Effect of the magnitude of the decrease of rumen ph on rumen fermentation on a dual flow continuous culture system

¹ Grup de Recerca en Nutrició, Maneig i Benestar Animal, Departament de Ciència Animal i dels Aliments, Universitat Autònoma de Barcelona 08193 - Bellaterra, Spain

^* To whom correspondence should be addressed. E-mail: sergio.calsamiglia{at}uab.es.

	Abstract

The negative effect of pH on rumen microbial fermentation has been associated with the total amount of time that pH is below a certain threshold. However, not only the time, but also the magnitude of pH reduction, is important. Eight 1,325-mL dual flow continuous culture fermenters were used in 2 replicated periods to determine the effect of the magnitude of pH reduction (5.6 vs 5.1) during 4 h/d on rumen microbial fermentation. Fermenters were maintained at constant temperature (38.5°C) and fed 97 g/d of a 60:40 forage:concentrate diet (19.2% CP, 29.0 % NDF, and 18.2% ADF, DM basis), and the solid and liquid dilution rates were controlled at 5.0 and 10.0 %/h, respectively. Treatments were a constant pH 6.4 (H); 4 h/d at pH 5.6 (L); 4 h/d at pH 5.1 (VL); and 2 h/d at pH 5.1 and 2 h/d at 7.1 (HL). Relative to H, L did not affect OM and NDF digestion, the VFA profile, ammonia N concentration, CP degradation, or the flow of dietary N. In contrast, VL tended (P < 0.10) to reduce true OM digestion, reduced the NDF digestibility and the acetate and branch-chained VFA proportions, and increased the propionate proportion. Compared with H, VL reduced the CP degradation and the flow of dietary N. Relative to H, treatment HL did not affect OM and NDF digestibility, the acetate proportion, CP degradation, or the flow of dietary N, but increased the propionate proportion and decreased the branch-chained VFA proportion and ammonia N concentration. There were no differences among treatments in the efficiency of microbial protein synthesis, the flow of bacterial N, or the flow of essential and nonessential AA. In summary, fermentation was not affected by either 4 h/d at pH 5.6, or fluctuating pH between 5.1 (2 h/d) and 7.1 (2 h/d), but when pH was at 5.1 for 4 h/d, rumen microbial fermentation was modified, suggesting that effects of low pH on rumen microbial fermentation are dependent on the magnitude of the pH decrease.

Key Words: acidosis, pH, rumen fermentation