PSII-16 - Comparison of two methods of inoculation of continuous culture fermentors fed different diets

Abstract: This study assessed how method of inoculating continuous culture fermentors with rumen particulate digesta affected in vitro fermentation. A 4-unit, single-flow fermentor system was used in a 4x4 Latin Square with treatments arranged in a 2x2 factorial design. Dietary treatments were: 1) a corn-silage-based total mixed ration (TMR) diet; and 2) an orchardgrass (Dactylis glomerata L.) and ground corn-based (ORG) diet balanced to support similar levels of milk production. Within each diet, two inoculation methods were used: 1) direct addition of particulate digesta (32 g) into the fermentor vessel (-S); or 2) blending an equivalent quantity of particulate digesta with 0.5 L of rumen liquid for 2 minutes, straining through two layers of cheesecloth, and adding only the strained liquid to the vessel (-B). All fermentors were inoculated with identical total quantities of rumen liquid (1.5 L). This resulted in four treatments, ORG-S, ORG-B, TMR-S, and TMR-B, which were randomly assigned to fermentors with 7 d of treatment adaptation and 3 d of sample collection. Effluent samples (24-h composite) were collected on d 8-10, composited by fermentor within period, and analyzed for volatile fatty acids (VFA), and organic matter (OM) and crude protein (CP) for determination of nutrient digestibilities. Data were analyzed using a mixed model with period and treatment as fixed effects, and fermentor as a random effect. Preplanned contrasts were used to compare the effect of inoculation method within each diet, and the interaction between inoculation method and diet. Apparent OM digestibility was 16% greater in ORG-S compared to ORG-B (P < 0.01), whereas no effect of inoculation method was observed in the TMR diet (P = 0.36), which resulted in an interaction of diet and inoculation method (P < 0.01). Concentration of total VFA was greater with direct addition of particulate digesta in both diets, though there was a significant interaction of diet and inoculation method (P < 0.01) due to a greater increase in VFA in the ORG diet (34%; P < 0.01) compared to TMR (10%; P = 0.01). Crude protein digestibility tended (P = 0.06) to be greater in ORG-S vs. ORG-B, whereas inoculation method did not affect CP digestibility in the TMR diet (P = 0.58). Inoculation method affected methane production (mmol/d) only in the ORG diet; total methane production was 58% greater in ORG-S compared to ORG-B (P < 0.01), whereas methane production was not different in TMR-S vs. TMR-B (P = 0.76), leading again to an interaction of diet and inoculation method (P = 0.04). In general, adding particulate digesta directly to fermentor vessels increased feed degradation and methane production in the ORG diet, but had a lesser effect in the TMR diet.