Effect of intercrops on actual yield
Considering all cultivars together, the mean actual yield of intercrops (lentil + spring wheat) was significantly higher than that of sole cropped lentil (Fig. 3a) in both 2015 (1.57 and 1.29 t ha−1, respectively; P < 0.01) and 2016 (2.26 vs. 1.93 t ha−1, respectively; P < 0.01). Mean actual yield of sole cropped spring wheat (1.46 and 1.60 t ha−1 in 2015 and 2016, respectively) was less than or equal to that of intercrops in almost all treatments, but the difference was significant only in 2016 (P = 0.26 and P < 0.001 for 2015 and 2016, respectively). There was no significant effect of cultivar or year on actual yield of spring wheat, allowing actual yields of the two wheat cultivars to be averaged together. Although actual yields of lentil included bruchid-damaged grain and small grain, the trends observed were the same when only the sound grain fraction of actual yield was considered (data not shown). Note that the amount of small grain turned out to be negligible.
These results indicate a grain yield advantage in intercrop vs. in sole crop under a wide range of conditions: two years with contrasting climates, four lentil varieties and two wheat varieties. This increase led to a land equivalent ratio (LER)—the relative land area of sole crops required to produce the same yield achieved in intercrop, and with the same species proportion in total grain (Willey and Osiru 1972)—ranging from 1.02–1.54 (mean = 1.24 ± 0.14) based on the actual yield. The LER illustrates the ability of lentil-spring wheat intercrops to increase total yields in low-input systems and organic farming, as reported in other studies (Carr et al. 1995) and for other legume-cereal intercrops (e.g. Bedoussac et al. 2015; Fletcher et al. 2016). The intercrop’s better performance can be explained by complementary use of N niches by lentil and spring wheat. As cereal forces legume to meet more of its N requirements by fixing N2 (e.g. Bedoussac et al. 2015), the lentil does not totally compete with spring wheat for soil mineral N when intercropped (e.g. Naudin et al. 2009). Furthermore, our results suggest that the lower the yield of sole cropped lentil, the higher the yield advantage in intercrop (Fig. 3a), indicating that this species mixture could also be a way to ensure a minimum grain yield for organic farmers among years, especially when lentil yields are low, for example due to dry spring conditions. Moreover, when lentil yields were high, intercrops produced more than sole cropped spring wheat, probably because N was limiting in both experimental years (Tosti et al. 2016). Our results also agree with those of Bedoussac and Justes (2010), who observed that total grain yields of cereal-wheat intercrops were higher than those of sole cropped wheat when N availability remained low, such as in stockless organic farming.
Mean actual yield of lentil was significantly lower in intercrop than in sole crop (Fig. 3b) in both 2015 (0.93 vs. 1.29 t ha−1, respectively; P < 0.01) and 2016 (1.20 vs. 1.93 t ha−1, respectively; P < 0.001). This highlights that spring wheat added at a low density (17% of sole crop density) was still dense enough to decrease the associated lentil yield, illustrating strong interspecific competition of spring wheat with lentil. Similar trends were observed in several previous studies of lentil-wheat intercrops (Akter et al. 2004; Carr et al. 1995; Wang et al. 2013). Actual yield of lentil in sole crop and intercrop was significantly higher in 2016 than in 2015 (P < 0.001 and P < 0.01, respectively) (Fig. 3b). Actual yield of intercropped lentil tended to be higher when that of sole cropped lentil was high, i.e. when conditions were favourable for lentil growth. These results can be explained in part by favourable temperature and rainfall conditions around flowering and early pod filling stages in 2016, greatly increasing the number of pods per plant (data not shown) and thus the yield, unlike in 2015, which had a dry spring.
Effect of intercropping on gross margin from actual yield
Mean actual gross margin of intercrops was significantly lower than that of sole cropped lentil but higher than that of sole cropped wheat (Fig. 3c) in 2015 (1427, 1778 and 304 € ha−1, respectively; P < 0.05) and 2016 (2117, 2965 and 346 € ha−1, respectively; P < 0.001). Despite the higher total yield of intercrops, the decrease in lentil yield in intercrop compared to that in sole crop was not economically offset by the actual yield of spring wheat in intercrop, given a selling price of lentil ca. four times that of spring wheat. Therefore, as lentil contributes more to intercrop gross margin, one should favour lentil yield to maximise gross margin of the actual yield of intercrops. Our results show that when hand-harvested—which corresponds to the actual yield—intercrop is less profitable than sole cropped lentil. Finally, actual gross margins ranged widely over the 2 years of experiments, from 734 to 2715 € ha−1 for intercrops and 1002–3188 € ha−1 for sole cropped lentil. Thus, intercrops with lentil can achieve high, albeit lower, actual gross margins, even with a strong decrease in the actual yield of lentil. Akter et al. (2004) observed an economic advantage in the actual yield of lentil-wheat intercrops for management strategies including irrigation, fertilisation and chemical control of biotic stresses. However, since lentil is intended to human food and harvested with combine harvesters, one should include the potential grain losses due to non-edible seeds (e.g. bruchid-damaged grains) and losses on field due to mechanical harvest to reveal marketable yield and marketable gross margin that reflect more accurately the reality of farmers.
Effects of intercrops on bruchid damage, lodging and mechanical harvest efficiency
Effect of intercrops on bruchid damage
Among all cultivars and years, mean percentage of bruchid-damaged grain was not significantly different for lentil in intercrop and sole crop (40 ± 15 vs. 42 ± 14%, respectively). Both treatments had a high mean percentage of bruchid-damaged grain in 2015 (49%) and a lower one in 2016 (33%). No difference in the bruchid damage was observed among cultivars except for Anicia, which was more sensitive (mean = 63 and 52% in 2015 and 2016, respectively, for sole crop and intercrop combined). Leroi et al. (1990) observed no significant difference in bruchid damage to cowpea intercropped with maize and that in sole crop. In contrast, Karel et al. (1982) and Olubayo and Port (1997) observed a significant decrease in bruchid infestation rate in cowpea-maize intercrops. These studies were carried out in East Africa, which has different bruchid species than those established in Europe. This result emphasises, however, that increased plant diversity in the field can decrease bruchid infestation rate. Under our conditions, we estimated that mean yield loss due to bruchids was 0.69 and 0.93 t ha−1 in 2015 and 0.52 and 0.93 t ha−1 in 2016 for lentil in intercrop and sole crop, respectively. The presence of B. lentis and B. signaticornis has been confirmed in southwestern France (Yus-Ramos et al. 2014), but to our knowledge, this is the first report of major damage by bruchids in this area in a scientific publication. Currently, the abundance of bruchids in southwestern France, coupled with the lack of effective agronomic or biological methods to control them, seriously hinders development of lentil in organic agriculture there. Unfortunately, our experiment cannot help to identify factors influencing bruchid damage, as it was not designed to do so, and no clear trend in damage was observed. Moreover, bruchid ecology is not well-known, but we can hypothesise that bruchid infestations are influenced by temperature or degree-days during the growing season, as well as by crop rotations, landscape pattern and biodiversity.
Effect of intercrops on lentil height at harvest, stem length and lowest pod height
Mean lentil height at harvest (Fig. 4a) was higher in intercrop than in sole crop, non-significantly in 2015 (28 vs. 23 cm, respectively; P = 0.23) but significantly in 2016 (36 vs. 25 cm, respectively; P < 0.01). Akter et al. (2004) observed a similar increase in lentil height in intercrop. Mean lentil height at harvest in intercrop was lower in 2015 than in 2016 (P < 0.05), while no difference was observed in sole crop (P = 0.38). In 2016, mean lentil stem length (Fig. 4b) was similar between intercrop and sole crop (42 cm; P = 0.75). Thus, the mean lodging was 15% in intercrop and 40% in sole crop (Fig. 4c). These results suggest a strong decrease in lentil lodging due to having spring wheat in the intercrop. Furthermore, mean height of the lowest pod was also significantly higher in intercrop than in sole crop (22 vs. 12 cm, respectively; P < 0.001; data not shown). Thus, sowing wheat at 17% of its sole cropped density in intercrop was sufficient to significantly increase lentil height and lowest pod height at harvest. Moreover, the lower the height of sole cropped lentil at harvest, the larger its difference with the height of intercropped lentil (Fig. 4a). Carr et al. (1995) observed an increase of 3.5 cm in the height of the lowest lentil pod (albeit smaller than our result) in intercrop compared to that in sole crop. Thus, intercrops could be a way to significantly decrease lentil lodging, thus increasing pod height and creating conditions in which combine harvesters are more likely to gather more of the actual yield of lentil.
Effect of intercrops on mechanical harvest efficiency
Our mechanical vs. hand-harvest experiment performed in 2016 (Fig. 5) confirmed that the mean hand-harvested yield of sound lentil grain was lower in intercrop than in sole crop (1.01 ± 0.19 vs. 1.29 ± 0.13 t ha−1, respectively; P < 0.05). In contrast, the mean yield of mechanically harvested lentil was similar in intercrop and sole crop (0.75 ± 0.11 vs. 0.64 ± 0.06 t ha−1, respectively; P = 0.81). Consequently, mechanical harvest efficiency was clearly higher for lentil in intercrop than in sole crop (75 vs. 50%, respectively, P < 0.05). The greater mechanical harvest efficiency in intercrop can be attributed mainly to the higher mean pod height in intercrop, confirming the importance of maintaining pod height as high as possible. The slight increase in the lowest pod height observed by Carr et al. (1995) decreased grain loss of lentil in intercrop by only 3% compared to that in sole crop. They provided no data on lentil lodging, however, making comparison with our experiment impossible. Breeding lentil cultivars for high mechanical harvest efficiency appears to be a viable long-term strategy for issues related to mechanical lentil harvest. Moreover, it would be interesting to determine the minimum relative density of spring wheat needed to increase mechanical harvest efficiency of lentil in intercrop and simultaneously decrease its strong interspecific competition with lentil. We hypothesise that densities below 17% of its sole crop density can reach these objectives, notably due to the ability of spring wheat to compensate for low density by growing more shoots. However, reducing wheat density at sowing could increase at least two risks: (1) that farmers would fail to obtain good spatial distribution of wheat seeds, even using a pneumatic precision drill, and (2) that unfavourable climatic conditions would decrease wheat density even further by decreasing its emergence rate.
Effect of intercrop gross margin from marketable yield
We applied mechanical harvest efficiency to the actual yields to estimate mechanically harvested yields. We first assumed that mechanical harvest efficiency was the same for all lentil cultivars in both years, which seemed acceptable based on our observations of lentil height at harvest and stem length. These observations suggested that, even though mechanical harvest efficiency can vary among cultivars and years, the relative difference in mechanical harvest efficiency between lentil in intercrop and sole crop remains large. We then assumed that loss of spring wheat grain during mechanical harvest was negligible, as confirmed by our field observations after harvest, and did not significantly affect marketable gross margins. Next, we assumed that mechanical harvest efficiency was the same for all grain fractions of actual yield (i.e. marketable, bruchid-damaged and small). Finally, the marketable yield was used to calculate marketable gross margin to compare intercrop vs. sole crop profitability.
Mean marketable gross margin (Fig. 3d) was significantly higher for intercrops (lentil + spring wheat) than sole cropped lentil or sole cropped wheat in both 2015 (629, 390 and 283 € ha−1, respectively; P < 0.05) and 2016 (1269, 987 and 325 € ha−1, respectively; P < 0.05). Furthermore, marketable gross margins, like actual gross margins, ranged widely over the 2 years (273–1773 and 23–1158 € ha−1 for intercrops and sole crops, respectively). The lowest marketable gross margin of intercrops was higher than that of sole cropped lentil (P < 0.05), meaning that intercrops can act as “harvest insurance” for farmers, especially when sole crop yields are low. On the other hand, the highest marketable gross margin of intercrops was also higher than that of sole cropped lentil (P < 0.001). Consequently, when lentil yield is high in sole crop, intercropping lentil may still be a way to increase gross margins. Intercropped lentil was thus found to be more profitable than sole cropped lentil in our experiments, under both favourable and unfavourable climatic conditions in organic farming. The decrease in lentil lodging due to support by wheat is an example of the “within-season benefit” concept developed by Fletcher et al. (2016) and helped to assess agronomic and economic performances of intercrops.
Yield gap analysis of all cultivars and years combined
Finally, we used our adaptation of the yield gap concept to detail lentil grain losses along the agronomic production stages in sole crop and in intercrop, for all cultivars and years combined. Mean attainable yield was 1.41 and 2.14 t ha−1 for lentil in intercrop and sole crop, respectively (Fig. 6). Mean attainable yield of sole cropped lentil was high and even higher with cv. Anicia in 2016 (3.11 t ha−1). This yield is consistent with that (3.0 t ha−1) observed by Wang et al. (2013) in an experiment conducted with cv. Anicia in organic farming in Germany without water stress. This strengthens our assumption that our growing conditions were favourable (i.e. no water stress) for lentil in 2016.
Although bruchids consumed ca. 25% of the attainable yield of lentil in both intercrop and sole crop, we observed a mean actual yield in both intercrop and sole crop that was relatively higher than the mean worldwide lentil yield (ca. 1.0 t ha−1, Erskine et al. (2011)). Subsequently, 25 and 50% of the actual yield were lost during the mechanical harvest of lentil in intercrop and sole crop, respectively. Finally, a large mass of bruchid-damaged grain residues, representing 25% of the mechanically harvested yield of lentil in both intercrop and sole crop, had to be removed from the mechanically harvested yield to obtain the marketable yield (Fig. 6). Note that additional downstream stages can be added if higher grain quality is required by agro-food industries.
Ultimately, the marketable yield of lentil in intercrop was only 42% of its attainable yield but was higher than that in sole crop, which was only 28% of its attainable yield. Intercropped lentil approaches attainable yield more closely than sole cropped lentil (and with less risk), but both systems currently lay far below optimum performances. The yield gap analysis (Fig. 6) illustrates that grain loss at mechanical harvest was an important issue for lentil but clearly highlights that bruchids were the major reducing factor in our experiments, as is the case for organically farmed lentil in southwestern France.