Effect of Varying Rumen Degradable and Undegradable Protein on Milk Production and Nitrogen Efficiency in Lactating Dairy Cows under Summer Conditions

The objective is to determine the effect of reducing nitrogen input through feeding low rumen degradable protein (RDP) and rumen undegradable protein (RUP) proportions on milk production, nitrogen efficiency and metabolism in heat-stressed cows. Forty-eight mid-lactating, Holstein cows were assigned to treatments using a randomized block design in a 2x2 factorial arrangement of treatments (n = 12/treatment). Treatments included two levels of RDP (10 and 8%) and two levels of RUP (8 and 6%). From d 1 to 21, a common diet (10% RDP-8% RUP) was fed to cows followed with their respective treatment diets fed from d 22 to 42 of the study. Cows were housed in a freestall barn and exposed to the prevailing temperature and humidity of July and August with no supplemental cooling. Milk samples were collected and analyzed, and plasma was harvested for analysis of metabolites from d 42. Treatment differences were tested using the MIXED procedure of SAS and reported as least square means ± [plus or minus] standard error of the mean. Rectal temperatures increased from a.m. to p.m., indicating cows were experiencing heat stress. The 10% RDP treatment decreased vaginal temperatures compared with 8% RDP in the 8% RUP (39.0 vs. 39.4 ± 0.14°C), but remained unchanged in the 6% RUP treatment (39.4 vs. 39.3 ± 0.14°C). The 8% RDP treatment increased energy-corrected milk (ECM) compared with 10% RDP in the 6% RUP treatment (31.7 vs. 29.4 ± 0.76 kg/d), but reduced ECM in the 8% RUP treatment (32.5 vs. 33.0 ± 0.76 kg/d). The 8% RDP treatment improved nitrogen utilization efficiency compared with 10% RDP (35.1 vs. 31.6 ± 0.76%). The 6% RUP treatment improved nitrogen utilization efficiency compared with 8% RUP (35.1 vs. 31.6 ± 0.76%). The 8% RDP treatment increased glucose concentrations compared with the 10% RDP treatment (3.13 vs. 2.98 ± 0.07 mmol/L). The 8% RDP treatment decreased insulin vi concentrations compared with the 10% RDP treatment (15.8 vs. 20.9 ± 1.55 μU/mL). Therefore, diets with low RDP and RUP may increase nitrogen utilization efficiency and metabolism without reducing milk production in heat-stressed dairy cows.