Effect of Dietary Metabolizable Protein Level on Energy Partitioning of Ayrshire and Holstein Dairy Cows.

This study aimed to determine the effect of dietary metabolizable protein (MP) level on energy partitioning of Ayrshire and Holstein dairy cows. Six Holstein and 6 Ayrshire lactating cows were used in a replicated 3 × 3 Latin square (35-d periods; 14-d adaptation) with a 2 × 3 factorial arrangement of treatments. Cows were fed (ad libitum) a total mixed ration formulated (NRC, 2001) to provide 85%, 100% or 115% of MP requirements. Energy expenditure in methane (respiration chambers; 5 d) and in milk, feces, and urine (7 days) were measured. 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. The breed × MP level interaction was not significant for measured variables. Gross energy (GE) intake was not affected by increasing MP level but was less for Ayrshire versus Holstein cows (93 vs. 117 Mcal/d). Loss of energy (Mcal/d) in methane and urine increased linearly with increasing dietary MP level, while fecal energy losses were not affected by MP level. When expressed as proportion of GE intake, only urinary energy losses increased with increasing level of MP, while proportional methane and fecal energy losses were not affected. Fecal energy losses (% GE intake) were less for Ayrshire than Holstein cows (30.0% vs. 32.2%), but urinary and methane energy losses were not affected by breed (2.5%, 5.4%, respectively). Milk energy expenditure (Mcal/d) or efficiency (% GE intake) increased linearly with increasing dietary MP level. Compared with Holstein cows, daily milk energy expenditure of Ayrshire cows was 20% less (24.0 vs 30.3 Mcal/d), but milk energy efficiency averaged 25.8% and was not affected by breed. Results from this study show that energy expenditure in Holstein and Ayrshire dairy cows responded similarly to increasing dietary MP level, even though energy intake and milk energy expenditure were very different between the two breeds.