ffect of dietary metabolizable protein level on enteric methane production and milk production of Holstein and Ayrshire dairy cows.

The objective of this study was to determine the effect of dietary metabolizable protein (MP) level on ruminal fermentation and apparent total-tract digestibility in Holstein and Ayrshire dairy cow breeds. Six Holstein (DIM = 91 ± 16; milk yield = 43.1 ± 5.1 kg; BW = 701 ± 66 kg) and Six Ayrshire (DIM = 93 ± 15; milk yield = 34.6 ± 3.0 kg; BW = 551 ± 48 kg) multiparous lactating cows were used in a replicated 3 × 3 Latin square (35-d period; 14-d adaptation) with a 2 × 3 factorial arrangement of treatments. Cows were fed (ad libitum) a TMR formulated (NRC, 2001) to provide 85%, 100% or 115% of MP requirements. Nutrients digestibility and rumen fermentation characteristics were determined over 7 and 2 consecutive days, respectively. Main effects of breed, MP level and interaction (breed × MP) were determined using the MIXED Procedure of SAS. Significance was declared at P ≤ 0.05. Apparent total-tract dry matter digestibility was lower in Holstein versus Ayrshire cows versus (68.3 vs. 70.3%) and was not affected by dietary MP level. Ruminal pH and total volatile fatty acid (VFA) concentration averaged 6.27 and 134 mM and were not affected by breed or MP level. Molar proportions of acetate, propionate, and butyrate were not affected by breed or MP level. Ruminal NH3 concentration and molar proportion of branched-chain VFA (iso-butyrate and iso-valerate) averaged 8.07 mM and 2.86 mol/100 mol, respectively and were not affected by breed or MP level. Results from this study show that for both breeds, MP level has no effect on ruminal fermentation characteristics (pH, VFA concentration and molar proportions, NH3 concentration) and DM digestibility. Ruminal fermentation characteristics did not differ between Holstein and Ayrshire dairy cows, but total-tract digestibility of DM was higher for Ayrshire compared with Holstein.