Herbage and Silage Quality Improved More by Mixing Barley and Faba Bean Than by N Fertilization or Stage of Harvest

Legume–cereal mixtures are pivotal in yielding a more balanced forage composition compared to the sole crops, due to the well-adjusted equilibrium of carbohydrate and protein. However, great attention is required in choosing the optimal ratio of the components for the mixtures and other agronomic practices (including N fertilization and stage of harvest), because they can sharply change the forage composition and quality. To fill this gap, the present research compared the herbage and silage qualities and key fermentative and nutritional traits of biomasses obtained by: (i) five mixtures (i.e., 100:0; 75:25, 50:50, 25:75, and 0:100) of hybrid barley (Hordeum vulgare L.) and faba bean (Vicia faba var. minor); (ii) two N barley fertilization rates (0 vs. 140 kg N ha−1); and (iii) three stages of harvest (milky, early dough and soft dough). We found that the presence of faba bean improved the forage quality, either herbage or silage, through different mechanisms. In the herbage mixtures, faba bean inclusion increased crude protein (CP), and, when compared to the fertilized barley, also water-soluble sugar (WSCs) concentration, with small effects on the relative feed values (RFV) and total digestible nutrients (TDN). In the silage, the higher the faba bean share in the mixture, the higher the RFV, the non-fibrous carbohydrates, and total and lactic acid concentrations, and the lower the pH and the hemicellulose and cellulose concentrations, particularly at the early and mid stages of harvest. These improvements were likely due to the high ability of faba bean to attain a vegetative status (i.e., low dry matter concentration) by the end of the cropping season, and to improve the organic acid production. Our results suggest that the inclusion of faba bean in a barley-based silage system can widen the window for silage harvesting, thanks to its ability to sustain the fermentation process, despite a lower WSC to CP ratio compared to the unfertilized barley. These results occurred almost irrespectively of the faba bean share in the mixture and barley fertilization. This implies that when designing intercrop mixtures, the effect of crop densities on yield should be considered more than on quality.