Effect of varying dietary starch and fiber levels and inoculum source (mule deer vs. dairy cow) on simulated rumen fermentation characteristics

This study measured starch and fiber digestion and microbial fermentation of three commercial exotic animal feeds using mule deer (MD) or dairy cow (DC) rumen inoculum. Diets were formulated to provide either high starch/low fiber (based on neutral detergent fiber fraction; NDF) with either alfalfa (diet A) or grain and oilseed byproducts (diet B) as the major fiber sources or low starch/high NDF (diet C). An initial batch culture incubation was run with diets inoculated with each rumen inoculum (n = 6; N = 36) over a 48 hr period with samples taken at different hour points for ammonia, pH, lactate, and volatile fatty acids (VFA). A second experiment was conducted where two continuous culture incubations (MD or DC) were run with six single-flow polycarbonate fermentation vessels per dietary treatment. Diets were fed two times a day over an 8-day period and sampled for ammonia, pH, and VFA before and after feeding on the last 3 days. On day 8, fermenter and effluent contents were collected and analyzed for nitrogen, dry matter digestibility (DMD), and organic matter digestibility (OMD). OMD was greater in MD (P = 0.02) and DMD tended to do the same (P = 0.06), but there were no differences due to diet (P > 0.05). Ammonia concentration was greater in DC (P < 0.01), and diets A and B had greater concentrations than diet C (P < 0.01). The greater digestibility, higher acetate:propionate (A:P) ratio and increased lactate levels prior to feeding likely led to diet C having a lower pH than diet A (6.59 vs. 6.66, respectively; P < 0.01) and led the tendency of A to be lower than C after feeding (P = 0.08). A:P ratio was greater in DC than MD before and after feeding (P < 0.01) and was greater in diet C than diets A or B (P < 0.01). Total VFA production tended to be greater in diets B and C in DC (P = 0.06). Rumen fluid source did affect fermentation. Increasing fiber level did not negatively affect fermentation and may increase OMD by removal of negative associative affects by starch on cellulolytic bacteria. Zoo Biol. 33:110-120, 2014. (c) 2014 Wiley Periodicals Inc.