Effects of time at suboptimal pH on rumen fermentation in a dual-flow continuous culture system

Ruminal pH varies considerably during the day, achieving values below 6.0 when cows consume large amounts of concentrates. Low ruminal pH has negative effects on ruminal fermentation. However, previous studies have indicated that rumen bacteria may resist short periods of low ruminal pH, and it is not clear how long this period may be before rumen microbial fermentation is negatively affected. Seven dual-flow continuous culture fermenters (1,320 mL) were used in 3 replicated periods with the same diet ( 97 g of dry matter/d of a 60:40 forage-to-concentrate diet, 18.3% crude protein, 35.9% neutral detergent fiber), temperature (39 degrees C), and solid (5%/h) and liquid (10%/h) dilution rates to study the effects of increasing time at suboptimal pH on rumen microbial fermentation and nutrient flow. Treatments were a constant pH of 6.4 and 6 different intervals of time during the day ( 4, 8, 12, 16, 20, 24 h) at suboptimal pH (5.5), with the rest of the day being at pH 6.4. Polynomial equations were derived using the Mixed procedure of SAS, and linear, quadratic and cubic terms were left in the equation if P < 0.10. True organic matter digestion decreased with increasing time at suboptimal pH and was best described by a cubic regression (TOMD = 58.5 - 2.15x + 0.16x(2)- 0.0037x(3); R-2 = 0.74). Digestion of NDF (DNDF = 55.1 - 1.00x; R-2 = 0.75) and digestion of ADF (DADF = 56.2 - 1.33x; R-2 = 0.78) decreased linearly with increasing time at suboptimal pH. Total VFA had a cubic response (VFA = 112.7 - 2.09x + 0.17x(2) - 0.0054x(3); R-2 = 0.82). The proportion of acetate decreased linearly ( acetate = 58.7 - 0.61x; R-2 = 0.79). The propionate proportion increased (propionate = 17.6 + 2.09 x - 0.044x(2); R-2 = 0.85) and branched-chain VFA decreased (BCVFA = 4.45 - 0.51x + 0.014x(2); R-2 = 0.75) quadratically. The ammonia N concentration (NH3-N = 5.85 - 0.13x; R-2 = 0.46) and flow (NH3-N flow = 0.18 - 0.0039x; R-2 = 0.43) decreased linearly as the time at suboptimal pH increased. Crude protein degradation (CPd = 41.9 - 1.60x + 0.060x(2); R-2 = 0.71), efficiency of microbial protein synthesis (EMPS = 26.6 - 0.33x + 0.021x(2); R-2 = 0.77), microbial N flow (MN flow = 1.38 - 0.036x + 0.0015x(2); R-2 = 0.77), and dietary N flow (DN flow = 1.49 + 0.041x - 0.0015x(2); R-2 = 0.65) had a quadratic response. The flow of essential, nonessential, and most individual AA increased linearly with increasing time at suboptimal pH. The effects of pH on rumen fermentation appear to start as soon as pH drops to suboptimal pH.