received 16 June 1998) SUMMARY The effects of plant density on the growth and yield of winter oilseed rape (Brassica napus) were examined in a series of five multifactorial experiments at Rothamsted Experimental Station between 1984 and 1989. Plant densities, manipulated by changing the seed rate and row spacing, or because of overwinter losses, ranged from 13 5 to 372 plants m. Normalized yields for the multifactorial plots increased with densities up to 50 60 plants m. In very high density plots in 1987 88, yield decreased as density increased 150 plants m. Plants grown at high density had fewer pod-bearing branches per plant but produced more branches m. Branch dry matter (DM) per plant was decreased by 42%, the number of fertile pods per plant and pod DM plant by 37%. There was no effect of density on the number or DM of pods m. Over 74% of the fertile pods were carried on the terminal and uppermost branches of plants grown at high density in 1987 88 compared with only 34% in plants grown at low density in 1988 89. Seed DM plant decreased with increase in density but seed size (1000-seed weight) increased. There was no effect of density on seed glucosinolate or oil contents. INTRODUCTION The ability of winter oilseed rape crops to compensate in yield for differences in plant density is well documented (e.g. Mendham et al. 1981; Ogilvy 1984; McGregor 1987; Leach et al. 1994). Although large differences in vegetative dry matter and plant structure are produced at different densities during early growth, seed yields are not greatly affected because of compensatory changes in individual components of yield. The recommended seed rate for winter sown oilseed rape in the UK is 6 7 kg ha. The aim is to establish 100 110 plants m going into winter and 80 plants m in the spring (Farman et al. 1989). The populations actually achieved generally vary depending on soil conditions and rainfall during the establishment phase or weather conditions, pests and diseases over winter. However, adequate yields can be achieved over a wide range of plant densities (e.g. 8 90 plants m ; Mendham et al. 1981) because the low density crops compensate by producing a greater leaf area, more branches and a greater pod number per plant. Most research has concentrated on the effect of low plant * To whom all correspondence should be addressed. Email: john.leach bbsrc.ac.uk populations on crop growth and yield to produce practical recommendations for the farming industry (e.g. Crop Failure Thresholds). It has been shown that plant populations of 20 30 plants m produce yields comparable to crops with 70 80 plants m ; in some instances a crop with only 9 10 plants m produced acceptable yields if the plants were healthy and evenly distributed (Mendham et al. 1981). With the shift in agricultural policy in the UK away from maximum yield production towards reduced agronomic inputs, improved seed quality and specific oils for industrial use, experiments were done to determine the effects of plant populations on components of yield that might affect seed quality, glucosinolate and oil content and the harvestability of the crop as well as its ability to achieve the potential yield for the crop. Low density populations produce more branches that carry fertile pods, thus prolonging the seed development phase. This results in a range of seed maturity at harvest which may affect seed quality and increase the risk of seed loss through pod shatter and poor harvestability. Plants grown at high densities are often more susceptible to lodging and increased disease incidence without the benefit of any yield increase, but the presence of fewer pod-bearing branches should produce more synchronous pod and seed development and result in more uniform seed

174 J. E. LEACH ET AL. Normalized yield 1 2 1 0 0 8 0 6 0 4 0 2 0 0 10 30 50 70 90 110 130 150 1 2 (b) 1 0 0 8 0 6 0 4 0 2 0 0 0 20 40 60 80 100 120 1 2 1 0 0 8 0 6 0 4 0 2 0 0 (a) (c) 40 50 60 70 80 90 100 110 Plants/m 2 (d) 40 60 80 100 120 140 (e) 140 50 100 150 200 250 300 350 400 ( f ) 10 30 50 70 90 110 Plants/m 2 Fig. 1. Normalized yields for individual plots of winter oilseed rape cv. Bienvenu grown at different densities at Rothamsted in (a) 1985 (maximum yield 7 68 t ha), (b) 1986 (maximum 5 97 t ha), (c) 1987 (maximum 7 30 t ha) and (d) 1988 (maximum 5 14 t ha) and cv. Ariana in (e) 1988 (maximum 4 23 t ha) and ( f ) 1989 (maximum 4 49 t ha). maturation, improved harvestability and possibly lower seed glucosinolate and higher oil contents. This study was designed to compare high density crops with crops having a wide range of plant densities produced in a series of multi-disciplinary multifactorial experiments, that examined the effects of different inputs on the growth and yield of winter oilseed rape (Leach et al. 1994). Additional plots, with higher seed rates and narrower row spacing, were included in some experiments to increase plant population densities. MATERIALS AND METHODS The observations were made on a series of multifactorial experiments at Rothamsted over five seasons between 1984 and 1989 that studied the effects of a wide range of agronomic and husbandry practices on the growth and yield of winter oilseed rape. Full details of the design, cultivations and factors tested in the experiments were given by Leach et al. (1994). In each season there were 64 main factorial plots (3 17 21 m), arranged in a half-replicated design in four blocks; in 1984 85 a fully randomized design was used. Additional plots were included in each experiment, randomized among the main factorial plots, for more detailed physiological studies of a wide range of treatments. Each of these plots included an 3 6 7 m area from which destructive samples were taken during crop growth. In 1987 88 and 1988 89, 42 additional plots were used for density studies. These plots were sown with cv. Ariana in early September at the time of the latest of the two sowing dates tested and were given full fungicide, insecticide and growth regulator treatments. Thirtysix plots tested three replicates of six rates of N (0, 50, 100, 150, 250 and 350 kg N ha in 1988 and 0, 50, 100, 150, 200 and 250 kg N ha in 1989) applied as divided dressing with seed rates of 8 or 16 kg ha sown at 17 cm row spacing. There were six additional plots with the same basal treatments as before but sown at 16 kg ha in 12 cm rows testing the lowest and highest

High plant populations of winter oilseed rape 175 4 50 4 00 Combine yield (t/ha) 3 50 3 00 2 50 2 00 50 100 150 200 250 300 Plants/m 2 Fig. 2. Combine yields (t ha) for individual plots of winter oilseed rape grown at different densities at Rothamsted in 1987 88 ( ) and 1988 89 ( ). Curves were fitted by GENSTAT, in the form y a b expkx, where y 4 471 (S.E. 0 691) 0 2 (S.E. 0 402) exp (S.E.= )x (D.F. 14) for 1987 88 and y a b x, where y 3 165 (S.E. 0 236) 0 00038 (S.E. 0 00269) x (D.F. 16) for 1988 89. N rates in 1987 88 and nil and the highest N rate in 1988 89. Assessments Every 2 weeks, from February to July, crop samples were cut at ground level from six 0 8 m row lengths per plot (0 85 m ) for growth analysis. The number and total fresh weight of plants in each sample was recorded and a random subsample of 5 10 plants taken for more detailed measurements of the percentage dry matter, numbers, green areas and dry weights of the stems, leaves, branches and pods in the different levels of branches. In 1988 89, five plants were marked in each plot to monitor the start and duration of flowering at each branch level. A few days before the crop was desiccated and combine harvested a final hand-harvest sample was taken from each plot from which a random subsample of 20 plants was taken to measure the components of yield (number and dry weight of pods and seeds, number of seeds pod and 1000-seed weight) for the different levels of branching. Samples of seed were dried at 45 C for glucosinolate analysis by high-performance liquid chromatography (following the procedure of Heaney et al. (1986)) and at 100 C for oil analysis by nuclear magnetic resonance (Conway & Earle 1963). Effects of treatments on variates measured were statistically assessed by analysis of variance. RESULTS Plant numbers and yield The effects of sowing date pest interactions and season resulted in a wide range of plant densities during the 5-year series of multifactorial rape experiments at Rothamsted. Final plant numbers in individual plots ranged from 13 5 to 372 plants m. The yields of these plots were normalized by expressing them as a ratio of the maximum plot yield for the year and are plotted against the plant densities (Fig. 1a f ). Allowing for the expected wide range of treatment effects, yields increased with density up to 50 60 plants m and thereafter remained constant. In the 1987 88 and 1988 89 seasons, plant density was included as an experimental factor by varying the seed rate and row spacing. At final harvest the earlysown main factorial plots in 1987 88 had a mean plant density of 74 plants m and later-sown ones 116 plants m. The differences in plant density resulted from better establishment and overwintering of the later-sown crop. The density study plots, which were all later sown, produced final mean plant populations of 116, 177 and 191 plants m for plots having 8 and 16 kg ha seed sown at 17 cm row spacing and 16 kg ha seed sown at 12 cm. In 1988 89 the main factorial experiment again had poor establishment of the earlier sown plots so that final mean plant

176 J. E. LEACH ET AL. 250 (a) 200 150 100 Plant number/m 2 50 1 Mar 15 Mar 250 (b) 29 Mar 12 Apr 26 Apr 10 May 24 May 7 Jun 21 Jun 5 Jul 19 Jul 200 150 100 50 15 Feb 1 Mar 15 Mar 29 Mar 12 Apr 26 Apr Fig. 3. Change in mean plant number m with time for winter oilseed rape grown at different densities at Rothamsted in (a) 1987 88 and (b) 1988 89. Sown with 8 kg ha seed at 17 cm row spacing ( ), 16 kg ha at 17 cm ( ) and 16 kg ha at 12 cm ( ). 10 May Date 24 May 7 Jun 21 Jun 5 Jul 19 Jul 2 Aug densities were 37 plants m for the early sown and 63 plants m for the later sown. As in the previous season, the density study plots established a range of plant densities so that by the time of the penultimate hand-harvest on 20 June the mean populations were 83, 109 and 132 plants m. However, 17 days later, at final hand-harvest, the populations had decreased to only 74, 80 and 87 plants m with no significant difference between treatments. The late reduction in plant number may have been the result of the hot dry weather at the end of the 1988 89 growing season or the effect of spatial variation within the crop. Analysis of variance showed no significant difference in yield between the treatment means for the two seasons

High plant populations of winter oilseed rape 177 1500 (a) 1250 1000 750 500 250 Total dry matter (g/m 2 ) 0 1 Mar 15 Mar 1750 (b) 29 Mar 12 Apr 26 Apr 10 May 24 May 7 Jun 21 Jun 5 Jul 19 Jul 1500 1250 1000 750 500 250 0 15 Feb 1 Mar 15 Mar 29 Mar 12 Apr 26 Apr Fig. 4. Change in mean total crop dry matter (g m ) with time for winter oilseed rape grown at different densities at Rothamsted in (a) 1987 88 and (b) 1988 89. Sown with 8 kg ha seed at 17 cm row spacing ( ), 16 kg ha at 17 cm ( ) and 16 kg ha at 12 cm ( ). 10 May Date 24 May 7 Jun 21 Jun 5 Jul 19 Jul 2 Aug (3 56, 3 45 and 3 62 0 13 t ha for 1987 88; 3 25, 3 18 and 3 17 0 10 t ha for 1988 89). A more detailed analysis of the individual plot values of the density study is shown in Fig. 2. In 1987 88, yield decreased as density increased above 150 plants m and the curve, fitted by GENSTAT, accounts for 70% of the variance in the data. In 1988 89, however, plant density did not reach 150 plants m and there was no effect of density on yield. For densities up to 150 plants m the mean yield in 1988 89 (3 22 0 19 t ha) was lower than in 1987 88 (3 96 0 14 t ha) which probably reflects the effect of water stress towards the end of that growing season. The progression of plant number and dry matter In both the 1987 88 and 1988 89 seasons, the winters were mild and there was good plant emergence and

178 J. E. LEACH ET AL. Table 1. Effects of plant density on hand-harvested components of yield of winter oilseed rape, sown with 8 and 16 kg ha seed rates and 17 and 12 cm row spacing, grown at Rothamsted in 1987 88 and 1988 89 Seed dry Pod number Pod dry matter weight 1000-seed Plants m (g plant) weight (g) m plant g m g plant 1987 88 Main factorial Sown early 74 3 3 78 4 46 4884 69 8 633 8 65 Sown late 116 6 2 74 4 35 5299 46 7 715 6 31 S.E. (D.F.) 9 4 (10) 0 33 (10) 0 06 (32) 571 (10) 9 7 (10) 47 (10) 0 62 (10) Density plots 8 kg, 17 cm 116 3 1 87 4 59 5515 50 4 702 6 30 16 kg, 17 cm 191 4 1 36 4 69 5724 34 0 693 4 09 16 kg, 12 cm 177 4 1 54 4 75 6063 38 2 735 4 70 S.E. (D.F.) 10 3 (24) 0 22 (23) 0 04 (24) 299 (24) 1 7 (24) 42 (24) 0 30 (24) 1988 89 Main factorial Sown early 37 5 7 04 4 30 5887 170 8 529 15 27 Sown late 62 7 4 81 4 43 6286 107 3 613 10 20 S.E. (D.F.) 3 1 (10) 0 48 (7) 0 05 (19) 546 (7) 3 9 (7) 62 (7) 0 87 (7) Density plots 8 kg, 17 cm 74 2 3 94 4 54 6522 89 7 652 8 98 16 kg, 17 cm 80 1 3 72 4 66 6064 79 5 616 8 02 16 kg, 12 cm 86 5 3 12 4 61 6144 72 7 606 7 17 S.E. (D.F.) 4 0 (22) 0 28 (21) 0 05 (24) 459 (24) 6 9 (20) 41 4 (24) 0 65 (24) 80 70 Percentage of total pod number 60 50 40 30 20 10 0 35 5 62 7 74 3 74 2 86 5 80 1 116 3 117 3 177 4 191 4 Number of plants/m 2 Fig. 5. Terminal and Branch 1 pod number as a percentage of total pod number for winter oilseed rape grown at different plant population densities at Rothamsted in 1987 88 and 1988 89. establishment in the autumn, especially in the fully replicated density plots which were sown late. Mean crop densities were 134, 259 and 270 plants m in November 1987 and despite a cold wet November, 127, 222 and 275 plants m in December 1988. Most of the plants survived overwinter in 1987 88 and mean plant densities in March 1988 were 149, 244 and 231 plants m, but during the rest of the season there

High plant populations of winter oilseed rape 179 Table 2. Effects of plant density on seed glucosinolate concentrations (µmol g at91% DM) and oil content (% oil in DM) of winter-sown oilseed rape, sown with 8 and 16 kg seed rates and 17 and 12 cm row spacing at Rothamsted in 1987 88 and 1988 89 Sowing date treatment Plants m Glucosinolate (µmol g DM) % oil in DM 1987 88 Main factorial Sown early 74 3 30 8 47 8 Sown late 116 6 25 6 48 3 S.E. (D.F.) 9 4 (10) 2 67 (24) 0 18 (26) Density plots 8 kg. 17 cm 116 3 24 1 49 3 16 kg. 17 cm 191 4 23 5 49 2 16 kg. 12 cm 177 4 24 7 48 8 S.E. (D.F.) 10 3 (24) 0 32 (24) 0 21 (24) 1988 89 Main factorial Sown early 37 5 36 3 45 0 Sown late 62 7 22 5 44 5 S.E. (D.F.) 3 1 (10) 1 95 (10) 0 11 (19) Density plots 8 kg. 17 cm 74 2 35 2 44 3 16 kg. 17 cm 80 1 37 2 44 7 16 kg. 12 cm 86 5 36 3 44 8 S.E. (D.F.) 4 0 (23) 2 46 (18) 0 17 (23) was a progressive decrease in plant number so that at final harvest the densities were 125, 161 and 182 plants m respectively. There were greater overwinter plant losses in 1988 89 and the mean densities by April 1989 were 108, 136 and 137 plants m. These plant losses were thought to have been due to damage by pathogen infection resulting from the cold wet conditions of November. Plant numbers further declined during the rest of the season to 83, 109 and 132 on 20 June 1989 and 74, 80 and 87 plants m at final harvest (Fig. 3a, b). Changes in total crop DM with time are shown in Fig. 4a, b. In 1987 88, crops grown at low density produced significantly less DM than at high density prior to mid-april. In 1988 89, crops produced c. 10% more DM, probably due to the warmer spring and summer, but there were no significant differences due to density at any stage of growth. Canopy structure and yield components Increasing plant density modified plant structure. Plants grown at the high densities produced more branches m but had fewer pod-bearing branches per plant, decreased branch DM per m and per plant and, in 1987 88, fewer fertile pods plant, less pod DM plant; there was no change in the number or DM of pods m (Table 1). Pod distribution within the crop canopy was also greatly influenced by plant density. At high density most pods were borne on the upper branches with threequarters of the fertile pods being carried on the terminal and uppermost branch in 1987 88 compared with only a third at low densities in 1988 89 (Fig. 5). Although the mean combine yields, for the three density treatments, were not significantly different, the weight of seed DM plant was significantly decreased by plant density and the mean weight per 1000-seed was increased (Table 1). Flowering, seed maturity and seed quality Flowering was monitored intensively in 1988 89 and less intensively in 1987 88. Increasing plant density had no effect on the date of the onset of flowering (the date at which 50% of the flowers were open) or the end of flowering in any of the branch categories within the canopy profile. At flowering in 1988 89, the plant densities ranged from 93 to 150 plants m and in this year for high plant densities the overall processes finished earlier and seeds matured earlier because there were fewer pod bearing branches plant a high proportion of which were on the uppermost branches. Plant density did not affect seed glucosinolate or oil contents in the late-sown density range plots. In the early-sown multifactorial plots there were large differences between years and a tendency for crops grown at low densities to have higher seed glucosinolate concentrations (Table 2).

180 J. E. LEACH ET AL. DISCUSSION It was thought that manipulation of plant densities of oilseed rape crops might be a means by which the uniformity of seed maturation, crop yield, seed glucosinolate and oil contents might be improved in oilseed rape if the potential problems of increased disease incidence and susceptibility to lodging were overcome by applications of fungicide and growth regulator sprays. Some progress towards the theoretical potential maximum crop yields of 7 5 t ha as proposed by Daniels et al. (1982) might also be expected. However, in the experiments reported here, yield stability was maintained across a wide range of plant densities up to about 150 plants m. This was because the increase in plant number per m was counteracted by plants producing fewer pods and smaller weights of pod and seed DM plant. At very high densities, 1000-seed weight was increased, which had some indirect effects on seed quality, and at densities 150 plants m, individual plot yields were decreased. The inability of the oilseed rape crop to maintain seed production at very high densities could have resulted from increased inter- and intra-plant competition for nutrition or radiation or from the effects of increased pests and diseases. However, in the present experiments the crops were fully protected with fungicides and insecticides and given growth regulator sprays and appeared to have adequate nitrogen nutrition since nitrogen fertilizer application had little effect on yield (Leach et al. 1994), although it had been shown to affect photosynthetic efficiency at different plant densities (Leach et al. 1989). The greatest effect of increasing plant density was to alter the distribution of pods within the canopy profile. At high densities a far greater proportion of the fertile pods was present on the terminal and uppermost branch, than at low density. Restricting pod production to the top of the canopy shortened the duration of pod development and seed production, hastened maturation, and resulted in earlier and more uniform crops at harvest and a more predictable and uniform seed quality. Harvest could be timed more accurately and there was less likelihood of serious seed loss due to pod shatter of over-mature pods. There was no great effect of density on either seed glucosinolate or oil content in these experiments, these were primarily influenced by the growing conditions in the different seasons. We thank the Ministry of Agriculture, Fisheries and Food and the Home Grown Cereals Authority for help in funding this work. The IACR receives grant aided support from the Biotechnology and Biological Sciences Research Council. CONWAY, T. F.& EARLE, F. R. (1963). Nuclear magnetic resonance for determining oil content of seeds. Journal of the American Oil Chemists Society 40, 265 268. DANIELS, R. W., SCARISBRICK, D. H., MAHAMUD, B. S., CHAPMAN, J.F.&ADDO-QUAYLE, A. (1982). Oilseed rape physiology. In Yield of Oilseed Rape: National Agricultural Centre Course Papers, pp. 1 20. London: Collins. FARMAN, C. D., HENMAN, A. P. & WARRY, P. J. (1989). Drilling. In Oilseed Rape Manual: Arable Unit, pp. 35 41. Stoneleigh: National Agricultural Centre. HEANEY, R. K., SPINKS, E. A., HANLEY, A. B.& FENWICK, G. R. (1986). Analysis of glucosinolates in rapeseed. Technical Bulletin, AFRC Food Research Institute. Norwich: AFRC. LEACH, J. E., MILFORD, G. F. J., MULLEN, L. A., SCOTT, T. & STEVENSON, H. J. (1989). Accumulation of dry matter in oilseed rape crops in relation to the reflection and absorption of solar radiation by different canopy structures. Aspects of Applied Biology 23, Production and REFERENCES Protection of Oilseed Rape and Other Brassica Crops, 117 123. LEACH, J. E., DARBY, R. J., WILLIAMS, I. H., FITT, B.D.L. & RAWLINSON, C. J. (1994). Factors affecting growth and yield of winter oilseed rape (Brassica napus), 1985 89. Journal of Agricultural Science, Cambridge 122, 405 413. MENDHAM, N. J., SHIPWAY, P. A.& SCOTT, R. K. (1981). The effects of seed size, autumn nitrogen and plant population density on the response to delayed sowing in winter oil-seed rape (Brassica napus). Journal of Agricultural Science, Cambridge 96, 417 428. MCGREGOR, D. I. (1987). Effect of plant density on development and yield of rapeseed and its significance to recovery from hail injury. Canadian Journal of Plant Science 67, 43 51. OGILVY, S. E. (1984). The influence of seed rate on population, structure and yield of winter oilseed rape. Aspects of Applied Biology 6, Agronomy, Physiology, Plant Breeding and Crop Protection of Oilseed Rape, 59 66.