The excessive use of nitrogen (N) has a negative impact on the environment and increases feed costs for dairy farmers. Work to identify the absolute metabolizable protein (MP) requirements for high producing lactating dairy cattle indicate that those requirements can be met at much lower dietary crude protein levels than are currently in use in the field. However, the intestinal digestibility (ID) of N should be known to formulate closer to requirements. The objective of this study was to evaluate the ID of animal proteins using the assay of Ross et al. (2013) when applied in the Cornell Net Carbohydrate and Protein System (CNCPS) v6.5. A blend of animal proteins were developed from blood meal with an ID of 74.6% N and a feather meal with an ID of 54.6% N. Both diets were isocaloric and supplied metabolizable energy allowable milk of 51 kg at 27 kg DMI and 15.8% CP. The high intestinal digestibility (HID) diet was formulated with 1.18 kg blood meal, whereas the low intestinal digestibility (LID) diet was formulated with 1.3 kg of a blend of 82.8% feather meal and 17.2% of the blood meal, and when the ID was used for formulation, the MP allowable milk was 45.8 kg/d and 43 kg/d, respectively. Multiparous and primiparous cows (n = 96) between 80 and 150 d in milk (DIM) were balanced between the 2 treatments in a replicated pen design of 16 cows per pen, 3 pens per treatment. Milk yield and dry matter intakes were measured daily, while components, body weight (BW), and body condition score (BCS) were measured weekly and the study was conducted for 63 d. Data were analyzed using JMP and the Mixed Model procedure. These results indicate that cattle responded to the difference in predicted ID digestibility and the ID assay can be useful in diet formulation to improve predictive capacity (Table 1). Table 1.

Item	HID	LID	SE	P-value
N	48	48
BW, kg	743	745	0.4	0.5
DMI, kg/d	26.2	25.7	0.4	0.5
Milk, kg/d	44.5	43.1	0.5	0.1
ECM, kg/d	49.5	46.2	1.7	0.04
Fat, %	4.08	3.81	0.10	0.04
Protein, %	3.03	2.93	0.04	0.06
Fat, kg/d	1.88	1.70	0.05	0.01
Protein, kg/d	1.39	1.31	0.01	0.008