152. Reducing Crude Protein in Dairy Cow Diets: Effects of Metabolizable Protein Supply on Performance, Nutrient Digestibility, and Nitrogen Efficiency.

Abstract Reducing crude protein (CP) in dairy cow diets improves N use efficiency and enhances the sustainability of dairy production. However, the minimum true metabolizable protein (TMP) supply required to sustain performance under low-CP conditions remains uncertain. Therefore, we evaluated whether lowering TMP supply within a low-CP diet affects performance and nutrient utilization in lactating dairy cows. Twenty-eight Holstein cows (115±26 days in milk) entered a 3-week adaptation period (AP) and were blocked based on AP fat- and protein-corrected milk (FPCM), parity, and body weight. Cows then received either a Control diet (150g CP/kg DM and 84g TMP/kg DM) or a Low-TMP diet (150g CP/kg DM and 70g TMP/kg DM) for 6 weeks. Data were analysed using repeated-measures mixed models including treatment and week as fixed effects, cow nested within block as a random effect, and the AP mean of each variable as a covariate. Dry matter intake was not affected by dietary treatment (P > 0.46). Energy intake did not differ between groups (P = 0.50), whereas TMP intake reflected the dietary treatments (P < 0.01; Control = 2,098 and Low-TMP = 1,742±18.8 g/d). Body weight tended to be lower under Low-TMP diets (P = 0.08; 655±2.56 kg) than under Control diets (663±2.56kg). Milk urea was lower under Control diets (P = 0.03; 20.3±0.47 mg/dL) than under Low-TMP diets (22.4±0.47 mg/dL). Treatment×week interactions were detected for several parameters (Treatment × week P < 0.01, unless otherwise stated). Milk yield was lower under Low-TMP diets than under Control diets in week 5, but did not differ otherwise. Although milk fat and protein contents did not differ statistically (P > 0.12), FPCM was lower under Low-TMP diets from week 3 onwards, driven by lower milk fat yield from week 3 onwards and lower milk protein yield from week 4 onwards. Milk lactose yield was greater under Low-TMP diets than under Control diets during weeks 1 and 2 but did not differ thereafter. Feed efficiency and N efficiency were greater under Low-TMP diets in week 1, but this pattern reversed from week 4 onwards, when both efficiencies were greater under Control diets (Treatment × week P = 0.03 for N efficiency). Apparent total tract digestibility of DM and CP did not differ between diets (P ≥ 0.24), whereas NDF digestibility was lower under Low-TMP diets (52.8±1.74%) than under Control diets (56.9±1.74%). Overall, reducing TMP supply within a low-CP diet resulted in progressive reductions in milk component yields and FPCM. These results indicate that TMP intake below ∼2,100 g/d under these dietary conditions compromises productive performance.