Yield of bolting winter beet (Beta vulgaris L.) as affected by plant density, genotype and environment

Europ. J. Agronomy 54 (2014) 1–8

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Yield of bolting winter beet (Beta vulgaris L.) as affected by plant density, genotype and environment E. Reinsdorf, H.-J. Koch ∗ , J. Loel, C.M. Hoffmann IfZ, Institute of Sugar Beet Research, Germany

a r t i c l e

i n f o

Article history: Received 12 August 2013 Received in revised form 18 November 2013 Accepted 24 November 2013 Keywords: Sugar beet Biomass yield Pruning Energy crop Taproot diameter

a b s t r a c t Winter beet roots and shoots might be a favorable substrate for biogas production in Central Europe. However, detailed information about the attainable yield of this crop is lacking. Thus, the impact of plant density, genotype and environmental conditions on total dry matter yield of winter beet crops that bolt after winter was investigated. A significant increase of the dry matter yield (esp. shoot) was expected by harvesting the 1st shoot after flowering in June followed by a final harvest of the whole plant in July. In 2009/10, 2010/11 and 2011/12, three series of field trials with (i) 3 target plant densities (148, 246, 370 thousand plants ha−1 ) and (ii) 3 different sugar beet genotypes were conducted at Göttingen (Lower Saxony, GER) and Kiel (Schleswig-Holstein, GER); (iii), additional field trials with 5 different sugar beet genotypes cultivated at 2 target plant densities (148, 246 thousand plants ha−1 ) were conducted in 2011/12, to investigate the relation between maximum taproot diameter and the shoot and taproot yield of bolting winter beet. The total dry matter yield considerably varied between 4 and 23 t ha−1 . It was predominantly affected by the environment and to a substantially lower extent by plant density. Increasing plant densities increased the total dry matter yield, resulting in a significantly higher total dry matter yield at plant densities ≥300,000 plants ha−1 compared with lower plant densities. Genotypic differences in total dry matter yield were negligibly small. Pruning in June substantially increased the total dry matter yield in July by ca. 8 t ha−1 only in one out of three environments. Final yield in June (without pruning) and July (pruning in June) was positively related with cumulated temperature and global radiation, but also with taproot dry matter yield before winter. The taproot, shoot (1st, 2nd) and total plant yield were positively correlated with maximum taproot diameter. In conclusion, high dry matter yields close to yields of established energy crops grown over winter were obtained with winter beet roots and shoots only under very favorable conditions (climate, single plant size). High yields can be achieved after good pre-winter development. However, for sufficient frost tolerance the taproot size of plants must be rather small. Hence, the cultivation of bolting winter beet under Central European climate conditions has to face a severe conflict of goals concerning winter survival and yield formation. © 2013 Elsevier B.V. All rights reserved.

1. Introduction In Central Europe sugar beet (Beta vulgaris L. subsp. vulgaris; Lange et al., 1999) is usually sown in spring (March to April) and harvested in autumn (September to November). Alternatively, in Mediterranean countries (e.g. South Spain, Morocco, Egypt), and in the USA (Imperial Valley), sugar beet is cultivated as a winter crop because of favorable growing conditions over winter followed by high temperature during summer. Winter sugar beet is sown in autumn and harvested in the summer of the following year.

∗ Corresponding author at: Institute of Sugar Beet Research, Holtenser Landstraße 77, 37079 Göttingen, Germany. Tel.: +49 551 50562 50; fax: +49 551 50562 99. E-mail addresses: [email protected], [email protected] (H.-J. Koch). 1161-0301/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.eja.2013.11.011

The aim of growing winter sugar beet under Central European climate conditions is to achieve a substantial yield increase by better synchronization of leaf development and irradiance (Hoffmann & Kluge-Severin, 2010), and to realize an early harvest date, allowing for a significant extension of the processing campaign of sugar factories (Kluge-Severin et al., 2009). Today, bolting is still a major constraint for the cultivation of winter sugar beet. Sugar beet is a biennial plant and the change from the vegetative to the floral stage (bolting) is induced by cold conditioning (vernalization) followed by long day conditions (photoperiod; Milford & Limb, 2008; Milford et al., 2010). Currently, non-bolting cultivars are not available and their registration and release is not expected in the short term. Under Central European climate conditions 100% of bolters are likely to occur (Hoffmann & Kluge-Severin, 2011). For sugar production, bolting must be avoided because of a serious reduction of taproot yield (Jaggard

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et al., 1983), and poor processing quality due to a lower sugar and a higher marc content (Hoffmann & Kluge-Severin, 2011). Consequently, bolting winter sugar beets are not suitable for sugar production. However, winter beet roots and shoots might be used as substrate for anaerobic digestion. Energy crops for biogas production should produce high biomass yields (Amon et al., 2007). Springsown sugar beet is one of the crops with the highest biomass and net energy yield in Europe (Hoffmann & Stockfisch, 2010), additionally offering excellent properties as substrate for biogas production (Starke & Hoffmann, 2011). Winter beet might also provide a high biomass yield. Including bolting winter beet could amend energy crop rotations by a crop species, which is completely different from common energy crops such as maize and cereals with regard to pests and diseases. Bolting winter beets could be harvested as whole plant in early summer and would ensure a continuous supply of sugar beet as substrate for anaerobic digestion. It is well known that autumn-sown bolting winter beet produces a high yield of shoot dry matter, but only little taproot dry matter, and that yield formation is closely related to the cumulated temperature (Hoffmann & Kluge-Severin, 2011). However, an essential prerequisite for successful winter beet cultivation is an adequate frost tolerance of the plant. The survival of B. vulgaris L. highly depends on climate conditions (Kirchhoff et al., 2012; Pohl-Orf et al., 1999; Reinsdorf & Koch, 2013), genetics (Kirchhoff et al., 2012) and phenotype, in particular the maximum taproot diameter, with an optimum of ca. 1–2.5 cm for highest frost tolerance (Kockelmann & Meyer, 2006; Reinsdorf et al., 2013; Senff, 1958, 1961). Among others, the taproot diameter is influenced by sowing date and plant density of the crop. The optimal taproot size of 1–2.5 cm before winter was achieved with target plant densities between 148,000 and 370,000 plants ha−1 when plants had been sown in August (Reinsdorf et al., 2013). It is not known, however, whether such small sized taproots, that offer maximum frost tolerance, also provide the highest yield in the following summer compared to bigger sized plants. The yield of bolting winter beet might be enhanced by increasing plant densities because of the increasing total number of plants. In contrast, yield might remain unchanged or even decrease due to intra-specific competition, decreasing single plant weight. Moreover, genotype and environmental conditions are likely to have a substantial effect on taproot and shoot yield of bolting winter beet, but the effect of such factors has not yet been investigated. Finally, it has not been studied up to date, whether the yield of bolting winter beet is increased by additional pruning and harvesting the 1st shoot after flowering, and a 2nd harvest of the whole plant consisting of the taproot and the 2nd shoot. A significant increase of the total shoot dry matter yield due to re-growth of the shoot after pruning was expected. The aim of our basic research study was thus to investigate (i) the impact of plant density (i.e. no. of plants and single plant size), genotype and environmental conditions and (ii) the effect of pruning and harvesting the 1st shoot in June, and conducting a 2nd harvest of the taproot and 2nd shoot in July, on total dry matter yield of bolting winter beet.

(54◦ 18 N, 9◦ 58 E). At Göttingen, the soil had a loamy texture (Luvisol); at Kiel, the soil was a sandy to clayey loam (Cambisol, Luvisol). The experiments were carried out as randomized block designs with 4 replicates. Plants were sown in August as pelleted seeds with a drilling machine in 6 row plots of 8 m length with a row width of 0.45 m. Three different series of field trials were conducted: (i) plant density trials, (ii) genotype trials and (iii) genotype x plant density trials (only 2011/12). In all experiments two or three harvest dates were included as the third factor. (i) The plant density field trials included the three target plant densities 148,000, 246,000 and 370,000 plants ha−1 . In the 2- to 4-leaf stage, the emerged plants (cultivar BSA No.: 1798) were manually thinned from denser stands (seed distance 3 cm) to the target plant densities. (ii) The genotype field trials consisted of 3 different sugar beet hybrids (Strube GmbH & Co. KG, Söllingen, GER), referred to as genotype 1, 2 and 3. In the 4- to 6-leaf stage, the emerged plants were manually thinned (seed distance 8 cm) to a plant density of 96,000 plants ha−1 . (iii) The genotype × plant density field trial included 5 sugar beet hybrids (Strube GmbH & Co. KG, Söllingen, GER) and the 2 target plant densities 148,000 and 246,000 plants ha−1 . Adjustment to the target plant densities was conducted as described for (i). In the Göttingen 2010/11 and 2011/12 experiments, all plots were covered with chopped wheat straw for frost protection. Straw was spread in late autumn before severe frost occurred to a ca. 10 cm thick layer. The genotype trial at Kiel 2011/12 was covered with fleece for frost protection. Plants were protected against frost to ensure winter survival and crop growth in the next year even at severe frost occurrence during winter, which had killed all sugar beet plants in several winter beet experiments conducted by Kirchhoff et al. (2012) and Reinsdorf & Koch (2013). At the beginning of the growing period after winter, straw and fleece were manually removed from the plots. Plant protection was carried out according to the regional standards of best professional practice to keep the crop free of weeds, pests and diseases. The winter beet crop was supplied with 60–100 kg N ha−1 after sowing in August, taking into account the soil mineral N, and 100 kg N ha−1 in spring, split in two applications of each 50 kg N ha−1 in April and May. Nitrogen fertilization was adapted from the standard practice for fertilizer application in transplanted sugar beet seed crops in France (Kockelmann & Meyer, 2006); the fertilization after sowing corresponded to the fertilization at transplantation. The monthly mean air temperature, cumulated global radiation and precipitation for Göttingen and Kiel are given in Fig. 1. All weather data were recorded near the trial fields. Cumulated daily mean temperature was calculated summing up the daily mean air temperature at 2 m above a base temperature of 3 ◦ C; daily mean temperatures below 3 ◦ C were set zero (Milford et al., 1985). 2.2. Harvest and plant analyses

2. Materials and methods 2.1. Field trials and site conditions In 2009/10, 2010/11 and 2011/12 field trials were conducted at Göttingen (Institute of Sugar Beet Research), Lower Saxony, GER (51◦ 28 N, 9◦ 54 E), and at the research farm Hohenschulen (Christian-Albrechts-University of Kiel), Schleswig-Holstein, GER

Two harvests were carried out in each experiment; one before and one after winter. The plants of three center rows per plot (10.8 m2 ) were manually harvested. In November, the young sugar beet plants were harvested for analyzing the pre-winter development. At the second half of June the bolted winter beets (100% bolting rate) were harvested at BBCH 69 to 71 (Meier et al., 1993), to determine the final dry matter yield. At each harvest, the total no. of plants was counted. The leaves including the shoot down to the oldest green petioles (crown), here

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Fig. 1. Monthly mean air temperature, cumulated global radiation and precipitation from August 2009 to July 2012 and long-term average from 1996 to 2011 at Göttingen (left) and Kiel (right).

defined as shoots, were separated from the taproots with kitchen knives. The fresh matter of the shoots and clean (washed) taproots was weighed. Shoot and taproot samples were mashed to a homogenous brei and dried at 105 ◦ C to constant weight for determination of the dry matter concentration and calculation of dry matter yield. In the trial series plant density (Göttingen 2010/11, 2011/12; Kiel 2011/12) and genotype x plant density (Göttingen, Kiel 2011/12), an additional harvest was conducted, where the 1st shoot was manually pruned with a hedge trimmer at ca. 20 cm above soil level in June. The 2nd shoot and taproot were harvested in July; pruning and final harvest were both conducted at BBCH 69 to 71 (Meier et al., 1993). Harvest and plant analyses of 1st, 2nd shoot and taproot were done as described above. At the harvests (June, July) of the plant density trials 2010/11, 2011/12 and the genotype × plant density trial 2011/12, a random sample of 25 (2010/11 trials) or 12 plants plot−1 (2011/12 trials) was taken from the harvest-rows. For each plant, fresh matter of shoot and (washed) taproot was determined and the maximum taproot diameter was measured using a caliper. The shoot and taproot fresh weights of these 12 or 25 plants were included in the total fresh and dry matter yield on plot level. 2.3. Statistical analyses All statistical analyses were carried out using SAS 9.3 (SAS Institute Inc., Cary, US-NC). Gaussian distribution of residuals was

tested, using Shapiro–Wilk-test at ˛ = 0.05 (SAS Univariate procedure). Analyses of variance were performed with the Mixed procedure. The different trial series (plant density, genotype) were analyzed separately. The effect of replication was assumed as random in the models. The effects of environment (site × year), target plant density, genotype and harvest date were assumed as fixed, and regarded significant at p < 0.05 (F-test). For separation of treatment means at ˛ = 0.05, Tukey’s t-test was applied. Pearson’s correlation analyses were carried out using the SAS Corr procedure with default settings.

3. Results 3.1. Plant density at harvest Plant density in June was significantly affected by the environment (site × year; Fig. 2). At Göttingen 2009/10 (all field trials) and Kiel 2010/11 (genotype trial), the entire crop stands were killed by frost. Genotype had no influence on the plant density measured in June, while it significantly different between the three target plant density treatments. Average plant densities were 22,000 (15%), 54,000 (22%) and 73,000 (20%) plants ha−1 lower than the target plant densities of 148,000, 246,000 and 370,000 plants ha−1 , respectively. No significant interactions between target plant density or genotype, and environment occurred (Fig. 2).

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Fig. 2. Plant density of bolting winter beet in June as affected by target plant density and genotype. Frost killing, total plant loss due to frost. Env, environment (site × year). Gö 09, Gö 10, Gö 11: Göttingen 2009/10, 2010/11, 2011/12. Ki 09, Ki 10, Ki 11: Kiel 2009/10, 2010/11, 2011/12. PDT , target plant density (1000 plants ha−1 ). GT, genotype. Error bars indicate standard deviations of treatment means. Different letters indicate significant differences between environments; p < 0.05 (Tukey’s t-test). Plant density: All PDT are significantly different from each other. Genotype: no significant differences between genotypes. n.s., not significant. *** p < 0.001.

3.2. Weather conditions The cumulated temperature and cumulated global radiation from sowing to harvest considerably differed among sites and years (Table 1). For the plant density trials temperature and radiation were highest at Göttingen 2010/11 and lowest at Kiel 2010/11, and for the genotype trials they were highest at Göttingen 2011/12 and lowest at Kiel 2011/12. On average, the cumulated precipitation at Kiel was higher than at Göttingen, with its maximum in 2011/12.

Table 2 Pearson’s correlation coefficients for the relation between cumulated daily mean temperature (base temperature 3 ◦ C), cumulated global radiation and cumulated precipitation from sowing to harvest in June and total, shoot and root dry matter yield of bolting winter beet harvested in June. Plant density trials: 5 environments (Göttingen 2010/11, 2011/12 and Kiel 2009/10, 2010/11, 2011/12). Target plant density 148,000 plants ha−1 . Genotype trials: 3 environments (Göttingen 2010/11 and Kiel 2009/10, 2011/12). 3 genotypes (1, 2, 3). n = 57. Dry matter yield (t ha−1 )

Temperature (◦ Cd) Radiation (MJ m−2 ) Precipitation (mm)

3.3. Dry matter yield On average of all environments, the shoot of bolting winter beet harvested in June accounted for 70–80% (7.7 t ha−1 ) of the total dry matter yield. Average taproot yield was 2.5 t ha−1 (Fig. 3). The total dry matter yield (4–16 t ha−1 ) of bolting winter beet harvested in June was highly influenced by the environment (Fig. 3). With increasing plant density the total dry matter yield increased, resulting in significantly higher yield at the highest target plant density of 370,000 plants ha−1 compared with target plant densities 148,000 and 246,000 plants ha−1 (Fig. 3). Genotypic differences and a significant genotype by environment interaction in total dry matter yield were observed (Fig. 3). On average, genotype 1 yielded significantly lower than genotypes 2 and 3.

Total

Shoot

Taproot

0.83*** 0.84*** −0.1 n.s.

0.77*** 0.77*** 0.01 n.s.

0.82*** 0.83*** −0.31*

n.s., not significant. * p < 0.05. *** p < 0.001.

At Göttingen 2010/11 the total dry matter yield was significantly increased from 14.5 to 23 t ha−1 by harvesting the 1st shoot in June plus final harvest of the whole plant (including the taproot and the 2nd shoot) in July (Fig. 4). In contrast, at Göttingen and Kiel 2011/12, pruning of the 1st shoot had no influence on total dry matter yield. The yield (total, shoot, taproot) of bolting winter beets harvested in June (Table 2) and July (Table 3), was positively correlated

Table 1 Cumulated daily mean air temperature (base temperature 3 ◦ C), cumulated global radiation and cumulated precipitation from sowing (S) to June-harvest (HJun ), July-harvest (HJul ) and from pruning in June (Pr) to harvest in July. Trial series

Period

Plant density Plant density Plant density Genotype

S–HJun S–HJul Pr–HJul S–HJun

Plant density Plant density Plant density Genotype

Plant density Plant density Plant density Genotype

Gö 09/10

Ki 09/10

Gö 10/11

Ki 10/11

Gö 11/12

Ki 11/12

Cumulated temperature>3 (◦ Cd) – 1338 – – – – – 1334

1654 2118 463 1588

1314 – – –

1376 1800 424 1811

1479 1927 448 1282

S–HJun S–HJul Pr–HJul S–HJun

Cumulated global radiation (MJ m−2 ) – 2484 – – – – – 2488

2945 3514 569 2819

2430 – – –

2484 3028 544 2941

2624 3218 594 2419

S–HJun S–HJul Pr–HJul S–HJun

Cumulated precipitation (mm) – 520 – – – – – 516

489 582 93 488

520 – – –

475 589 114 537

749 909 160 621

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Fig. 3. Dry matter yield of bolting winter beet in June as affected by target plant density and genotype. Frost killing, total plant loss due to frost. Env, environment (site × year). Gö 09, Gö 10, Gö 11: Göttingen 2009/10, 2010/11, 2011/12. Ki 09, Ki 10, Ki 11: Kiel 2009/10, 2020/11, 2011/12. PDT , target plant density (1000 plants ha−1 ). GT, genotype. Different letters indicate significant differences between environments; p < 0.05 (Tukey’s t-test). Plant density: PDT 370 is significantly different from PDT s 148 and 246. Genotype: GT 1 is significantly different from GTs 2 and 3. n.s., not significant. * p < 0.05, ** p < 0.01, *** p < 0.001.

Fig. 4. Total dry matter yield of bolting winter beet in June, and in July after pruning of the 1st shoot in June. Crop density trials: 3 environments (Göttingen 2010/11, 2011/12, Kiel 2011/12). Mean of 3 target plant densities: 148, 246 and 370 (1000 plants ha−1 ). Env, environment (site × year). Gö 10, Gö 11: Göttingen 2010/11, 2011/12. Ki 11, Kiel 2011/12. Different letters indicate significant differences between treatment means; p < 0.05 (Tukey’s t-test). *** p < 0.001.

The dry matter yield of the 2nd shoot correlated positively (r = 0.74***) with cumulated temperature and negatively (r = −0.73***) with cumulated precipitation from June to July (Table 3iv). The formation of the 2nd shoot was not correlated with cumulated global radiation from June to July. Fresh matter yield of single plants (total, 1st shoot, taproot) harvested in June increased with increasing maximum taproot diameter (Table 4). The second shoot, taproot and total plant fresh matter yield of winter beets harvested in July were also positively correlated with maximum taproot diameter (Table 4). The correlations between maximum taproot diameter and taproot fresh matter versus shoot and total fresh matter plant−1 were closer when harvested in July after pruning compared to the winter beets harvested in June without pruning (Table 4). Total dry matter yield of bolting winter beet in June and July was positively correlated with taproot dry matter yield in November, showing correlation coefficients of r = 0.90*** for the yield data of the genotype trials, and r = 0.74*** and 0.90*** for the June and July harvested winter beets of the plant density trials excluding the environments Kiel 2009/10 and 2010/11, respectively (Table 5). However, when correlating total dry matter yield of pre-winter and June harvests of plant density trials including the environments Kiel 2009/10 and 2010/11, the correlation coefficient was only r = 0.41**.

(r = 0.77***–0.92***) with cumulated temperature and global radiation from sowing to harvest. Dry matter yield was either negatively correlated with the cumulated precipitation or showed no relation (Tables 2 and 3).

3.4. Dry matter concentration

Table 3 Pearson’s correlation coefficients for the relation between cumulated daily mean temperature (base temperature 3 ◦ C), cumulated global radiation and cumulated precipitation from sowing to harvest in July and (i) total, (ii) shoot, (iii) root dry matter yield of bolting winter beet in July after pruning of 1st shoot in June. (iv) Cumulated daily mean temperature (base temperature 3 ◦ C), cumulated global radiation and cumulated precipitation from pruning to harvest versus dry matter yield of the 2nd shoot. Plant density trials: 3 environments (Göttingen 2010/11, 2011/12 and Kiel 2011/12). 3 target plant densities: 148, 246, 370 (1000 plants ha−1 ). n = 28.

Table 4 Pearson’s correlation coefficients for the relation between maximum taproot diameter (MTD), taproot, shoot and total fresh matter yield per plant of bolting winter beet in June and in July. Plant density and genotype × plant density trials: [June] 4 environments (Göttingen 2010/11, 2011/12 and Kiel 2010/11, 2011/12). [July] 3 environments (Göttingen 2010/11, 2011/12 and Kiel 2011/12). 3 target plant densities: 148, 246, 370 (1000 plants ha−1 ). n = 1268 [June], n = 746 [July].

Dry matter yield (t ha−1 )

Temperature (◦ Cd) Radiation (MJ m−2 ) Precipitation (mm) n.s., not significant. *** p < 0.001.

(i) Total

(ii) Shoot (1st + 2nd)

(iii) Taproot

(iv) 2nd shoot

0.87*** 0.88*** −0.74***

0.83*** 0.84*** −0.77***

0.91*** 0.92*** −0.66***

0.74*** −0.18 n.s. −0.73***

The dry matter concentration of shoot and taproot significantly differed among the environments (15–19% 1st shoot harvested in

[June]

Taproot

1st shoot

Total

MTD Taproot 1st shoot

0.92***

0.67*** 0.65***

0.80*** 0.80*** 0.97***

[July]

Taproot

2nd shoot

Total

MTD Taproot 2nd shoot

0.92***

0.77*** 0.83***

0.87*** 0.93*** 0.97***

***

p < 0.001.

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Fig. 5. Dry matter concentration of shoot and taproot of bolting winter beet in June, and in July after pruning of the 1st shoot in June. Crop density trials: 3 environments (Göttingen 2010/11, 2011/12, Kiel 2011/12). Mean of 3 target plant densities: 148, 246, and 370 (1000 plants ha−1 ). Env, environment (site × year). Gö 10, Gö 11: Göttingen 2010/11, 2011/12. Ki 11, Kiel 2011/12. H, Harvest date. Error bars indicate standard deviations of treatment means. Different letters indicate significant differences between treatment means; p < 0.05 (Tukey’s t-test). n.s.: not significant. * p < 0.05, ** p < 0.01, *** p < 0.001.

Table 5 Pearson’s correlation coefficients for the relation between taproot dry matter yield of winter beet in November (HNov ) and total dry matter yield in June (HJun ) and in July after pruning of the 1st shoot in June (HJul ). Plant density trials: 5 environments (Göttingen 2010/11, 2011/12 and Kiel 2009/10, 2010/11, 2011/12). 3 target plant densities: 148, 246, 370 (1000 plants ha−1 ). Genotype trials: 4 environments (Göttingen 2010/11, 2011/12 and Kiel 2009/10, 2011/12). 3 genotypes (1, 2, 3). Trial series

n

Plant density

50 (29)a 36

Genotype

Taproot HNov Taproot HNov

Total HJun

Total HJul

0.41** (0.74*** )a 0.90***

(0.90*** )a –

a 3 environments, incl. June and July harvest (Göttingen 2010/11, 2011/12 and Kiel 2011/12). ** p < 0.01. *** p < 0.001.

June, 18–21% 2nd shoot harvested in July, 18–22% taproot; Fig. 5). While dry matter concentration of the 2nd shoot was 1.5–3.3% higher compared to the 1st shoot, the dry matter concentration of taproots was not affected by harvest date in the field trials 2011/12, but increased by 1.5% from June to July at Göttingen 2010/11. 4. Discussion 4.1. Survival/frost protection In the field trials Göttingen 2009/10 and Kiel 2010/11 (genotype) the winter beet crop was entirely killed by frost, from which the crop of the other experiments was protected by covering with chopped wheat straw or fleece. Without artificial protection, or a solid snow cover as at Kiel 2009/10 and 2011/12, the crop probably would have been lost in all experiments (Reinsdorf & Koch, 2013; Reinsdorf et al., 2013). However, artificial covering of the crop can only be an adequate measure to enhance survival in research work. In commercial winter beet cultivation, any artificial covering of the crop will not be practicable, because of the extraordinary high costs and a considerable amount of work for application. However, despite a high risk of frost killing in large parts of Central Europe, high survival rates of winter beet can be achieved in regions with a mild winter climate or a high probability of a solid snow cover during frost periods, as has been confirmed by Pohl-Orf et al. (1999), Kirchhoff et al. (2012) and Reinsdorf & Koch (2013). Besides environmental conditions, the plant density and the maximum taproot diameter have significant influence on the survival rate of winter beet. Survival rates were observed to increase with increasing plant densities, which was due to a decrease in taproot diameter

(Reinsdorf et al., 2013). The frost tolerance of sugar beet is highest at maximum taproot diameters of 1–2.5 cm (Kockelmann & Meyer, 2006); Senff, 1958). 4.2. Total dry matter yield The total dry matter yield of bolting winter beet showed high variation, ranging from 4 to 16 t ha−1 without pruning and 10 to 23 t ha−1 after pruning of the 1st shoot. The plants formed a high shoot yield, but only little taproot dry matter, which was also observed by Hoffmann & Kluge-Severin (2011). Compared with common energy crops such as whole crop winter wheat and barley, which are sown in autumn and harvested in the following summer like bolting winter beet, the maximum yield of 16 t ha−1 (without pruning) was at a similar level (Eder & Krieg, 2012). The top yield of 23 t ha−1 , which was obtained only at one out of three environments (Göttingen 2010/11) after pruning and final harvest in July, is even competitive with the total dry matter yield of maize (spring crop), which is the most important and high yielding energy crop (Eder & Krieg, 2012). Hence, under favorable conditions, a high dry matter yield can be achieved with bolting winter beet, but high yields are less secure compared with other energy crops. The total dry matter yield in June and July was positively affected by taproot dry matter yield before winter. The correlation was closest when data from Kiel 2009/10 and 2010/11 were excluded. At these environments, growth after winter was probably limited because of bare frosts, which reversibly damaged the leaf buds, cool temperatures at the time of sprouting in April and bolting in May, and drought in spring. Consistently, Senff (1961) observed increasing seed yield of direct sown winter sugar beet with increasing pre-winter development due to earlier sowing dates. In oilseed rape, Mendham et al. (1981) found lower seed yield in late sown crops with little growth before winter compared to early sown crops that overwintered with a large leaf area. 4.3. Environment, genotype and plant density The total dry matter yield of bolting winter beet was influenced by plant density, genotype and the environment, from which the environment caused the greatest variation in dry matter yield of both taproot and shoot. Environmental differences were due to differences in cumulated temperature, global radiation and precipitation. The dry matter yield increased with increasing cumulated temperature and global radiation, but decreased with increasing cumulated precipitation. A positive relation between cumulated

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temperature as well as absorbed solar radiation and dry matter formation of autumn-sown sugar beet was also described by Hoffmann & Kluge-Severin (2010, 2011). Such correlations are very likely, since most metabolic reactions such as photosynthesis, being a prerequisite for biomass formation, depend on temperature (Hew et al., 1969; Taiz & Zeiger, 2006). Increasing precipitation most likely retarded growth because of lower radiation and temperature during rainfall. Stockfisch et al. (2002) also found decreasing taproot growth rates of spring-sown sugar beet with increasing precipitation, but no directed correlation between cumulated precipitation and taproot dry matter yield was detected. Similarly, rainfall and climatic water balance did not adequately describe the growth of spring-sown sugar beet (Kenter et al., 2006). In our study, cumulated precipitation from sowing to harvest was between 489 and 749 mm. However, total water consumption of spring-sown sugar beet in temperate climates only amounts to 385–492 mm (Morillo-Velarde & Ober, 2006). Provided that the total water consumption of autumn-sown winter beet is similar, water most likely was not the limiting factor for the crop in this study. However, very low precipitation in spring 2011 (March to May) might have restricted yield formation at Kiel, as well as temporary water logging, caused by extensive rainfall in August 2011. Genotypic differences in total dry matter yield were negligibly small compared to the environmental effects, which was also observed for the yield and quality of spring-sown sugar beet taproots (Hoffmann & Märländer, 2002; Hoffmann et al., 2009). In June, the total dry matter yield of winter beets of the plant density and the genotype trials grown at the same environment corresponded well, when plant density and cumulated temperature and global radiation were similar. The observed genotypic differences in total dry matter yield were mainly caused by different shoot dry matter yields among genotypes (Göttingen 2010/11, 2011/12), which also caused a significant genotype by environment interaction. Differences among genotypes might be due to different plant densities or a different vitality of the genotypes after winter. However, analyses of non-killing frost events on plant vitality are missing and the mechanisms of frost tolerance in sugar beet are widely unknown at present. In addition to environmental and genotypic factors, plant density influenced the dry matter yield of bolting winter beet. Although plant densities in June were below the initial target plant densities in most field trials, all actual plant densities were significantly different from each other. Deviations from target plant densities were partly due to frost killing during winter, and partly due to a reduced number of plants row−1 already after sowing. At plant densities below 80,000 plants ha−1 , total dry matter yield of spring-sown sugar beet crops declines with decreasing plant density (Märländer, 1990). Thus, total dry matter yield in the genotype trial Göttingen was lower in 2011/12 than in 2010/11, despite a higher cumulated temperature in 2011/12. However, a positive effect of increasing plant density was also observed at values considerably higher than 100,000 plants ha−1 . The total dry matter yield was significantly higher at target plant density 370,000 plants ha−1 compared to the target plant densities 148,000 and 246,000 plants ha−1 . Obviously, the impact of the total number of plants on yield was larger than the impact of single plant weights, which decreased at increasing plant densities and thus, decreasing taproot diameters; this effect was observed across different genotypes grown at different environments and plant densities. Nevertheless, extraordinary high plant densities ≥300,000 plants ha−1 will not be relevant for the cultivation of commercial crops, because of very high seeding costs combined with a comparatively low extra yield. On the contrary, a target plant density of 96,000 plants ha−1 (genotype trials), which meets the standard at spring-sown sugar beet crops, seems to be too low because of a higher risk of

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an insufficient crop stand after winter as observed at Göttingen 2011/12. Hence, a target plant density between 148,000 and 246,000 plants ha−1 should be aimed when cultivating sugar beet as a winter crop. 4.4. Pruning While pruning significantly increased the total dry matter yield at Göttingen 2010/11, no effect was observed in 2011/12. These differences can partly be explained by differences in plant size/taproot diameter. Plants with bigger taproots were probably better capable to form higher amounts of shoot biomass because of higher amounts of stored carbohydrates (sucrose), that were translocated from the taproot to the growing shoot during re-growth of the bolters. In addition to plant size, (i) a substantial increase in taproot dry matter concentration from pruning in June until July only in 2010/11, (ii) a lower cumulated precipitation, esp. from pruning until harvest, and (iii) a higher cumulated temperature in 2010/11 might have contributed to the differences in yield formation observed between the 2010/11 and the 2011/12 field trials. Environmental conditions were also found to affect plant recovery and grain-yield formation in barley and triticale after pruning for forage production (Royo, 1999). However, while dry matter yield of the 2nd shoot was positively related with temperature and negatively with precipitation from pruning to harvest, radiation had no effect on the yield of the 2nd shoot, indicating that radiation has not limited growth during the summer months. Kenter et al. (2006) did not observe an effect of solar radiation during summer on growth rates of sugar beet either. Furthermore, yield formation of the 2nd shoot might depend on other factors, such as the pruning height and the physiological plant developmental stage at the time of pruning, known to affect yield formation of forage grasses (Dovel, 1996; Willms, 1991; Woodis & Jackson, 2008) and cereals (Royo, 1999). However, in our study the pruning heights were always the same and pruning was made at very similar growth stages. To conclude, the total dry matter yield of bolting winter beet was significantly increased by pruning, but only under favorable conditions (single plant size, weather conditions). More research is needed to clearly identify the important factors controlling the gain in total dry matter yield by pruning the 1st shoot before final harvest. 5. Conclusions The total dry matter yield of bolting winter beet was predominantly influenced by the environment, but also by plant density and genotype, and revealed high variation. Under favorable conditions, high dry matter yields that are similar to yields of established energy crops grown in the same period of the growing season (winter cereals) were attained with bolting winter beet. However, to obtain the highest possible frost tolerance in winter beet, the taproot size of single plants before winter should be preferably small. This can be ensured by high plant densities or late sowing dates. Conversely, the pre-winter development of single plants has to be preferably high to enhance yield formation after winter, especially of bolting winter beets including pruning of the 1st shoot. Today, the cultivation of bolting winter beet under Central European climate conditions has to face severe constraints concerning winter survival and yield. Regarding the total dry matter yield per unit acreage, bolting winter beet presently seem not to be a convincing alternative for the well-established cropping systems of energy crops such as winter cereal whole plant silage or springsown maize.

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Acknowledgements This project was funded by the Federal Ministry of Education and Research under grant no.: 0315465A. The excellent technical assistance in the field trials by the staff of the Institute of Sugar Beet Research (Göttingen) and the research farm Hohenschulen of Christian-Albrechts-University of Kiel is gratefully acknowledged. We also express our thanks to Strube Research GmbH & Co. KG for the selection of different sugar beet hybrids and their free provision as pelleted seeds for our field trials. References Amon, T., Amon, B., Kryvoruchko, V., Zollitsch, W., Mayer, K., Gruber, L., 2007. Biogas production from maize and dairy cattle manure – influence of biomass composition on the methane yield. Agric. Ecosyst. Environ. 118, 173–182. Dovel, R.L., 1996. Cutting height effects on wetland meadow forage yield and quality. J. Range Manage. 49, 151–156. Eder, B., Krieg, A., 2012. Substrate. In: Eder, B. (Ed.), Biogas-Praxis. ökobuch Verlag, Staufen bei Freiburg, pp. 47–84. Hew, C.-S., Krotkov, G., Canvin, D.T., 1969. Effects of temperature on photosynthesis and CO2 evolution in light and darkness by green leaves. Plant Physiol. 44, 671–677. Hoffmann, C.M., Huijbregts, T., van Swaaij, N., Jansen, R., 2009. Impact of different environments in Europe on yield and quality of sugar beet genotypes. Eur. J. Agron. 30, 17–26. Hoffmann, C.M., Kluge-Severin, S., 2010. Light absorption and radiation use efficiency of autumn and spring sown sugar beets. Field Crops Res. 119, 238–244. Hoffmann, C.M., Kluge-Severin, S., 2011. Growth analysis of autumn and spring sown sugar beet. Eur. J. Agron. 34, 1–9. Hoffmann, C.M., Märländer, B., 2002. Züchterischer Fortschritt bei Ertrag und Qualität von Zuckerrüben. Zuckerindustrie 127, 425–429. Hoffmann, C., Stockfisch, N., 2010. Effiziente Nutzung von Ressourcen im Zuckerrübenanbau. Zuckerindustrie 135, 37–43. Jaggard, K.W., Wickens, R., Webb, D.J., Scott, R.K., 1983. Effects of sowing date on plant establishment and bolting and the influence of these factors on yields of sugar beet. J. Agric. Sci. Cambridge 101, 147–161. Kenter, C., Hoffmann, C.M., Märländer, B., 2006. Effects of weather variables on sugar beet yield development (Beta vulgaris L.). Eur. J. Agron. 24, 62–69. Kirchhoff, M., Svirshchevskaya, A., Hoffmann, C., Schechert, A., Jung, C., KopischObuch, F.J., 2012. High degree of genetic variation of winter hardiness in a panel of Beta vulgaris L. Crop Sci. 52, 179–188. Kluge-Severin, S., Hoffmann, C., Märländer, B., 2009. Yield and quality of winter beets – prospects for sugarbeet production? Sugar Ind. 134 (5), 366–376. Kockelmann, A., Meyer, U., 2006. Seed production and quality. In: Draycott, A.P. (Ed.), Sugar Beet. World Agriculture Series. Blackwell Publishing, Iowa, pp. 89–113.

Lange, W., Brandenburg, W.A., De Bock, T.S.M., 1999. Taxonomy and cultonomy of beet (Beta vulgaris L.). Bot. J. Linn. Soc. 130, 81–96. Märländer, B., 1990. Einfluß der Bestandesdichte auf Ertrags- und Qualitätskriterien sowie über mögliche Ursachen der Konkurrenz in Zuckerrübenbeständen. J. Agron. Crop Sci. 164, 120–130. Meier, U., Bachmann, L., Buhtz, E., Hack, H., Klose, R., Märländer, B., Weber, E., 1993. Phänologische Entwicklungsstadien der Beta-Rüben (Beta vulgaris L. ssp.). Codierung und Beschreibung nach der erweiterten BBCH-Skala mit Abbildungen. Nachrichtenbl. Deut. Pflanzenschutzd. 45, 37–41. Mendham, N.J., Shipway, P.A., Scott, R.K., 1981. The effects of delayed sowing and weather on growth, development and yield of winter oil-seed rape (Brassica napus). J. Agric. Sci. 96, 389–416. Milford, G.F.J., Jarvis, P.J., Walters, C., 2010. A vernalization-intensity model to predict bolting in sugar beet. J. Agric. Sci. 148, 127–137. Milford, G.F.J., Limb, R., 2008. Bolting in sugar beet – time to re-evaluate our advice? British Sugar Beet Rev. 76, 3–5. Milford, G.F.J., Pocock, T.O., Riley, J., 1985. An analysis of leaf growth in sugar beet. I. Leaf appearance and expansion in relation to temperature under controlled conditions. Ann. Appl. Biol. 106, 163–172. Morillo-Velarde, R., Ober, E.S., 2006. Water use and irrigation. In: Draycott, A.P. (Ed.), Sugar Beet. World Agriculture Series. Blackwell Publishing, Iowa, pp. 221–255. Pohl-Orf, M., Brand, U., Driessen, S., Hesse, P.R., Lehnen, M., Morak, C., Mucher, T., Saeglitz, C., von Soosten, C., Bartsch, D., 1999. Overwintering of genetically modified sugar beet, Beta vulgaris L. subsp. vulgaris, as a source for dispersal of transgenic pollen. Euphytica 108, 181–186. Reinsdorf, E., Koch, H.-J., 2013. Modeling crown temperature of winter sugar beet and its application in risk assessment for frost killing in Central Europe. Agric. Forest Meteorol. 182-183, 21–30. Reinsdorf, E., Koch, H.-J., Märländer, B., 2013. Phenotype related differences in frost tolerance of winter sugar beet (Beta vulgaris L.). Field Crops Res. 151, 27–34. Royo, C., 1999. Plant recovery and grain-yield formation in barley and triticale following forage removal at two cutting stages. J. Agron. Crop Sci. 182, 175–183. SAS Institute, 2011. SAS 9.3 Help and Documentation. SAS Institute, Cary, NC. Senff, G., 1958. Versuche mit der Feldüberwinterung von Zuckerrüben zur Saatguterzeugung. Zuckererzeugung 2, 279–281. Senff, G., 1961. Der direkte Zuckerrübensaatgutbau nach Sommeraussaat unter besonderer Berücksichtigung von Maßnahmen zur Beseitigung seiner Nachteile. Zeitschrift für Acker- und Pflanzenbau 112, 10–38. Starke, P., Hoffmann, C., 2011. Zuckerrüben als Substrat für die Biogaserzeugung. Zuckerindustrie 136, 242–250. Stockfisch, N., Koch, H.-J., Märländer, B., 2002. Einfluss der Witterung auf die Trockenmassebildung von Zuckerrüben. Pflanzenbauwiss 6, 63–71. Taiz, L., Zeiger, E., 2006. Photosynthesis: physiological and ecological considerations. In: Taiz, L., Zeiger, E. (Eds.), Plant Physiology. , 4th ed. Sinauer Associates, Inc., Publishers, Sunderland, Massachusetts, pp. 197–220. Willms, W.D., 1991. Cutting frequency and cutting height effects on rough fescus and Parry oat grass yields. J. Range Manage. 44, 82–86. Woodis, J.E., Jackson, R.D., 2008. The effect of clipping height and frequency on net primary production of Andropogon geradii (C4 grass) and Bromus inermis (C3 grass) in greenhouse experiments. Grass Forage Sci. 63, 458–466.