Influence of soybean-crop proportion on the conservation of maize-soybean bi-crop silage

Our objective was to determine the influence of soybean proportion on the conservation of maize-soybean bi-crop silages. We hypothesized that the aerobic stability of bi-crop silages would increase with the level of legume in the mixture, without affecting fermentative losses. Maize (Zea mays) and soybean (Glycine max) were cultivated in alternating rows within four 0.5-ha plots and manually harvested when the maize crop reached 330 g/kg of dry matter (DM). After cut, maize and soybean plants from each plot were chopped separately. Five proportions of whole-plant soybean were mixed with whole-plant maize, ensiled in laboratory silos (0, 125, 250, 375 and 500 g/kg of fresh matter) and stored for 90 d. As expected, crude protein increased linearly with the inclusion of soybean crop in the mixed silages (from 106 to 154 g/kg DM), mainly at the expense of soluble N. On the other hand, the addition of soybean increased DM loss during fermentation (from 59 to 109 g/kg) without altering yeast counts and aerobic stability of the silages. The increase in 2,3-butanediol, ethanol and acetic acid concentrations suggested that soybean addition favored the development of enterobacteria. In vitro DM and fiber digestibility linearly decreased with the inclusion of soybean, due to an increase in acid detergent fiber content. In conclusion, despite the expected increase in crude protein, mixing up to 500 g/kg of whole-plant soybean in whole-plant maize at ensiling did not improve aerobic stability, increased fermentative losses and reduced the in vitro digestibility of the silages compared with whole-plant maize silage.