Evaluation of grain legume cropping systems for animal fodder potential and impacts on subsequent wheat yield under less favourable soil conditions in organic agriculture in Luxembourg

Untersuchungen von Körnerleguminosen-Anbausystemen als proteinreiches Futtermittel und deren Einfluss auf den nachfolgenden Weizenertrag unter ungünstigen Bodenbedingungen im ökologischen Landbau in Luxemburg

2 Materials and Methods

In a first step, eight different grain legume cropping systems and a non-nitrogen fixing control crop were cultivated as previous crops (PC) (Table 1): winter and spring peas in pure stand and in mixture with cereals, winter and spring faba beans, soybean, blue lupin and triticale (control) were tested in a randomized complete block design with four replicates and a plot size of 15 m² (1.5 × 10 m). Borders of 15 m² were established on either side of each trial plot to avoid border effects. In the borders the same crop was sown than in the main plot and management was also the same than in the main plot. For each grain legume cropping system, sowing density was chosen according to official recommendations (Table 1). Distance between rows was 25 cm for crops in pure stand and 13.8 cm for crops in mixture and for triticale. Soybean and blue lupin were inoculated with their specific Bradyrhizobia. Weed control was performed by hand-weeding and with a mechanical hoe (only in plots sown with 25 cm distance between rows).

After harvesting the previous crops, plots were marked and later plowed. Afterwards, winter wheat (variety: Achat, sowing density: 350 germinable kernels m^–2, distance between rows: 13.8 cm) was sown in the plots of the different grain legume cropping systems, to test the impact of the previous crop (seasons in 2012/13 and 2013/14, respectively).

Hence, an experimental sequence consisted of a grain legume cropping system (previous crop) and a winter wheat crop, which were sown in two consecutive seasons (2011/12 – 2012/13 and 2012/13 – 2013/14). In Table 2, the management of the field trial and the nutrient soil status are summarized.

In the tables, means significantly differing from each other are followed by different letter(s) at the 5% level of probability using the Tukey-test. For description of stochastic variability, the average of the standard error of differences between two means (mean SED) are shown.

For the data on protein yield and C:N ratio of straw on previous crops in both experimental sequences and for weeds at harvest of winter wheat in the second experimental sequence, a logarithmic transformation was used to conform to the ANOVA model’s assumption of a normal distribution. Adjusted treatment means were transformed back for presentation of the data, representing median estimates on the original scale (Piepho, 2009).

Due to technical problems and predation damage in certain cropping systems in both experimental sequences, the authors decided not to analyze data over the two experimental sequences but to analyze each sequence separately.

3 Results

In the first experimental sequence, grain yield was highest for spring faba bean and winter faba bean (Table 4). Grain yield was lowest for winter peas in pure stand and in mixture and for soybean, which can be explained by technical problems that occurred at harvest of winter peas, due to severe lodging of these crops. Consequently, many grains fell out and could not be harvested with the plot harvester, but remained on the field. For soybean, predation damage by hares was observed in the first leaf stage. In the second experimental sequence, the highest grain yield was reached by winter faba bean, spring pea in pure stand and spring pea in mixture. Grain yield was lowest for blue lupin. At flowering, severe predation damage by hares was observed on blue lupins. As a consequence, most lupin plants developed only a single pod.

Protein content was significantly highest for soybean (420 and 414 g kg^–1) followed by blue lupin (299 and 333 g kg^–1), spring faba bean (278 and 272 g kg^–1) and winter faba bean (269 and 284 g kg^–1) in the two experimental sequences. Lowest protein content was measured for spring peas in pure stand and in mixture.

In the first experimental sequence, protein yield was significantly highest for spring faba bean and lowest for soybean and winter triticale. In the second experimental year, the highest protein yield was observed for winter and spring faba beans as well as soybean, which did not differ significantly from spring peas in pure stand and in mixture or winter pea in mixture. Winter triticale and blue lupin had the lowest protein yield. Overall, the differences between previous crops were less pronounced for their protein yield when compared with their protein content.

In the first experimental sequence, the highest C:N ratio in straw was determined for winter triticale, followed by blue lupin, soybean and winter faba bean. The lowest C:N ratios were found for spring peas in mixture and in pure stand. C:N ratio was significantly highest for winter triticale and soybean in the second experimental sequence.

Nmin1 did not show significant difference in the first experimental sequence but did so in the second experimental sequence (Table 3). The highest Nmin1 was measured for winter pea.

In the first experimental sequence, the highest winter wheat grain yield was measured after spring pea, followed by winter wheat after blue lupin and after spring faba bean, which did not differ significantly from each other (Table 6). There was no significant difference between the grain yields of winter wheat after the remaining previous crops. The lowest yield was measured after winter triticale. In the second experimental sequence, grain yield of winter wheat after winter pea in mixture, spring pea in mixture and winter triticale were significantly lower than after the other previous crops.

Protein content of winter wheat did not differ significantly among treatments in the first experimental sequence. In the second experimental sequence, a significant difference in protein content of winter wheat was observed between winter wheat grown after winter pea in mixture (highest protein content) and winter wheat grown after soybean (lowest protein content).

In the first experimental sequence, protein yield of winter wheat was highest after spring pea, followed by blue lupin and spring faba bean. The other previous crops did not show a significant difference in protein yield of winter wheat. In the second experimental sequence, protein yield of winter wheat was lowest after winter triticale and both spring and winter pea in mixture.

According to the F-test (Table 5), thousand kernel weight (TKW) of winter wheat was significantly influenced by the previous crop in the first experimental sequence (p = 0.0385), but pairwise comparisons by the Tukey-test were not significant (Table 6). In the second experimental sequence TKW was significantly lower after winter pea in mixture, spring pea in mixture and winter triticale.

In the second experimental sequence, a significant difference in tillers m^–2 of winter wheat was observed between winter wheat following winter pea or spring faba bean (highest number of tillers m^–2) and winter wheat following winter pea in mixture (lowest number of tillers m^–2; Table 7). Yara N of winter wheat was significantly lowest for winter triticale, winter pea in mixture and spring pea in mixture. In this second experimental year, wheat was affected by Puccinia striiformis and black head molds. These diseases were much more pronounced when the previous crops were peas grown in mixture with cereals or winter triticale. Furthermore, plant length and density of wheat at harvest were significantly lower for the same previous crops (Table 7). Additionally, weed biomass at harvest was highest for these three previous crops. The number of kernels per ear was lowest for winter triticale and spring pea in mixture.

4 Discussion

Fig. 1. Precipitation, air temperature and soil temperature at 8 cm depth in the vegetation period 2011/12 at the experimental site Karelshaff, Luxemburg (coordinates: 49°49’44,7” N, 6°03’42,4” E).

Fig. 2. Precipitation, air temperature and soil temperature at 8 cm depth in the vegetation period 2012/13 at the experimental site Karelshaff, Luxemburg (coordinates: 49°49’44,7” N, 6°03’42,4” E).

Yields of spring peas (pure stand and in mixture) tended to be higher than yields of winter peas, which contradicts findings by Urbatzka et al. (2011). These researchers also reported that the sensitivity to environmental conditions of spring peas is higher compared with winter peas. As weather conditions (Fig. 1 and Fig. 2) were favourable in both experimental sequences, which positively influenced soil moisture, there was little or no environmental stress and spring peas could perform well. Urbatzka et al. (2011) reported comparable protein content for winter and spring peas, whereas in the present study, protein content of winter peas (mixture and pure stand) was higher than spring peas in both experimental sequences. There was no clear trend visible for performance of winter and spring faba beans. The slightly better performance of winter faba bean in the second experimental sequence could be explained by the milder winter in that season and the resulting higher winter survival rate (99%) compared with the first experimental sequence (75%) (Fig. 1 and Fig. 2). 

In the first experimental sequence, spring pea in pure stand clearly had the highest previous crop value. C:N ratio in the straw of spring pea in pure stand was significantly lowest and hence, immobilization of N during decomposition of the residues was low compared with the remaining cropping systems. Nevertheless, yield forming parameters of winter wheat such as tillering, plant density, Yara N, plant length at harvest and TKW, mainly influenced by N supply (Geisler, 1983), did not differ in the first experimental sequence. Impacts on soil structure and nutrient and water availability could have resulted in the observed differences in wheat yields. For example, peas use less water than other crops. Merrill et al. (2007) and Miller et al. (2002) reported a carry-over of available soil water following legumes as an important factor contributing to higher yields by the following wheat crops. The good performance of wheat succeeding blue lupin may also be explained by an increase of the access to water resources for wheat due to the tap-root penetration of lupin loosening the soil structure in deeper layers.

In the second experimental sequence, grain legumes in pure stand had a significantly higher previous crop value regarding wheat yield compared with grain legumes in mixtures and winter triticale. Neither C:N ratio in straw of the previous crops nor Nmin content after harvest of the previous crops explained the differences in wheat yield. Yield forming parameters such as tillering, plant density, Yara N, plant length at harvest and TKW are influenced by the N supply (Geisler, 1983) and were significantly lower for these three previous crops. Furthermore, weed biomass at harvest was higher for these three previous crops. Thus, the lower yields of winter wheat after winter triticale, winter pea in mixture and spring pea in mixture could be explained by a different mineralization of residues of these previous crops, and a resulting lower nitrogen supply compared with the other previous crops. Furthermore, the higher infestation rate of black head molds and Puccinia striiformis of winter wheat following these three crops may have been due to weakened plants caused by the lower nitrogen supply.

It was also observed that when deciding to cultivate peas in pure stand or not, it is important to consider the leaf type of the pea. Semi-leafless peas that can support themselves and are not prone to lodging reach a better yield when sown in pure stand, whereas full-leaf types should be cultivated with a cereal partner, as they need a supporting crop to achieve good yields. For the decision on whether to cultivate a winter or spring form of pea, it remains unclear whether the better performance of the spring form was due to the sowing time or the leaf type; here, further studies are necessary. For the tested winter pea variety, winter-kill is not a high risk, but it may be for the winter faba bean variety tested. Hence, the cultivation of the spring type should be preferred, at least until new winter hart varieties are available.

The reduced yields of mono-cropped cereals compared with cereals in rotation with grain legumes were confirmed. In the first experimental sequence, non-nitrogen related beneficial effects of grain legumes prevailed, with wheat succeeding spring pea in pure stand performing best. In the second experimental year, with the infestation of Puccinia striiformis, grain legumes grown in pure stand showed a higher previous crop value due to a difference in mineralization of residues and, consequently, a higher nitrogen supply compared with winter triticale or grain legumes grown in mixture with a cereal partner.

In summary, growing legume grain crops is a promising option for generating protein fodder, even under less favourable soil conditions in organic agriculture in Luxembourg.