Reducing the forage content of the ration using the undigested forage neutral detergent fiber: The effects on production, rumen environment, and digestibility.

Several factors influence the effectiveness of forage in ruminant rations, including NDF content and the physical nature, fragility, digestibility, and more of the forage. Recently, several studies suggested using the undigestible NDF (uNDF) fraction as a possible approach to achieve a more precise ration. The objective of the current study was to reduce the forage content of the diet by using the in vitro forage uNDF for diet formulation and to determine the effects on production, rumen environment, and digestibility. Thirty-four cows were divided into 2 groups in a crossover design study. Treatments consisted of either (1) a control (CTL) diet containing 35.8% forage (DM basis), 11.8% forage uNDF estimated through 30 h of in vitro fermentation (uNDF₃₀), and 10% forage uNDF estimated through 48 h of in vitro fermentation (uNDF₄₈) based on 20% wheat silage and 15.8% wheat hay, or (2) a low-forage (LF) diet containing 30.6% forage, 11.8% forage uNDF₃₀, and 10.4% forage uNDF₄₈ based on 20% wheat silage, 2.2% wheat hay, and 8.3% wheat straw. Each period lasted 35 d, and data collection occurred during the final 21 d. Milk yields were recorded daily, and milk samples were taken weekly. Two rumen samples were collected twice for VFA, pH, and ammonia, and 8 fecal samples were collected for total-tract digestibility measurements. No differences were observed in rumen pH, ammonia, and VFA concentrations. Apparent total-tract digestibility of DM, OM, protein, ether extracts, and NDF was higher in the CTL diet. Milk yields (52.2 and 51.7 kg/d, respectively), 4% FCM, and ECM yields were higher in the CTL than in the LF treatment. The milk fat and protein content did not differ, the milk fat tended to be higher, and the milk protein yield was higher in the CTL treatment. The DMI was 3.2% higher in the CTL than in the LF treatment (32.7 vs. 31.7 kg/d, respectively), and the milk-to-DMI ratio was higher in the LF than in the CTL treatment. In summary, reducing the forage content by balancing the diet for forage uNDF reduced the DMI, milk, fat, and protein yields and digestibility, probably due to the high inclusion rate of wheat straw in the LF diet. However, the production efficiency of milk was higher in the LF treatment. Further research is required to fine-tune the proportion of forage uNDF and to determine the optimal exchange of forage feedstuffs in the diet.