Nitrogen partitioning and microbial protein synthesis in lactating dairy cows with different phenotypic residual feed intake

Table 5 The urinary purine derivatives (PD) and estimated MP supply to the dairy cows selected for phenotypic divergence in residual feed intake (RFI)

Items	Low RFI	High RFI	SEM	P value^a
Items	Low RFI	High RFI	SEM	RFI	Day	RFI × Day
Urine volume^b, L/d	33.6	33.4	2.64	0.96	< 0.01	0.07
Urinary PD, mmol/d
Allantoin	489	520	33.1	0.51	0.60	0.16
Uric acid	39.4	51.1	4.80	0.11	0.09	0.40
Endogenous PD	54.2	49.9	2.87	0.31	0.29	0.23
Sum	474	484	40.2	0.86	0.70	0.77
MCP^c, g/d	2152	2198	183	0.86	0.70	0.78
IADP^d, g/d	695.5	760.8	22.5	0.06	0.07	0.80
MP^e, g/d	2065	2171	100.2	0.47	0.36	0.58

^aP value associated with RFI, time, and the interaction of RFI and time
^bUrine volume (L/d) = BW (kg) × 29 (mg/d)/creatinine (mg/L) [24]
^cMicrobial protein (MCP) was indirectly estimated by the equation [21]: MCP = (allantoin + uric acid - endogenous PD) × 70 × 6.25/ (0.116 × 0.83 × 1000)
^dIntestinally absorbable dietary protein (IADP) = RUP × CP intake × IDP, where IDP is the measured intestinal digestibility of rumen undegraded protein (RUP). The feedstuff incubated in the rumen for 16 h was used to determine the IDP according to a modified 3-step procedure [25]
^eMP = IAMCP + IADP; IAMCP = Intestinally absorbable MCP = MCP × 0.64 [13]

ISSN: 2049-1891