Effect of dietary metabolizable protein level on digestion and ruminal fermentation of Holstein and Ayrshire dairy cows.

The objective of this study was to determine the effect of dietary metabolizable protein (MP) level on enteric methane production and milk production of Holstein and Ayrshire dairy cow breeds. Nine Holstein (DIM = 81 ± 20; milk yield = 42.3 ± 4.2 kg; BW = 701 ± 71 kg) and nine Ayrshire (DIM = 91 ± 13; milk yield = 35.0 ± 3.4 kg; BW = 561 ± 58 kg) 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. Enteric CH4 (i.e., respiration chambers) and animal performance (intake and milk production) were determined over 5 and 7 days, respectively. Main effects of breed, MP level and interaction (breed × MP) were determined using the MIXED Procedure of SAS. Linear and quadratic contrasts were used to determine effects of MP level on variable responses. Significance was declared at P ≤ 0.05 and tendencies at 0.05 < P ≤ 0.10. Interaction breed × MP level was not significant for dry matter intake (DMI) or milk production. Intake of DM tended to increase linearly with increasing MP level and was higher for Holstein versus Ayrshire cows (25.7 vs, 20.5 kg/d). Milk yield increased linearly with MP level and was higher for Holstein versus Ayrshire cows (43.3. vs. 32.8, kg/d). Holstein cows produced more milk fat and milk protein (1.65, 1.32 kg/d, respectively) than did Ayrshire cows (1.33, 1.11 kg/d, respectively). Yields of milk fat and milk protein increased linearly and quadratically, respectively, with increasing MP level. Feed efficiency increased linearly with increasing MP level, but was not affected by breed. Methane emission was higher for Holstein versus Ayrshire cows (480 vs. 386 g/d) and increased linearly with MP level. Increasing MP level in the diet of Holstein cows increased linearly CH4 yield (17.5, 18.9, 19.6 g/kg DMI) whereas not effect was observed in Ayrshire cows (averaging 19.0 g/kg DMI; breed × MP level interaction, P ≤ 0.07). Results from this study show that milk production of Holstein and Ayrshire dairy cows responded similarly to increasing dietary MP level, even though DMI and production levels were very different between the two breeds. However, response of CH4 yield to increasing dietary MP level differed between the two breeds.