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 ~200g/kg, wilted to ~300 and ~400g/kg; palisade grass: direct cut at ~250g/kg and wilted to ~400g/kg) while the SC:BC ratio was modified by addition of glucose [0, 5 and 10g/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 3.0g/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.

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 ad-ditive 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 aerobi-cally 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.

The benzoate equivalent (BEq) concept is a potential tool to compare silage additives that differ in active ingredients and their respective concentrations. Thus, our objective was to define BEq coefficients for potassium sorbate and sodium propionate in comparison to sodium benzoate, and establish an application rate based on the BEq to extend the aerobic stability of rehydrated corn grain silage. Dry corn kernels were ground (hammer mill, 8-mm screen), rehydrated to 350 g/kg moisture, and treated as follows: no additive (control), potassium sorbate (PS) at 125, 250, 500 and 1000 g/t, sodium benzoate (SB) at 250, 500, 1000 and 2000 g/t, sodium propionate (SP) at 500, 1000, 2000 and 4000 g/t, and a mixture of SB, PS and SP (BPS) at 250 + 150 + 53 g/t, 375 + 225 + 79 g/t, 500 + 300 + 105 g/t and 625 + 375 + 132 g/t, respectively. Treated grain was stored for 60 d in laboratory silos. All additives, alone or mixed, decreased the yeast and mold counts, ethanol and ethyl ester concentrations, and DM loss during fermentation, whereas more water-soluble carbohydrates were preserved in the treated silages. The untreated (control) corn grain silage rapidly deteriorated upon air exposure (similar to 41 h of aerobic stability), and PS, SB, and SP, alone or mixed, linearly improved the aerobic stability with increasing application rate. PS (BEq = 1.99) was more effective than SB (BEq = 1), and SB was more effective than SP (BEq = 0.35) in enhancing the aerobic stability. An application rate of approximately 1400 g/t of BEq could extend the aerobic stability of rehydrated corn grain silage for 3 d over that of untreated silage. In conclusion, the BEq concept is a plausible tool to compare commercial silage additive preparations and define their application rates in corn grain silage.

As producing tropical grass silage with high feeding value is still a challenge, we examined the effectiveness of sodium nitrite-based additives on guinea grass (Megathyrsus maximum cv. Mombasa) silage quality. The forage was mechanically harvested from four 3-ha fields and divided into 5 piles per field to receive one of the following treatments (fresh matter basis): no additive (control), soybean hulls (100 g/kg; SH), sodium nitrite (1 g/kg; NIT), sodium nitrite (1 g/kg) + hexamine (0.65 g/kg; NIT+HEX), and formic acid (85%) (4 mL/kg; FA). Sodium nitrite-based additives (sodium nitrite applied alone or in combination with hexamine) and FA were capable of curtailing clostridial development, resulting in lower concentrations of NH3-N and n-butyric acid and reduced dry matter (DM) loss during fermentation, whereas protein quality and hygienic quality (reflected by lower Clostridium counts) were improved. A strong linear relationship was detected between the concentrations of butyric and valeric acids and DM losses during fermentation (R-2 = 0.87, P < 0.01). Addition of SH improved DM degradability and slightly decreased fermentation losses, but it did not lead to butyric acid-free silages. The use of FA and sodium nitrite-based additives was effective in improving the fermentation quality of tropical grass silage, and the combination with hexamine was superior to the sole use of sodium nitrite. All treatments improved in vitro DM degradability over untreated silage. Only NIT, NIT+HEX and FA increased the concentration of rumen-undegradable protein, with NIT+HEX and FA outperforming NIT. Sodium nitrite-based additives have the potential to improve the fermentation and the nutritive value of guinea grass silage.