Effect of application rate of sodium nitrite and hexamine on the fermentation and the chemical composition of guinea grass silage harvested at different stages of maturity

Grass ensilability varies with maturity stage, mainly due to changing concentrations of dry matter and soluble carbohydrates with progressing forage maturity. Consequently, the required dose of silage additive to prevent the development of undesirable microorganisms may change with maturity stage. The objective of this study was to verify whether the application rate of an additive containing sodium nitrite and hexamine interacts with guinea grass maturity to alter silage fermentation and chemical composition. Four fields of guinea grass (0.5–0.7 ha each field) were mowed and divided into two plots per field. After 5 wk, one plot of each field was mowed again to establish differences in stage of maturity. Ten weeks after the first mowing, the grass plots with 5- and 10-wk regrowth were manually harvested and used for the trial. The grass from each plot (approx. 30 kg) was chopped and divided into 3 piles, totaling to 24 piles, as result of four fields, two maturities, and three additive treatments: control (without additive), low dose of sodium nitrite (0.5 g/kg) + hexamine (0.325 g/kg) (NHL), and high dose of sodium nitrite (1 g/kg) + hexamine (0.65 g/kg) (NHH). After 90 d of storage, the silos were opened and silages sampled to determine dry matter (DM) loss, microbial counts, fermentation end-products, aerobic stability, chemical composition, and in vitro DM digestibility. Guinea grass harvested at 10-wk regrowth had a lower content of crude protein (P < 0.001) and a greater content of cell wall components (P < 0.001), resulting in a more lignified (P < 0.001) and less digestible (P < 0.001) forage than that harvested at 5 wk. There were interactions between plant maturity and additive dose for several silage traits (P < 0.05), likely due to the slightly greater fermentability coefficient (+5.1 points) for the more mature grass (P < 0.001). Within each maturity stage, silage pH and fermentation end-products associated with clostridia metabolism (i.e., n-butyric acid, propionic acid, i-butyric acid, i-valeric acid, n-valeric acid, ammonia, and 2,3-butanediol) linearly decreased (P < 0.001) with additive application rate, but the magnitude of improvement was slightly greater for 5-wk than 10-wk regrowth. Application of additive linearly decreased silage DM loss at both 5-wk (95.2, 46.7, and 20.6 g/kg DM, P < 0.001) and 10-wk (66.5, 31.7, and 13.6 g/kg DM, P < 0.001) regrowth stages, but only silages treated with NHH had n-butyric acid concentration < 3 g/kg DM. The proportion of rumen undegradable protein (P < 0.001), soluble carbohydrates concentration (P < 0.001), and in vitro DM digestibility (P < 0.001) were linearly increased with additive dose within each maturity stage. As treated silages were better conserved, silage aerobic stability was linearly reduced (P < 0.001) with additive dose, although all silages were aerobically stable for ≥ 4.7 d. In conclusion, the additive based on sodium nitrite and hexamine, applied at a regular dose, was able to largely restrict Clostridium development and DM losses during fermentation of guinea grass silage at both maturity stages. However, harvesting more mature grass markedly impaired its chemical composition and digestibility, rendering it no feasible strategy to reduce the additive application rate by half.