Einfluss der Bestandesarchitektur auf die Ertragsbildung im Winterraps (Brassica napus L.)

Abstract

In the present study, a three-factorial field trial, consisting of the factors cultivar, seed rate and growth regulating fungicides was conducted to analyze the relationship between crop architecture of a winter oilseed rape (WOSR) crop and yield. The aim was to identify the optimal crop architecture to achieve the maximum possible yield and to explore how it was affected by the three cropping factors. The field trial was carried out from 2010/11 to 2012/13 on experimental fields of the University of Göttingen. Only the data from 2010/11 and 2012/13 were used for the final statistical data analysis. Due to strong damage induced by winterkill and a subsequent massive attack by B. cinerea the experiment was abandoned in the second year in spring 2012. The field trial consisted of 32 treatments which were composed of two WOSR hybrid cultivars (2010/11: NK Petrol + SY Merlot; 2012/13: NK Petrol + SY Vesuvio), two seeding rates (30 seeds/m² or 60 seeds/m²) and eight growth-regulating fungicide treatments. The fungicide applications varied in frequency per season (autumn/spring), and were compared with an untreated control. In order to be able to describe and explain the crop architecture of WOSR and to assess their influence on the yield formation of WOSR, a lot of different crop parameters were examined including grain yield, straw biomass, oil content, oil yield, harvest index, plants/m², number of pods/plant, number of seeds/pod, thousand grain weight (TGW), number of side branches, crop height, pod set height, thickness of pod layer, photosynthetic active radiation (PAR) in the crop, culture coverage, green colour of the canopy during flowering and during ripening [Normalized Difference Vegetation Index (NDVI)] root crown diameter, number of leaves/plant, length of leaves and leaf area index (LAI). In addition, the occurrence of diseases and pests was assessed with special emphasis on blackleg (Leptosphaeria maculans/Leptosphaeria biglobosa, anamorph Phoma lingam) and cabbage root fly (Delia radicum). The data of the individual parameters were evaluated separately for each year of the trial. Using multiple regressions was the best method to determine the crop parameter, which had the greatest impact on grain yield. The first experimental year 2010/11 was characterized by a cold, wet autumn, which was followed by a cold and snowy winter. The main growth period of oilseed rape (March to May) was characterized by below-average rainfall (-98.8 mm) and slightly above average temperatures (+0.5°C). Sufficient rainfall in June prevented stronger yield reductions. The maturation took place under cool and dry conditions. The weather of the year 2012/13 was different from the weather of the year 2010/11 to the effect that the autumn was milder and drier, the winter was longer and drier, the main growth period of rape was much more wetter (+98.0 mm) and cooler ( 2,8°C) and the maturation period was warmer and drier. The year-specific weather conditions had a major impact on the plant density and the growth-regulating effect of the fungicides used. So more plants/m² were lost over winter in 2010/11 than in 2012/13. The growth-regulatory fungicides shortened the WOSR plants under the dry conditions in spring 2011 not as much as under the humid conditions in spring 2013. Moreover, the variable weather conditions were responsible for different average yield levels of WOSR. In 2010/11, the average yield level was around 4.47 t/ha and in 2012/13 it was around 5.58 t/ha. In 2010/11, the radiation absorption by the green plant canopy during the middle of pod development (BBCH 75) had the strongest positive influence on grain yield. How much PAR was absorbed by the plant canopy in BBCH 75 was primarily dependent on the plant density in spring in BBCH 16/18. The plant density influenced mainly the cultural coverage in BBCH 32/34 (65.5%), which rose with increasing plant density. Dense culture coverage resulted in the further development into a dense, green pod layer, which absorbed more PAR while only a small amount of unused radiation reached the ground. At the high seed rate of 60 seeds/m² plant density in BBCH 16/18, culture coverage in BBCH 32/34 and green colour of the plant canopy in BBCH 73 were significantly higher than at the low rate of 30 seeds/m². The proportion of PAR under the pods as well as on the ground in BBCH 75 at the high seed rate was significantly lower than at the low seed rate. The grain yield of SY Merlot was only significantly higher at the high sowing rate than at the low sowing rate. In case of NK Petrol, the grain yield even at the high seed rate was higher than at the low seed rate, but these differences were not significant. The fungicide treatments had little effects on the yield-relevant crop parameters. Due to a poorer crop emergence and higher plant losses over winter in SY Merlot, the two varieties SY Merlot and NK Petrol significantly influenced some crop parameters, such as the plant density in autumn in BBCH 14/15 as well as in spring in BBCH 16/18 and the culture coverage in BBCH 32/34. Overall, the seed rate in 2010/11 had the greatest impact on crop architecture and thus on yield formation. All tested yield factors had a significant impact on grain yield, whereas the number of plants/m² had the strongest effect. With rising plant density, the grain yield increased. The number of pods/plant, number of seeds/pod, the TGW and the number of side branches/plant were negatively correlated with grain yield, which was due to the negative correlation between these factors and plants/m². In 2010/11, phoma crown canker and phoma stem lesions (first 30 cm of the stem) were weakly negatively correlated with grain yield. Hereby, disease severity (DS) of phoma crown canker in BBCH 83/84 had the strongest negative correlation with grain yield (rp = 0.363). The weak negative relationship between root collar infestation with P. lingam and the green colour of the plant canopy in BBCH 73 (3.9%) explained how the grain yield was affected by the infestation. In 2012/13, there was no significant negative relationship between phoma crown canker or phoma stem lesions and grain yield, because the infection with P. lingam was significantly lower than in 2010/11. In 2010/11 the mean DS of blackleg was between 3 and 5 and in 2012/13 between 1 and 2. DS of phoma crown canker in BBCH 83/84 was on the one hand dependent on the plant density in spring in BBCH 16/18 and on the other hand on DS with phoma stem lesions. The plants at the low seed rate were more infested with P. lingam at the root collar than plants at the high seed rate, because at the low seed rate more plants were damaged by larvae of the cabbage root fly than at the high seed rate. In 2010/11, the infection with phoma stem lesions and phoma crown canker were significantly reduced by the fungicide variants in which the combination of the active ingredients of paclobutrazol and difenoconazole was used. For phoma crown canker, it was crucial that the mentioned substance combination was applied for the first time in autumn (var. 3, 4, 6 and 7). The common practice fungicide treatment, in which the combination of the active ingredients of mepiquat chloride and metconazole and the compound tebuconazole were used, did not significantly reduce the infestation with P. lingam. In the experimental year 2012/13, the pod set height in BBCH 75 had the strongest positive effect on grain yield (30.6%). In BBCH 75, the pod set height offered the strongest interaction with the canopy height in BBCH 65/67 (96.2%). In BBCH 65/67, the canopy height was primarily affected by the pod set height of the same growth stage (95.5%). Therefore in 2012/13, the growth of the crop and the growth-regulating fungicides had a substantial influence on yield formation. The more frequent the combination of the active ingredients of paclobutrazol and difenoconazole was applied, the stronger the pod set height in BBCH 75 was shortened. In case of the canopy height in BBCH 65/67, this effect could be observed in a similar manner. The stronger the pod set height was shortened, the stronger the straw biomass yield was reduced. This reduction was significant in variant 4, in which the substance combination of paclobutrazol and difenoconazole was used twice in autumn and once in spring, and in variant 6, wherein the same combination of active ingredients was used twice in autumn and twice in spring. This significant reduction of the stem tissue induced by the fungicide treatments also significantly decreased the grain yield. The significant height differences between NK Petrol and SY Vesuvio had no influence on the straw biomass yield or grain yield. In 2012/13, the seed rate did not present a yield limiting factor. Thus, in both seed densities similar grain yields were obtained. In summary, in 2012/13 the growth-regulating fungicide treatments had the greatest impact on the crop architecture and thus on the yield formation. In 2012/13, the side branches/plant, the plant density in spring and the TGW were the only yield factors which had a significant effect on grain yield. Hereby, the side branches/plant had the strongest influence, whereas the grain yield decreased with increasing number of side branches. The plant density and the TGW influenced the grain yield positively. Due to very low coefficients of determination (max. 5%), these relationships were not very distinct. This meant that the yield factors played a minor role in the yield formation of WOSR. The optimum ratio between the plant density in spring and the number of side branches/plant in 2010/11 was approximately 40 plants/m² and circa 8 side branches/plant and in 2012/13 approximately 45 plants/m² and circa 7 side branches/plant. In general it was found that with an increase in the seed rate by a factor of two, on the average, two side branches/plant were less developed. In both years, most of the growth-regulating fungicide treatments did not increase the number of side branches/plant. Therefore, the formation of side branches of individual plants could not be enhanced by growth-regulating fungicides. Overall, it can be noted that, depending on annual weather conditions, the seed rate, partly in combination with the variety, or the growth-regulating fungicide treatments have affected the crop architecture of WOSR and therefore these production factors were important in yield formation of WOSR. Hence, especially by the seed rate or by the growth-regulating fungicides, cultivation tools are available to the farmers to specifically manipulate the crop architecture of WOSR. According to the results, the seed rate should be about 40 to 50 seeds/m² in order to compensate a crop failure in adverse weather conditions. To avoid yield reductions due to inhibition of growth, growth-regulating fungicides should not be used across-the-board and in excess, but according to the rules of integrated pest management.