Histidine requirement of dairy cows determined by the indicator amino acid oxidation (AAO) technique

Abstract Text:

The indicator AAO technique has been used successfully to quantify AA requirements in pigs and poultry. This technique was used to evaluate His requirement in dairy cows. Six lactating dairy cows were used in a 6×6 Latin square design, with 7-d periods. Cows were fed a TMR balanced to provide 110% and 75% of energy and metabolizable protein (MP; 1653 g/d) requirement, respectively. All AA (813 g/d; casein profile), excluding His, were infused into the abomasum to supply 105% of MP requirement. Treatments were abomasal infusion of His at 0, 7.6, 15.2, 22.8, 30.4 and 38.0 g/d, representing 1.50, 1.83, 2.15, 2.46, 2.78, and 3.09 % of MP. On d6, [1-¹³C]leucine was infused intravenously for 5 h (4.0 mmol/h; prime dose 4.0 mmol). Six blood samples were collected every 20 min during the last 2 h of infusion and the isotopic enrichment of plasma keto-isocaproic acid and CO₂ used to estimate whole body (WB) Leu irreversible loss rate (ILR) and oxidation. On d7, NaH¹³CO₂ was infused (2 mmol/h; prime dose 2.8 mmol) intravenously, with a similar schedule as d6, to estimate the WB ILR of CO₂.

	His supply (% of MP)						SEM	P value
	1.50	1.83	2.15	2.46	2.78	3.09		Linear	Quad.
DMI, kg/d	19.5	20.2	20.1	20.3	20.3	20.3	0.18	0.01	0.06
Yields
Milk, kg/d	38.6	42.1	42.0	42.5	42.8	42.4	0.66	0.002	0.02
True protein, g/d	973	1141	1152	1168	1163	1139	20.7	<0.001	<0.001
Leu oxidation, mmol/h	34.2	24.9	25.2	21.9	24.5	23.3	1.88	0.01	0.02
Plasma concentrations
His, µM	18.0	24.7	46.4	63.3	61.2	68.4	3.42	<0.001	<0.01
Carnosine, µM	6.4	6.9	8.4	9.2	9.3	9.8	0.56	<0.001	0.29
Anserine, µM	4.8	6.5	5.5	6.6	5.5	5.5	0.71	0.77	0.18

 Leucine and CO₂WB ILR were not affected by treatments and averaged 126.8 ± 2.15 mmol/h and 23.4 ± 0.98 mol/h, respectively. Yields of milk and milk protein, and Leu oxidation (indicator AAO) all indicated that a His supply of 1.83%MP was sufficient to meet requirement. However, the consistent decrease in plasma concentrations of carnosine (ß-alanyl-His dipeptide) below 2.46%MP suggests provision of His from endogenous pools; this could temporarily mask a dietary deficiency. The indicator AAO method is sensitive and provides similar estimates of requirement to those based on milk protein yield. Nonetheless, to accurately estimate His requirements needs consideration of changes in endogenous pools of His.

Keywords: Dairy cow, amino acid oxidation, histidine