To determine the effect of a limited supply of His, 5 multiparous Holstein cows fitted with catheters were used in a 4 × 4 Latin square plus one cow, with 14-d periods. Cows were fed a diet balanced to supply 103% of NE_L and 72% of MP requirements (1610 g/d; NRC, 2001). Treatments were abomasal infusion of His at 0, 7.6, 15.2 or 22.8 g/d plus a mixture of AA (595 g/d, CN profile; from Ajinomoto). At each period, 6 sets of blood samples were collected from arterial, portal, hepatic and mammary sources. Splanchnic plasma flow was calculated by downstream dilution of deacetylated para-amino-hippurate and mammary plasma flow, using the Fick principle. Results are given in Table 1. Mammary plasma flow was not affected (P = 0.75) by His deletion, averaging 690, 644, 682, and 690 ± 34 L/h, for incremental doses of His. Portal absorption accounted for 108% of the infused His at the highest dose suggesting very little catabolism of His across the gut. The mammary uptake:ouput ratio did not differ from unity across infusion rates. However, the splanchnic release was not sufficient to account for MG uptake at the 0 dose, in line with decreased muscle carnosine (a labile pool of His dipeptide) concentration at low His supply (JDS 97 E-Suppl.1:757). The increased hepatic removal observed as His supply increases was the sole responsible of the decreased His efficiency and was directly related to liver inflow of His. Table 1.

Item	His infused (g/d)				SEM	P-value
His flow (mmol/h)	0	7.6	15.2	22.8	Linear	Quadratic
Digestive flow1	8.9	8.9	9.1	9.1	0.12	0.08	0.92
Total supply (TSupply)	8.9	10.9	13.2	15.2	0.12	<0.001	0.92
Portal absorption	7.2	8.7	11.2	14.2	0.88	<0.001	0.39
Hepatic removal	−1.2	−1.4	−3.8	−5.0	0.83	0.005	0.53
Splanchnic release	5.9	7.5	7.5	9.8	1.48	0.11	0.82
Mammary (MG) uptake (U)	−6.7	−7.7	−7.5	−9.0	0.49	0.01	0.64
Milk output (O)2	6.3	7.5	7.8	8.3	0.24	<0.001	0.21
Liver fractional removal	0.04	0.02	0.04	0.04	0.016	0.72	0.55
MG fractional removal	0.65	0.40	0.22	0.23	0.033	<0.001	0.004
U:O	1.06	1.02	0.97	1.08	0.054	0.92	0.21
Splanchnic/MG	0.92	0.98	0.99	1.09	0.174	0.50	0.91
Maintenance net rqt3	1.9	1.9	2.0	2.0	0.02	0.08	0.91
Efficiency4	0.95	0.90	0.76	0.70	0.022	<0.001	0.93

¹NRC (2001). ²In protein synthesized within the MG, 28.9 mg His/g true protein. ³Maintenance requirement = scurf + endogenous urinary + metabolic fecal (Florida Symposium, 2014:166) excluding efficiency of utilization. ⁴(Milk output + maintenance)/Total supply.