Effects of previous cropping on seed yield and yield components of oil-seed rape (Brassica napus L.)

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ELSEVIER European Journal of Agronomy 6 (1997) 215-223

European Journd of

AsronOmY

Effects of previous cropping on seed yield and yield components of oil-seed rape (Brassica napus L.)

Klaus Sieling *, Olaf Christen, Bahadour Nemati, Herbert Hanus

Institute of Crop Science and Plant Breeding, Christiun-Albrechts-University Kiel, Olshausenstr. 40, 24118 Kiel, Germany

Abstract

Information about the effect of the preceding crop or crop combination on the seed yield of oil-seed rape is extremely scarce. Experiments were carried out in northwest Germany to investigate the effect of different preceding crops on the growth, seed yield and yield components of oil-seed rape. The two directly preceding crops, wheat and oil-seed rape, had only a negligible and non-significant effect on the seed yield of the following oil-seed rape crop.

Oil-seed rape grown after wheat had more pods per plant, due to an increase in the number of pods on the higher category branches.

In contrast, the seed yield and yield components were more affected by the cropping sequence, i.e. the crops 2 years before. Averaged over two experimental years, the greatest yields were observed in oil-seed rape following the sequence peas-wheat (694 g mm’), whereas the smallest seed yield occurred after 2 years of oil-seed rape cropping (371 g m-‘). The differences in the seed yield were again associated with more pods per plant, which compensated for the lower number of plantsm-‘, whereas the number of seeds per pod and the mean seed weight were almost unaffected by the previous cropping. It was not possible to relate the described differences to the crop development, since differences in the biomass caused by the previous cropping were only significant at maturity. Oil-seed rape grown after 2 years of oil-seed rape had the highest ratings of stem canker (Leptosphaeria maculans) as well as verticillium wilt (Verticillz’um dahliae). But the general level of the diseases was low, and therefore other causes for the effects described must be considered. 0 1997 Elsevier Science B.V.

Keywordy; Oil-seed rape; Seed yield; Yield components; Preceding crop; Cropping sequence

1. Introduction

The proportion of oil-seed rape in crop rotations in northern Europe has increased considerably throughout the last decade but the effect of different preceding crops on yield and yield components of a following oil-seed rape has been reported only occasionally. Based on 3 years of survey-data from former East-Germany, Mijller and Makowski

* Corresponding author.

PII S1161-0301(96)02049-7

(1977) reported that the highest yields of oil-seed rape (2.35 t ha-‘) were obtained following a clover or lucerne-dominated pasture. Rape grown after a cereal crop yielded 2.19 t ha-‘. The authors, however, attributed the yield differences solely to the later planting, by an average of 5 days, of oil-seed rape following cereals compared with growing after pasture. George et al. (1985), again on the basis of survey data from former East-Germany, reported a 13% yield decrease of oil-seed rape grown after rapeseed compared with that after a 4-year break in the rotation. Contrasting results

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216 K. Siding et ul. II Europun Journul of Agrononq 6 ( 1997) 215-223

were reported by Gonet and Ploszynska ( 1987 ) from Poland. In a rotational experiment they observed no yield response of oil-seed rape to the proportion of the crop in the rotation. Even in continuous cropping no significant yield decrease occurred compared with an oil-seed rape grown in a five-course cereal rotation. The authors considered that oil-seed rape could be grown continuously in the coastal areas in Poland close to the Baltic Sea.

The yield components of oil-seed rape have often been studied in experiments involving either different plant densities and/or seed dates (Mendham et al., 198la; Jenkins and Leitch, 1986;

Lutman and Dixon, 1987; Habekotte, 1993).

fertilizer or plant growth regulator treatments (Mendham et al., 1981b; Scarisbrick et al., 1985) or cultivars (Mendham et al., 1984; Chay and Thurling, 1989). In general, the most variable yield components, regardless of the factors compared, proved to be the number of pods per plant or number of pods m-‘. The number of plants mm * and the seed weight was in most experiments more influenced by the season than by treatments and had little effect on seed yield.

The most important constraints on yields of brassica crops grown in short rotations seem to be the incidence of various fungal diseases. Tahvonen et al. (1984) observed an increase from 2 to 38%

with damping-off caused by Rhizoctonia solani when turnip rape had been grown on the same held for three consecutive years. Survey data from former East-Germany indicate that stem canker (Leptosphaeria maculuns) as well as verticillium wilt (Verticillium dahliae) increase with a higher proportion of oil-seed rape in the crop rotations.

According to Seidel et al. ( 1985) a severe infection with stem canker could cause yield losses of up to 50%. Svensson and Lerenius (1987) reported a considerable increase in verticillium wilt in oil- seed rape grown continuously compared with rape grown in a three- or five-course rotation.

Additional small-plot experiments with an artificial inoculation confirmed that a 50% yield decrease could be caused by verticillium infection.

The objective of work reported here was to determine the influence of different preceding crops and crop combinations on the yield and the yield

components of oil-seed rape and relate this to changes in the crop development.

2. Materials and methods 2.1. Description of the site

A long-term rotation experiment was established in 1985 on a sandy loam (Luvisol) at the Hohenschulen Experimental Farm of the University of Kiel, located in the northwest of Germany some 15 km north-west of Kiel

( Schleswig-Holstein). Soil analyses indicated that macro nutrients in the top 60 cm of soil were above the critical levels for oil-seed rape production.

2.2. Experimental details

Becduse this experiment has been described else- where (Christen et al., 1992; Christen and Sieling, 1993, 1995), we present here only a brief overview, with details pertinent to the oil-seed rape study.

The field experiment was originally laid out to compare 15 different crop rotations of which two did not include oil-seed rape (Table 1). These rotations included winter wheat, winter barley, oil-seed rape, peas and oats, and ranged from

Table I

Crop rotations compared in the long-term experiment Rotation

I. Oil-seed rape-monoculture 2. Wheat-monocuhure 3. Barleyymonoculture 4. Oil-seed rape-wheat 5. Oil-seed rape-barley

6. Oil-seed rape-oil-seed rape-wheat 7. Oil-seed rape-wheat-barley 8. Oil-seed rape-wheat-wheat 9. Oil-seed rape-wheat-peas

10. Oil-seed rape-oil-seed rapeewheattwheat 11. Oil-seed rape-oil-seed rape-wheat-peas 12. Oil-seed rape-wheat-peas-wheat 13. Oil-seed rape-wheat-oatssbarley 14. Oil-seed rape-wheat-peas-wheat-barley

15. Oil-seed rape-wheat-peas-oil-seed rape-wheat

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K. Sieling et al. J European Journal of Agronomy 6 (1997) 215-223 211

continuous cropping to five-course rotations.

According to the principles of crop rotation experiments, each single component of these 15 rotations should be present every season, i.e., 45 components randomly arranged in one block. With a total of three blocks, this required 135 plots for the entire trial. The design was also suitable for comparing the influence of different preceding crops on a subsequent oil-seed rape crop, because oil-seed rape either followed wheat (rotations 4, 6, 8, 10, 12 and 15) or rapeseed (rotations 1, 6, 10 and 11) as a direct preceding crop, or wheat, oil-seed rape or peas the year before. This analysis is restricted to the above-mentioned preceding crop combinations, since it allows a quantification of the interaction between the direct preceding crop (R-l) and the crop which had been grown on that particular plot 2 years before (R-2). The number of replicates of these comparisons is indicated in the first line of the respective tables.

2.3. Weather conditions

Precipitation averages 716 mm annually at the experimental site with about 400 mm received from April to September, the main growing season, and some 300 mm during October to March. Both experimental years reported herein were unusual due to the warm winter months of December, January and February with temperatures 5 to 7°C above the 15-year average. Favourable rainfall distribution and temperatures prevailed during the 1988 growing season, giving a high yield potential in that year. In 1989, a severe period of drought during May, June and July limited yields. Despite the differences in the seasonal weather, interactions between the cropping history and the harvest years were not statistically significant on yield and therefore only average figures are presented.

2.4. Crop management

Standard cultural practices were employed in both years. After harvest all plots were disc-har- rowed and the seed bed preparation consisted of ploughing and harrowing. Further agronomic details are given in Table 2.

The oil-seed rape cultivar Ceres was chosen for

Table 2

Experimental details

Season 1987/88 1988/89

Seed rate, kg ha-’ 5.0 3.5

Sowing date 4 September 19 August

Row spacing 14cm 14 cm

Herbicides 5 October 24 August

Metazachlor + Metazachlor Fluazifop-butyl

9 March 7 September Carbetamin + Metazachlor + Dimefuron Fluazifop-butyl Nitrogen fertilization

Nl (80 or 120 kg N ha-‘) 9 March 24 February

N2(100kgNha-‘) 5 April 28 March

Fungicides

Prochloraz - 3 November

14 April 9 March

Vinclozolin 16 May 5 May

Insecticides 22 April 20 March

Harvest date 27 July 28 July

this experiment because of its outstanding commer- cial importance in northern Germany in 1985 at the beginning of this study. All oil-seed rape plots were planted on the same date, regardless of the preceding crop, in order to avoid any interactions due to different sowing dates. The seed dates and the seed rates differ in the two experimental years due to general differences in the soil conditions.

2.5. Treatments

In agreement with common cultural practice, nitrogen fertilizer was given in two applications, at the beginning of the growing season in spring and at the beginning of stem elongation.

Three blocks were treated differently with regard to the nitrogen fertilization and the fungicide application. In the first block (N-, F +) the oil-seed rape received a total of 180 kg N ha-’ (80 kg N ha-’ and 100 kg N ha-‘) and fungicide treatments to control fungal diseases. In the second block (N+, F+), the nitrogen fertilization was increased to a total of 220 kg N ha- ’ ( 120 and 100 kg N ha-‘) again including a fungicide treatment as described above. The third block (N+, F -) received 220 kg N ha- ‘) but no fungicide treatments. These treatments were chosen to repre-

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218 K. Sieling et al. i European Journd qf’iigronomy 6 i 1997) 215-223

sent the major farming practices in northwest Germany in oil-seed rape production. However, since the fungicide and fertilizer treatments had no statistically significant effects on seed yield in either of the 2 years the effects of these treatments are not reported in this paper.

2.6. Data collection and disease assessment

Plant samples of 2 x 1 m row in each plot were taken before winter (GS 25) (growth stages according to Schtitte et al. (1982), Table 3), at the beginning of the growing season (GS 31), depend- ing on the year in late March or beginning of April, during bud development (GS 57), after flowering (GS 69) and at maturity (GS 87).

The biomass was determined after drying in a forced air-drying oven at 70°C for 48 h. The samples from the last date (GS 87) were used to count the numbers of branches, the number of pods per branch and the average number of seeds per pod. Biomass data was calculated per m2. The branches were classified as follows: main stem and the following three branches (l-3); all following branches, i.e., branches of a higher category than 4, were included in the category branches 4 ff. The plots had a size of 18 m’. The estimate of seed yield was based on the single plant analysis (2 x 1 m row) at GS 87 (maturity). For oil-seed rape with its high variability a larger sampling area would be desirable, but since the sampling area in this field trial was restricted due to the original design, we could not increase the number of plants used for the different sampling dates.

Table 3

Growth stages of oil-seed rape (according to Schtitte et al..

1982)

Growth stage Phenological development

01-09 Germination

IO-19 Emergence

20-29 Leave development

30-49 Stem elongation

50-59 Bud development

60-69 Flowering

70-79 Pot formation

80-89 Beginning of maturity

90 Harvest

The stem canker rating (Leptosphaeria maculans) was based on a scale introduced by the Biologische Bundesanstalt fur Land- und Forstwirtschaft (BBA) and ranged from 0 (no visible symptoms) to 9 (total browning and destruction of the root).

Accordingly, the rating for verticillium wilt (Verticillium dahliae) ranged from 0, no visible symptoms, to 9 where the plant was completely covered with microsclerotia. The disease assessments were carried out on the plants which were subsequently used for the single plant analyses at maturity (GS 87). Symptoms caused by other pathogens were recorded, but none reached a severe level in the two experimental years reported here.

Prior to statistical analysis, data based on the single plant analysis as well as the disease assessments were averaged for each plot. Data were subjected to a statistical analysis using PROG GLM (Generalised Linear Models), option LSMEANS of the SAS package. The statistical procedure GLM was applied to allow for the unequal numbers of observations.

3. Results 3.1. Biomass

The growth in biomass of the oil-seed rape over the 2 years was similar and therefore only average results are given (Table 4). During the period between establishment before winter to the sampling after flowering (GS 69), the different preceding crops caused only small and non-significant changes in the pattern of the biomass accumula- tion. A response to the different preceding crop combinations was only significant at the sampling date at maturity (GS 87). At maturity, oil-seed rape grown after the crop combination peas followed by wheat had accumulated 1727 g m-‘, whereas oil-seed rape grown after 2 years of rapeseed only produced an above ground biomass of

984gmp2.

3.2. Yield

The effect of the two directly preceding crops and the preceding crop combination on the seed

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Table 4

K. Sieling et al. / European Journal of Agronomy 6 (1997) 215-223 219

Effect of different preceding crops on the biomass (g m-‘) of oil-seed rape, average of 1987/88-1988/89 Pre-previous Previous crop

crop (R-2) to rape (R-l)

Code GS 25 before winter Wheat

Oil-seed rape Peas Wheat Oil-seed rape Peas LSD,=,.,,

Wheat Wheat Wheat Oil-seed rape Oil-seed rape Oil-seed rape

ww 84 128 219 890 1065

RW 82 97 249 902 1108

PW 78 122 282 996 1727

WR 79 133 226 9.53 1249

RR 14 101 229 1001 984

PR 102 110 202 858 1199

25 40 69 203 336

GS 27 after GS 57 winter

GS 69 GS 81

yields based on the single plant analysis is given in Table 5. Since the interaction year x preceding crop was not significant, only the average figures for the harvest years 1988 and 1989 are given. Oil- seed rape following winter wheat yielded slightly more than following oil-seed rape, but the difference was not statistically significant. The greatest seed yield, 694 g rnp2, averaged over the two experimental years, was obtained from the treatment with oil-seed rape following peas-wheat (PW).

The smaller seed yield, 371 g rne2, was obtained when oil-seed rape followed two consecutive years of oil-seed rape (RR). A comparison of the oil- seed crops grown directly after wheat with those following oil-seed rape reveals the effect of the preceding two crops. Regardless of the directly preceding crop, oil-seed rape grown after peas two seasons before had a greater seed yield than crops with wheat or oil-seed rape 2 years before.

Table 5

Effect of the interaction of the different preceding crops on the seed yield (g m-*) of oil-seed rape, average of 1987/88-1988/89 Pre-previous

crop (R-2)

Preceding crop (R- 1) Wheat Oil-seed rape

Mean

Wheat 415 490 453

Oil-seed rape 423 371 387

Peas 694 483 589

Mean 511 448

LSDp=,,,, preceding crops (R-1)=89; LSD,=,.s, pre-previous crops (R-2) = 126; LSD r = ,,05 preceding crop x pre-previous crops = 179.

3.3. Yield components

Since the number of plants rnp2 was not significantly affected by the direct previous cropping, the differences in single plant yield were responsible for the small differences in seed yield (Table 6).

The major reason for the not significant, though pronounced, differences was the seed yield of the 4 ff branches which was greater in plants grown after wheat as compared to oil-seed rape. With an almost constant number of seeds per pod as well as mean seed weight in the respective branches, the greater seed yield in higher category branches (4 ff) was associated solely with more pods at these branches.

In contrast, in the cases of the different crop combinations, the number of plants me2 was more affected by previous cropping (Table 7). The fewest plants rnw2 were observed in the treatment following peas-wheat (PW) and wheat-wheat (WW), i.e., treatments without oil-seed rape in the cropping history for the last 2 years. All other treatments which had been cropped with oil-seed rape at least once during the two previous years, had more plants rnm2, perhaps due to volunteers, although the differences were not statistically significant.

Oil-seed rape following directly peas-wheat (PW) had the greatest single plant yield, as well as the greatest seed yield on all branch categories.

In the treatments following oil-seed rape the differences in seed yield were smaller. The oil-seed rape following peas-rapeseed (PR) only produced greater seed yields through a greater seed yield in

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220 K. Siding et al. : European Journal qf Agronomy 6 i 1997) 215-223 Table 6

Effect of different preceding crops on yield components and fungal diseases of oil-seed rape, average of 1987/88- 1988/89

Preceding crop (R-l ) LSD,=,, ,15 Wheat Oil-seed rape

Number of plots 36 24

No. of plants me2 73 80 ns*

No. of fertile branches 4.5 4.0 II!, Seed yield main branch (g) 4.0 3.7 ns Seed yield branches I- 3 (g) 2.9 2.3 ns Seed yield branches 4 ff (g) 3.8 2.0 ns Single plant yield (g) 10.7 8.0 tls No. of pods on main stem 47.8 43.4 Ilh No. of pods on branches 35.3 29.6 n:,

l-3

No. of pods on branches 4ff 43.9 25.6 No. of pods per plant 127 99 No. of seeds per pod on 19.8 19.5

main stem

ns ns ns No. of seeds per pod on

branches 1-3

17.4 16.S ^IlS

No. of seeds per pod on branches 4ff

17.3 16.0 ^llS

Mean seed weight on main stem (mg)

4.18 4.28 IlS

Mean seed weight on branches l-3 (mg)

4.13 4.15 ns

Mean seed weight on branches 4lT (mg)

4.15 4.20 ns

Stem canker rating (O-9) Verticillium rating (O-9 )

1.6 2.5 0.3

1.7 2.0 ^nS

*Not significant at P10.05.

the category 4 ff branches. In the oil-seed rape grown after peas-wheat (PW) these differences were associated with an exceptionally large number of pods per plant, which was significantly different to most other treatments. The largest relative difference, however, also occurred on the branches of the higher order categories (4 ff ), whereas the differences on the main stem and branches l-3 were smaller. A similar trend occurred in the oil- seed rape grown directly after oil-seed rape. In these treatments the relative yield difference between the rapeseed-rapeseed (RR) cropping sequence compared with the peas-rapeseed (PR) sequence increased in the higher order branch categories. This being the main reason for the differences in the seed yield, the effect of the previous cropping on the number of seed per

pod as well as the mean seed weight in the different branch categories was smaller.

3.4. Disrases

Both diseases assessed in this experiment showed higher ratings in oil-seed rape directly following oil-seed rape instead of grown after wheat (Table 6). However, the differences for verticillium wilt were negligible. The disease assessments of phoma canker and verticillium wilt for the interaction of the direct preceding crop and the preceding crop combination mirror the results described above on the effect of the directly preceding crop (Table 7). The highest ratings for both diseases occurred in the oil-seed rape following 2 years of rapeseed (RR) and the lowest disease assessments were observed in oil-seed rape after the cropping sequence peas-wheat (PW). With only one excep- tion, the ratings for both diseases were higher in the oil-seed rape directly following oil-seed rape instead of wheat.

4. Discussion

The different preceding crops and crop combinations in this experiment caused differences in seed yield and particularly in the number of pods per plant in both years, showing that yield and yield components of oil-seed rape can be affected by the cropping history. In general, the seed yield of oil- seed rape increased with the length of the break between two oil-seed rape crops. Therefore, oil- seed rape shows a similar yield response to different preceding crops and cropping history as cereals, of approximately 10% of the grain yield (see, e.g., Prew et al., 1986; McEwen et al., 1989; Christen et al.. 1992; Jenkyn et al., 1993; Christen and Sieling, 1993). The results reported for oil-seed rape support findings reported by Moller and Makowski (1977) as well as George et al. (1985).

Our experiment confirms that oil-seed rape shows a response to different preceding crops even when all the plots are planted on the same date. The seed yield in our experiment, however, was more influenced by the cropping history, i.e., the crop grown 2 years before, than by the directly preced-

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K. Sieling et al. / European Journal of Agronomy 6 (1997) 215-223 221 Table I

Effect of different preceding crop combinations on yield components and fimgal diseases of oil-seed rape, average of 1988-1989 Preceding crop combination

WW RW PW WR RR PR

LSD,=,,,,

No. of plots 12 18 6 12 6 6

No. of plants m-’ 68 85 66 13 80 86 ns

No. of fertile branches 3.87 3.80 5.91 4.37 3.52 4.16 1.30

Seed yield main branch (g) 3.76 3.46 4.94 3.91 3.10 3.98 ns

Seed yield branches l-3 (g) 1.96 2.21 4.40 2.58 1.57 2.72 ns

Seed yield branches 4 ff (g) 1.16 2.11 7.48 1.80 1.46 2.86 ns

Single plant yield (g) 6.88 8.50 16.8 8.35 6.13 9.58 ns

No. of pods on main stem 49 41 54 47 38 45 ns

No. of pods on branches 1-3 29 28 49 36 21 32 15

No. of pods on branches 4ff 17 31 83 26 17 33 ns

No. of pods per plant 95 100 186 109 76 110 62

No. of seeds per pod on main stem 19.3 19.7 20.3 19.8 18.5 20.2 ns

No. of seeds per pod on branches l-3 17.1 17.0 17.9 16.4 15.9 17.2 ns

No. of seeds per pod on branches 4ff 16.4 18.4 17.0 15.9 15.8 16.3 ns

Mean seed weight on main stem (mg) 3.99 4.08 4.46 4.24 4.30 4.29 ns

Mean seed weight on branches l-3 (mg) 3.93 4.08 4.37 4.07 4.19 4.18 ns

Mean seed weight on branches 4ff (mg) 3.96 4.16 4.31 4.09 4.39 4.10

Stem canker rating (O-9) 1.3 1.9 1.5 2.1 3.8 1.7 Y9

Verticullium rating (O-9) 1.8 2.1 1.3 1.6 2.8 1.7 ns

ing crop. This effect may be explained by the crop residues which were ploughed back on the soil surface after 2 years. Our results do not support the opinion of Gonet and Ploszynska (1987) that oil-seed rape does not show a yield decrease when grown continuously or in short rotations.

Since this study is to our knowledge the first report on the effects of different preceding crops on the yield components of oil-seed rape, we can only compare our results with experiments dealing with the effect of various husbandry factors on the yield components of oil-seed rape. Such experimental work has revealed that the number of pods per plant is generally regarded as the most variable yield component in oil-seed rape. In contrast, and this finding also supports other reports on the effect of management practices on the yield components of oil-seed rape, mean seed weight was not affected by the different preceding crops. The ability of the oil-seed rape crop to compensate for early damage due to pests and/or diseases or different treatments often complicates the inter- pretation of such experiments. However, the development and yield component data reported here support the view expressed by Tayo and Morgan

(1979), that stress around flowering has a severe effect on the seed yield of oil-seed rape, due to a restricted capacity for a later compensation.

Whether the differences in the two diseases assessed in this experiment are sufficient to explain the yield differences remains open to discussion. Rawlinson et al. (1984) investigated the effect of different fungicide treatments on the incidence of stem canker. In their experiments, a single application of benomyl decreased the incidence of stem canker and resulted in a yield increase of 0.63 t ha-‘. But since the fungicide application also affected other diseases, such as light leaf spot (Pyrenopezia bras- sica), it was not possible to relate the increase in seed yield solely to the decrease of stem canker.

Rawlinson et al. (1984) did not compare different preceding crops in their experiments. However, in one of their field trials chopped rape straw was scattered over all plots before emergence of oil- seed rape, whereas in a parallel trial the oil-seed rape was grown without inoculation. The seed yield of the controls which did not receive a fungicide treatment between the two trials differed by almost 1.0 t ha-‘. Though conclusions from these two experiments have to be drawn carefully,

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222 K. Siding et ul. I European Journal of Agronomy 6 i 1997) 215-223

since Rawlinson et al. (1984) used two different cultivars, this finding indicates that the incidence of stem canker and light leaf spot can have a substantial effect on the seed yield of oil-seed rape.

Tahvonen et al. ( 1984) observed a considerable increase with damping-off caused by Rhkoctonim soluni on spring turnip rape in short rotations compared with a long break in rotation. A detailed seed yield response, however, is not given in their study.

Results of this field experiment clearly indicate that seed yield and yield components of oil-seed rape show a response to the previous cropping.

Differences in the nitrogen nutrition of oil-seed rape seem not to be the reason for the yield differences due to the preceding crop combinations, especially the higher yield of oil-seed rape after a pea-wheat crop sequence. N supply in autumn (from fertilization or mineralization) does not or only slightly affects seed yield of oil-seed rape (Ogilvy and Bastiman, 1992). Under the climatic conditions of Schleswig-Holstein, mineralization of soil N normally starts at the end of April/beginning of May, when N uptake by oil- seed rape decreased, so that oil-seed rape appa- rently uses little mineralized soil N for its yield formation (Teebken and Sieling, 1995).

In general, our results have to be interpreted very carefully for various reasons. First, the two experimental years were unusually warm during the winter, i.e., winter-kill of oil-seed rape plants did not occur in this experiment. Since normally in most seasons frost damage and/or the complete loss of plants is a major factor in our environment, the effect of different preceding crops or crop combinations will be modified due to a lower number of plants in spring. And second, oil-seed rape plants show an extreme variability and hetero- geneity, making it absolutely necessary to increase the number of plants subjected to detailed yield component analysis in future experiments. We should add that further studies should include a greater number of preceding crops as well as differences in crop management. In future studies it is also important to assess carefully the incidence and severity of diseases during the entire crop development as well as other non-pathogenic

causes for yield effects in short oil-seed rape rotations.

Acknowledgment

We would like thank H. Jager for technical assistance. This project was funded by the Deutsche Forschungsgemeinschaft (German Research Foundation).

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