Applying the fermentability coefficient concept in tropical grass silages

Two experiments were carried out to evaluate whether the fermentability coefficient (FC) concept is applicable to Megathyrsus and Urochloa genera and whether the forage nitrate content alters the fermentation pattern of the respective silages. Guinea grass [Megathyrsus maximus (Jacq.) B.K. Simon & S.W.L. Jacobs (Syn. Panicum maximum Jacq.) cv. MG18 Aries II] and palisade grass [Urochloa brizantha (Hochst. ex A. Rich) R.D. Webster (syn. Brachiaria brizantha (A. Rich) Stapf) cv. Marandu] were used in both experiments. In experiment 1, a broad range of FC was induced by different dry matter (DM) content and soluble carbohydrates (SC):buffering capacity (BC) ratio. The DM content was modified by wilting (i.e., guinea grass: direct cut at ∼200 g/kg, wilted to ∼300 and ∼400 g/kg; palisade grass: direct cut at ∼250 g/kg and wilted to ∼400 g/kg) while the SC:BC ratio was modified by addition of glucose [0, 5 and 10 g/kg of fresh matter (FM)] resulting in a factorial arrangement of 3 × 3 for guinea grass and 2 × 3 for palisade grass, with three replications per treatment. In experiment 2, five doses of sodium nitrate (0, 0.15, 0.30, 1.5 and 3.0 g/kg FM) were added factorially to DM and glucose levels used in experiment 1, leading to a factorial arrangement of 3 × 3 × 5 for guinea grass and 2 × 3 × 5 for palisade grass, with three replications per treatment. Regression analyses were performed using the REG procedure of SAS. An independent dataset of tropical grasses and their silages (n = 226) was used to validate the FC model. The FC equation obtained for tropical grasses in the current study did not diverge from the model derived from temperate forages (i.e., FC = DM + 80 × SC:BC), as the regression slopes were similar (79 vs. 80; P = 0.92). There was a linear negative correlation between FC and minimum content of nitrate required to inhibit butyric fermentation, confirming the clostridia-inhibiting role of nitrate in tropical grass silages too. However, the effect size differed between grass genera. In conclusion, as proposed for temperate forages, FC values ≥350 markedly decreases the risk of butyric fermentation, whereas FC values ≥400 completely suppress butyric fermentation in tropical grasses with moderate levels of nitrate and epiphytic lactic acid bacteria (LAB). The FC required to curtail butyric fermentation decreased linearly with nitrate concentration. Nevertheless, the minimum nitrate content to inhibit Clostridium development during silage fermentation depended on the forage species.