Effects of diets balanced for undigestible neutral detergent fiber varying in forage inclusion rate and particle length on rumen environment, digestibility, production, and efficiency in dairy cows.

Several parameters are used to evaluate the physical characteristics and effectiveness of forage in ruminant nutrition, including NDF content, fiber length, and digestibility. In our recent study, we examined the use of undigestible NDF (uNDF) as a factor for diet formulation to better evaluate the forage quality. The objective of this study was to evaluate the interaction between forage inclusion rate and forage particle length (FPL) in diets formulated to contain similar concentrations of forage 48-h undigestible neutral detergent fiber (forage-uNDF48), and to determine their effects on DMI, rumen environment, chewing activity, nutrient digestibility, and production performance of high-producing dairy cows. Forty-two multiparous Israeli-Holstein cows (averaged 43.4 ± 5.9 kg milk/d and 115 ± 68 DIM) were assigned to a 2 × 2 factorial arrangement of treatments with forage inclusion rate (35 vs. 30% of DM) and wheat hay FPL (long vs. short), resulting in 4 diets balanced for forage-uNDF48 (12.3-12.4% of DM), NDF (∼31%), CP (∼16.9%), and starch (∼29.1%) content. In the low-forage (LF) diets, wheat straw replaced part of the wheat hay to maintain a fixed forage-uNDF48 in the diets; however, only the wheat hay was subjected to the FPL treatment. Reducing the forage inclusion rate increased DMI and intake of forage-uNDF48, whereas total daily rumination and chewing time were not affected by treatments; however, low forage diets reduced the rumination, eating, and chewing times per kg of DM, NDF, forage-uNDF30, and forage-uNDF48. Unexpectedly, the mean rumen pH was higher in LF than in high forage (HF) diets (6.48 vs. 6.29) and was also higher with short FPL than with long FPL diets. The apparent total-tract digestibility of DM, OM, starch, and NDF was not affected by treatments. Milk yield was not affected by forage inclusion rate but was higher for the long compared with the short FPL diets (44.3 vs. 42.9, respectively). The fat percentage (4.25 vs. 3.98%), 4% FCM (45.0 vs. 42.4 kg/d), and the ECM (45.4 vs. 43.0 kg/d) yields were greater in HF diets, whereas the LF diets increased energy balance and body weight gain. Feed efficiency for 4% FCM and ECM was higher in the HF than in the LF diets, with no effect of FPL. These results indicate that under the conditions of the present study, forage inclusion rate exerted a greater influence than FPL on feed intake, rumination, milk fat concentration, 4% FCM, and ECM. However, interpretation of FPL effects should be made with caution because the particle length treatment was applied only to the wheat hay fraction. The results of this study highlight the complex interplay among diet composition, rumination time, and ruminal pH, limiting the ability to reliably predict the relationships among these 3 factors. The findings underscore the potential value of incorporating forage-uNDF as an additional parameter in diet formulation to more precisely evaluate forage quality and utilization. However, further research is needed to assess the applicability of this approach across a wider range of dietary compositions and forage inclusion levels.