There is limited research regarding the response of circulating His to post-ruminal His infusion. Histidine is unique in the sense that cows can rely on endogenous pools of His (anserine, carnosine, hemoglobin) to meet requirements. The objective of this study was to investigate whether abomasal infusion of incremental amounts of His would result in a linear response in plasma His concentration. Five multiparous lactating Holstein cows (149 ± 92 DIM) fitted with ruminal cannulas were used in a replicated 5 × 5 Latin squares with 7-d experimental periods. All the cows were fed a basal diet to provide adequate energy and metabolizable protein (NRC, 2001). Treatments were 0, 6, 12, 18, and 24 g/d of abomasally infused His. Blood samples were collected at 2, 4, 6, and 8 h after the morning feeding on the last 3 d of each period and composited by day. Plasma AA were quantified using a ultra performance liquid chromatography tandem mass spectrometer. Relationships between plasma His and His infused were described by the PROC REG procedure of SAS (Table 1). Incremental amounts of His infused increased plasma His and the sum of plasma His + carnosine + 3-methly His (R² = 0.93 and 0.92, respectively). However, plasma His expressed as the percentage of total AA or total AA minus plasma His improved the linear relationship (R² = 0.97 and 0.97, respectively). Data from this study showed a strong linear response between abomasal infusion of His and plasma His concentration, particularly when His was expressed as proportion of total AA (TAA) or TAA minus His metabolites. Table 1. Regression equations in cows abomasallyinfused with incremental amounts of His

Iteration	Equation	SE	CI	CV	R2
His, µM	Y = 0.6437x + 55.73	0.103	0.2011	3.07	0.93
His, %TAA	Y = 0.0249x + 2.35	0.003	0.0053	1.92	0.97
His, %TAA-His	Y = 0.0263x + 2.41	0.003	0.0049	1.88	0.97
His+Car+3MHis, µM	Y = 0.6674x + 79.68	0.117	0.2295	2.53	0.92
His+Car+3MHis, %TAA	Y = 0.0255x + 3.36	0.002	0.0042	1.36	0.97
His+Car+3MHis, %TAA − His-Car-3MHis	Y = 0.0276x + 3.48	0.002	0.0038	1.30	0.97