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Effects of casein and varying protein sources on in vitro fermentation of forages by mixed cecal microorganisms.
Halpin, M. Y., J. S. Drouillard, L. K. Fehlberg, T. L. Douthit, J. M. Lattimer
Kansas State University, Manhattan, KS
While it is known protein impacts VFA production in the rumen, our objectives were to 1) identify optimal concentration of added casein to maximize fiber digestion by equine cecal microorganisms fed alfalfa or prairie hay, and 2) evaluate varying protein sources for impact on fermentation of prairie hay in cecal cultures. Cecal digesta obtained from 4 cannulated horses was used to inoculate in vitro cultures. Casein was combined with alfalfa or prairie hay (5 g, DM basis) to provide 0, 0.5, 1, 2, or 4% additional crude protein. Cellulose was included in inversely proportional amounts relative to casein to equalize DM between treatments. Substrate, casein, and cellulose were combined with 140 mL of McDougall’s buffer and 10 mL of strained cecal fluid and incubated at 39 ̊C for 48 h. Terminal pH, IVDMD, and disappearance of neutral detergent fiber (NDFD) and acid detergent fiber (ADFD) were influenced by combinations of forage source and casein [linear forage x casein effect, and linear and quadratic effects of casein (P ≤ 0.05)], but gas production was unaffected. There was a linear effect of casein on concentrations of acetate, propionate, butyrate, isobutyrate, isovalerate, valerate, isocaproate, and acetate:propionate ratio (P ≤ 0.044). There was a linear forage x casein effect on total VFA concentration (P < 0.05). Both alfalfa and increasing casein were associated with increased IVDMD, NDFD, and ADFD, while alfalfa was also associated with increased VFA concentration and decreased pH (P < 0.0001). For trial 2, casein, fishmeal, soybean meal, whey, plasma, and l-lysine were added to supply 2% additional protein to cultures with prairie hay. Compared to control, all protein sources led to decreased pH and increased IVDMD, NDFD and ADFD (P ≤ 0.006), with the largest effects elicited by casein, lysine, and whey. Supplemental protein, regardless of source, resulted in increased propionate and butyrate concentrations, and decreased acetate:propionate ratio (P ≤ 0.022). Acetate concentration increased with casein, l-lysine and whey (P ≤ 0.029). Total VFA production increased with l-lysine, whey, casein, soybean meal, and fishmeal (P ≤ 0.04) and tended to increase with plasma (P = 0.0564). Gas production increased only with casein (P = 0.0056). Supplemental protein, regardless of source, impacted fermentation parameters of prairie hay cultured with cecal microorganisms, with the greatest changes occurring with the more soluble casein, whey, and l-lysine.