A meta analysis (22 studies, 83 dietary treatments) evaluated the impact of neutral detergent fiber (NDF) content and whole tract NDF digestibility (NDFD) on dry matter intake (DMI) and milk yield (MY) in high producing dairy cows fed corn silage and alfalfa based diets. Experimental diets were ingested at high levels to support high MY (24.1 and 37.3 kg/d, respectively of DM and MY). The average NDFD was of 42.9% (range: 20.0-68.0 %; within study standard error: ±4.0 %). DMI was negatively related with the dietary NDF (e.g. ±1.0 % of NDF caused ±160 g/d of DMI; r 2=0.51; P<0.01), while NDFD and NDF were positively related (r 2=0.74; P<0.01; ±1.0 % of NDF caused approximately ±1.0 % of NDFD). MY was not influenced by dietary NDF. Considering a subset of 5 studies, the regression between NDFD and corn silage:alfalfa ratio in the diet (r 2=0.84; P<0.01) allowed to estimate the NDFD of diets containing only alfalfa hay (55%) or corn silage (44%). In conclusion, the dietary NDF concentration influences the NDFD and this has implications both for DMI and MY of cows. Diets based on alfalfa hay have higher NDFD than those based on corn silage.
