Degradation of two protein sources at three solids retention times in continuous culture

Effects of solids retention times (SRT) of 10, 20, and 30 h on protein degr adation and microbial metabolism were studied in continuous cultures of rum inal contents. Liquid dilution rate was constant across all retention times at .12 h(-1) (8.3 h mean retention time). Two semipurified diets that cont ained either soybean meal (SBM) or alfalfa hay (ALFH) as the sole nitrogen source were provided in amounts that decreased as SRT was increased. Digest ion coefficients for DM, NDF, and ADF increased with increasing SRT. Digest ion coefficients for nonstructural carbohydrates were higher in the SBM die t than in the ALFH diet but were not affected by SRT. Protein degradation i n the ALFH diet averaged 51% and was unaffected by retention time. In the S BM diet, digestion of protein was 77, 78, and 96% at 10-, 20-, and 30-h ret ention times, respectively. Microbial efficiency decreased with increasing SRT and was greater for the SBM than for the ALFH diet. Efficiencies ranged from 30.6 to 35.7 and 20.8 to 29.2 g of N/kg of digested DM for the SBM an d ALFH diets, respectively, as SRT decreased from 30 to 10 h. The diaminopi melic acid content of the microbes increased as SRT increased, indicating t hat changes in microbial species occurred owing to passage rates. From thes e results, we concluded that the digestibility decreases associated with in creased ruminal turnover rates may be less for nonstructural carbohydrates and protein than for the fiber fractions.