PSII-23 Evaluation of diets varying in rumen available protein on methane production in continuous culture fermenters.

The objective of this study was to evaluate methane emissions and fermentation parameters in in vitro continuous culture (IVCC) fermenters receiving feedlot diets with varying levels of rumen available protein (RAP, sum of degradable intake CP and recycled nitrogen). Diets were formulated with the Cargill MAXTM system, which utilizes a model to estimate microbial CP (MCP) based on type and amount of dietary carbohydrates fermented in the rumen. Twelve fermenters were used in a randomized block design experiment with two 10-d periods. Dietary treatments were: DEF (RAP deficient by 20% compared to MCP requirements), BAL (balanced to MCP), EXNPN, (20% excess RAP compared to MCP), EXTP (20% excess RAP compared to MCP) but replacing NPP by true protein (urea vs soybean meal). Fermenters (~ 2 L capacity) were fed (~ 80) a steam flaked corn, DDGS and wheat hay-based TMR diet once daily. Effluent was collected the last 2 days of the experiment to determine nutrient degradation, VFA, and microbiome composition. Methane in the headspace was monitored continuously in all fermenters using a Micro-Oxymax Respirometer (Columbus Instrument Inc., Columbus OH). Data was analyzed with the lmer package in R with a model that contained the fixed effects of diet, their interaction with time (if applicable), and the random effects of period and fermenter. The DEF diet increased methane production (µmol/h) by approximately 43% compared to all other treatments (P < 0.01), and the effect was consistent across all hours, with no difference between the other diets. There was a tendency (P = 0.07) for excess diets to increase NDF degradation compared to DEF and BAL (34.0, 35.0, 43.1, 43.6% for DEF, BAL, EXNPN, and EXTP respectively, SEM=4.8%). There was no effect of diet on VFA production or proportion except for isovalerate which decreased (P = 0.03) only in EXNPN. Ammonia concentration was higher (P < 0.05) in EXNPN at 1-4 h post feeding. There was a tendency (P = 0.09) for the diet to affect bacterial DM flow with BAL having the highest flow (6.46, 7.24, 6.88, 5.82 g/d for DEF, BAL, EXNPN, and EXTP respectively, SEM = 0.72). There were no effects of diets on methanogens. Microbiome diversity was different (P = 0.03) for the DEF compared to the other diets. The BAL and excess diets had a more diverse carbohydrate-active potential than the DEF diet, suggesting that the DEF diet supports complex fermenting bacteria, while the BAL and Excess diets encourage a more diverse set of fermenting bacteria. These findings suggest that diets varying in RAP modulate the metabolic potential of the microbiome, which in turn affects methane production. This study shows that changes in RAP levels can have an impact on methane production in feedlot diets.