The cultivation of cowpeas (Vigna unguiculata (L.) Walp) is of great economic and social importance in developing countries and areas with low human development indexes (Moussa et al., 2011) because this crop is a source of income for small producers, and its seed is the main source of protein and minerals for many people living in these regions (Ahamefule & Peter, 2014; Boukar et al., 2011).

Although cowpeas are economically and socially important, this crop still has a low production yield. The main causes for this low productivity are the limited technology used by producers, the use of varieties not adapted to the edaphoclimatic conditions and with low productive potential, phytosanitary problems, and inadequate management of fertilisation and soil fertility (Aliyu & Makinde, 2016).

To address the problem of low productivity, more productive varieties and varieties that are adapted to regional conditions should be identified and selected (Teixeira et al., 2010). Regarding the management of the fertility of different varieties, the use of nitrogen- fixing is recommended (Farias et al., 2016; Haro et al., 2018).

Bradyrhizobium japonicum is a nitrogen fixing bacterium that has been widely studied in soybeans (Glycine max), and tested in cowpeas (Wongphatcharachai et al., 2015). Previous studies have reported increases in cowpea yields of up to 70 % when plants are inoculated with strains of this bacterium (Haro et al., 2018).

One way of evaluating the performance of varieties of agricultural crops under different conditions is by using a crop life table. Crop life tables make it possible to identify and quantify the loss factors and the different production components (Pereira et al., 2017). By using crop life tables, the critical production components and key loss factors of agricultural crops can be identified.

Knowledge of critical components and limiting factors enable the development of management strategies aimed at reducing crop losses. Thus, it becomes feasible to plan and develop breeding programmes, make fertility recommendations for cultivation, and identify varieties and strains that are better adapted to local edaphoclimatic conditions. It is worth mentioning that the present work was carried out in Brazil, there was no assessment of losses, and the data was only summarised in the production report.

Considering the many factors that may lead to low cowpea productivity in tropical regions, the present study aimed to identify and quantify the losses in the critical production component and the key loss factors for three varieties of cowpea in the presence or absence of inoculation with B. japonicum.

Significant differences in yield were observed between cowpea varieties (x² = 16.03, Gl = 2, p < 0.01), but there were no differences between treatments with and without inoculant (x² = 0.40, Gl = 1, p = 0.52). However, the vinegar (1410.02 kg ha^-1) and milk (1377.28 kg ha^-1) varieties were more productive than the buttermilk variety (1064.18 kg ha^-1) (Figure 2). Using the Wilcoxon test, we found that there was no significant difference between the yields of the vinegar and milk varieties (Z = 1.14, Gl = 1, p = 0.26). However, there were significant differences in productivity between the vinegar and the buttermilk varieties (Z = -3.14, Gl = 1, p < 0.01), and the milk and buttermilk varieties (Z = 3.71, Gl = 1, p < 0.01).

Significant differences in total losses were observed between the three cowpea varieties (X² = 33.10, df = 2, p < 0.01), but no significant differences were observed between treatments with and without inoculant (x² = 7.51, df = 1, p = 0.06).

The largest losses occurred in the milk variety (89.38 %), followed by the buttermilk (84.73 %) and vinegar (81.76 %) varieties (Figure 1). However, in the buttermilk variety, inoculation with B. japonicum did result in differences between yield losses of treatments, and in non-inoculated cowpeas the total loss was 84.53 %, whereas in inoculated cowpeas it was 87.28 % (Figure 1).

Figure 1.

According to the Wilcoxon test, the losses of the vinegar variety differed significantly from those of the milk variety (Z = 7.21, Gl = 1, p < 0.01) and the buttermilk variety (Z = 3.97, Gl = 1, p < 0.01). The productivity of the milk variety was different from that of the buttermilk variety (Z = -2.97, Gl = 1, p < 0.01). Losses varied between the production components. Thus, the greatest losses were observed in the flower component, followed by the grain and pod components (Tables 1, 2, 3).

Table 1

Note: N = 60.

Table 2

Note: N = 60.

Table 3

Note: N = 60.

All path diagrams of each variety had a good model adjustment, with Gfi > 0.90 (Figures 2, 3, and 4). In all three cowpea varieties (buttermilk, milk, and vinegar), the flower component was considered a critical production component. Regarding the flower component, flower abortion was the only loss factor (Figures 2, 3, and 4).

Figure 2.

Figure 3

Figure 4

The partial losses of the grain component significantly contributed to the total losses. The milk variety had the highest trail coefficient, followed by the buttermilk and vinegar varieties (Figures 2, 3, and 4). The factor that contributed the most to the partial losses of the grain component for the milk variety was poor grain formation. In the buttermilk and vinegar varieties, the non-fertilisation of eggs contributed the most to the partial losses of the grain component (Figures 2, 3, and 4).

Inoculation increased yield losses due to increased rates of flower abortion in the buttermilk and milk varieties, but did not affect yield losses in the vinegar variety (Figures 2, 3, and 4). Inoculation also increased the rate by which cowpea plants were damaged by cowpea weevils in the buttermilk variety. Indirectly, inoculation increased the total losses in the three varieties studied (Figures 2, 3, and 4).

The vinegar and milk cowpea varieties had higher productivity than the buttermilk cowpea variety under the experimental conditions described above, which may be due to the adaptability of these varieties to local edaphoclimatic conditions. Thus, it is necessary to pay attention to the climatic variables of the región, as these may affect the performance and productivity of different cowpea varieties (Santos, 2011).

Abortion of flowers was the key factor behind losses in the critical flower component for the three varieties studied. Flower abortion is common in some plant species. In such plants, a large number of flowers are formed, after which the flowers with greater adaptive power are selected to be fertilised and the others are aborted (Burd, 1998; Kozlowski & Stearns, 1989).

Losses in the grain component were due to poor grain formation and non-fertilisation of eggs. These losses are also affected by climatic variables. Stress caused by high temperatures, for example, can compromise the viability of eggs, cause infertility in the female and male reproductive system, and affect grain filling (Hedhly, 2011; Snider et al., 2011).

Inoculating cowpea plants with B. japonicum did not increase the productivity of the tested varieties, however, it increased the rate of abortion of flowers in the buttermilk and milk varieties. Indirectly, inoculation increased the total losses in the three cowpea varieties studied. It may be that the strain of inoculant used was not adapted to these cowpea varieties. There are several Bradyrhizobium strains that are well-adapted to cowpeas, including the

INPA 03-11B, UFLA 3-84, BR3267, UFLA03-153, and UFLA03-164 strains (Farias, 2016). Inoculation with B. japonicum affected the rate of damage caused by cowpea weevils in the buttermilk variety. Cowpea weevils are an important insect pest of stored grains, and resistance to this insect pest varies among different cowpea varieties (Lima et al., 2001).

The vinegar and milk cowpea varieties were found to be more productive and better adapted to the local conditions of our study site than the buttermilk variety. The yield losses were greater in the milk variety, followed by the buttermilk and vinegar varieties. The critical production component for the three varieties was found to be the flowers, and the key loss factor was found to be the abortion of the flowers. The grain components of the three varieties affected the total losses. The factors that contributed the most to the loss of the grain components for the three varieties were the malformation of the grains and the non-fertilisation of the eggs. Inoculation with B. japonicum directly affected losses due to the increased abortion of flowers in the buttermilk and milk varieties. In addition, inoculation also affected the rate of cowpea weevil infestation in the buttermilk variety. Indirectly, inoculation with B. japonicum affected the total losses in the three cowpea varieties studied.