Relationship Between Supplemental Protein and Ruminal Fermentation of an Orchardgrass-Based Herbage Diet

Even though many vegetative, cool-season pastures contain more than 25% CP on a DM basis, lactating dairy cows are commonly overfed supplemental protein while grazing. A dual-flow continuous culture fermenter system was used to assess the effect of supplemental CP on ruminal fermentation of an orchardgrass (Dactylis glomerata L.) herbage diet. Treatments were 30 g DM of a 16, 14, 12, and 10% CP (corn- and soybean-based) supplement plus 45 g DM of herbage. Fermenters were sampled for pH, NH3-N, and VFA 4 times daily at feeding (0700, 1030, 1430, and 2030 h). Effluents were analyzed for DM, CP, NDF, OM, nonstructural carbohydrates (NSC), purine concentration, and bacterial production. The DM, OM, NSC, and CP digestibility values were calculated. Neither NDF nor CP digestibility was affected (P > 0.05) by supplemental CP. Apparent DM, OM, and NSC digestibility values, as well as the true OM digestibility value, increased (P < 0.05) linearly with decreasing proportional content of supplemental CP. Mean ruminal pH decreased (P < 0.05) from 6.46 to 5.99 as proportional content of CP decreased. Propionate concentration increased (P < 0.05) with decreased supplemental CP. Ruminal NH3-N concentration decreased (P < 0.05) linearly with decreasing supplemental CP. The percentage of total N flows of NH3-N and non-NH-N decreased (P < 0.05) with decreasing supplemental CP. Bacterial efficiency increased (P = 0.05) linearly with decreasing supplemental CP. A nonproportional increase in supplemental CP in relation to NSC may impair the ruminal fermentation and bacterial efficiency of orchardgrass herbage, resulting in decreased ruminal digestive efficiency.