MINIMUM VERSUS OPTIMUM CONCENTRATIONS OF FIBER IN DAIRY-COW DIETS BASED ON BARLEY SILAGE AND CONCENTRATES OF BARLEY OR CORN

Six primiparous and 6 multiparous lactating Holstein cows were used in a double 6 x 6 Latin square to determine the effects of the percentag e of forage fiber in diets containing concentrates based on barley or corn. Cows received one of six diets that provided three percentages o f neutral detergent fiber (NDF) from barley silage [19.5, 25.0, and 40 .9% of dry matter intake (DMI)] combined with concentrates based on ei ther barley or corn. The DMI (18.6 vs. 18.3 kg/d) and net energy for l actation (27.2 vs. 26.8 Mcal/d) were similar for cows fed barley and c orn diets, respectively. Consequently, milk yield (25.7 vs. 25.0 kg/d) and fat-corrected milk yield (22.3 vs. 21.7 kg/d) were not affected b y type of grain. Milk yields were similar for cows fed low and medium NDF diets, but yields declined when NDF from forage exceeded 25% of DM I because of declining intake of net energy for lactation. Effects of increased fiber from forage on milk fat content differed depending on diet; the low NDF diet based on barley caused milk fat to decline. Rum ination time was higher for diets based on barley than for diets based on corn (516 vs. 469 min/d), and, as the percentage of forage fiber i n the diet increased from low to high, rumination time increased more for cows fed diets based on corn than for cows fed diets based on barl ey. The minimum amount of forage fiber necessary in diets to avoid mil k fat depression appeared to be higher for barley diets than for corn diets, although milk fat depression may not be a valid criterion by wh ich to assess minimum fiber concentrations. Further research is needed to determine the minimum concentration of forage fiber needed to ensu re healthy ruminal function and cow longevity.