Passage rate and efficiency of microbial protein synthesis in buffaloes fed increasing levels of crude protein
Optimization of the ruminal passage rate improves the conditions for growth of rumen microorganisms; however, if the dilution rate is too fast, the microbial growth can be reduced. The objective was to evaluate the effect of increasing levels of crude protein (CP) in the diet of buffaloes on the passage rate and efficiency of microbial protein synthesis (EMPS). Four crossbred growing buffaloes were used, weighing 355 ± 3.5 kg of body weight, cannulated in the rumen and distributed in a 4×4 Latin square design. The total mixed ration consisted of corn silage (850 g.kg-1) and concentrate (150 g.kg-1) and was formulated to meet the proposed levels of CP (70, 90, 110 and 130 g.kg-1). To determine the dilution rate, a Co-EDTA solution (32 g of Co-EDTA in 500 mL of distilled water) was added via ruminal cannula before the first feeding. Ruminal fluid was collected at time zero (before the first feeding), and 2, 4, 6, 8, 10, 12, 14, 16 and 24 hours post-feeding. To estimate the EMPS, spot urine samples were collected. From the concentration of creatinine in the spot urine sample, the urinary volume was estimated and the production of microbial nitrogen was calculated from the amount of absorbed purines, which was estimated as the excretion of purine derivatives in urine using the following equation: Y = 0.74X + (0.117 W0,75). The synthesis of microbial nitrogen in the rumen was calculated as a function of absorbed purines: Y = X70/0.116 × 0.83 × 1000. Data were interpreted using the SAS statistical software (version 9.0). There was a linear effect (P<0.01) for the levels of CP on microbial production, which averaged 244.8, 342.0, 394.8 and 425.2 g of microbial protein for 70, 90, 110 and 130 g.kg-1PB, respectively. According to the regression equation (Y = 54.62 + 4.70X, R² = 0.4555), we observed an increase of 4.70 units in the microbial production for each percentage unit of dietary CP. However, this effect is not observed on the dilution rate and on the EMPS when expressed per unit of fermented carbohydrate, probably due to the relationship between these two parameters. Therefore, if the increase in the dietary CP did not alter the EMPS, it can be concluded that the rumen microorganisms’ requirements were met at the lowest level of dietary protein.
Keywords: transit kinetics, nitrogen, purine derivatives