Impact of decreasing undigested neutral detergent fiber concentration in corn silage-based diets for dairy cows: Nutrient digestibility, ruminal fermentation, feeding behavior, and performance.

The objective of this study was to evaluate the effects of decreasing concentrations of undigested NDF after 288 h of incubation (uNDF288) in corn silage-based diets for dairy cows on nutrient intake and digestibility, feed sorting index, ingestive behavior, ruminal fermentation, milk yield, and milk composition. Twenty-four Holstein cows were used, including 8 primiparous and 16 multiparous cows, in a replicated 4 × 4 Latin square experiment. Cows were blocked based on milk yield (37.1 ± 4.28 kg/d), DIM (110 ± 60.0 d), and BW (669 ± 74.2 kg). Each period lasted 21 d, with the last 7 d designated for data collection. Treatments consisted of dietary uNDF at 11.1%, 10.4%, 9.7%, and 9.0% diet DM. The dietary uNDF concentration was manipulated by ensiling corn plants cut at 40 cm and 65 cm above the ground, provided in different proportions in the diet. Increasing the cutting height of corn silage from 40 to 65 cm resulted in a reduction in the uNDF288 concentration from 21.2% to 16.8% DM, and increased estimated in vitro ruminal NDF digestibility and total-tract NDF digestibility for silage with a cutting height of 65 cm. Results showed that reducing dietary uNDF concentration linearly increased intake and apparent total-tract digestibility of DM, OM, CP, and NDF. Cows fed diets with greater uNDF concentration rejected larger particles, whereas lower uNDF diets tended to decrease ruminal ammonia concentration. Milk yield and FCM yield increased linearly with decreasing diet uNDF. A 1 unit increase in uNDF was negatively associated with -0.59 kg/d in DMI and -0.48 kg/DIM yield and -0.75 kg/d in FCM yield, as well as decreases of 1.79% in DM digestibility and 1.73% in amylase-treated NDF digestibility. In conclusion, reducing the dietary uNDF concentration from 11.1% to 9.0% improved performance due to increased nutrient intake and digestibility, as well as altering the ruminal fermentation.