Lysine is typically a limiting AA for milk production in North American dairy cow corn-based diets. The objective of this study was to evaluate the effects of a rumen-protected lysine prototype (RP-Lys; Balchem Corp., New Hampton, NY) on milk production in dairy cows. Ten multiparous Holstein cows (94 ± 25 DIM) were used in a 5 × 5 replicated Latin Square with 18-d treatment periods. Treatments were 1) control (CTL; 0 g of Lys fed as RP- Lys), 2) 17.5 g of Lys fed as RP-Lys (low dose; RPLL), 3) 21.8 g Lys fed as RP-Lys (medium dose; RPLM), 4) 31.51 g of Lys fed as RP-Lys (high dose; RPLH) and 5) 30 g of Lys fed as rumen protected Lys (APL, Aji-Pro L, generation 2, Ajinomoto, Chicago, IL). The basal diet was composed of 52% forage and 48% concentrate (DM basis) and was balanced assuming 27 kg/d DMI, 54 kg/d milk, 3.55% milk fat, 3.06% milk protein, and 669 kg BW. The basal diet was formulated to have MP and MP Lys balances of −93 and −15 g/d, respectively. Feed samples were collected to determine chemical and nutritional compositions. Milk samples were collected on d 15–18 of each period and analyzed for milk components. Data were analyzed using the Mixed procedure in SAS 9.4. ANOVA indicated no treatment effects (Table 1). Regression analyses showed linear increases in milk and lactose yields of 9 and 0.5 g for every 1 g of supplemental Lys prototype (P ≤ 0.05). There was a trend for a quadratic effect on milk protein percent. The results of this study indicate that supplementation with RP-Lys prototype promoted an increase in milk and lactose yields in mid-lactation multiparous cows. Table 1. Effects of rumen-protected Lys on milk production and milk composition

Item	Treatment					SEM
CTL	APL	RPLL	RPLM	RPLH
DMI, kg/d	29.5	29.8	30.2	29.3	30.0	0.52
Milk yield, kg/d	44.2	44.8	45.9	46.3	45.3	1.18
ECM, kg/d	50.0	49.7	52.3	51.4	50.4	1.81
Lactose, %	4.70	4.73	4.75	4.72	4.71	0.02
Lactose, kg/d	2.10	2.10	2.21	2.19	2.12	0.07
True Protein, %	3.15	3.12	3.10	3.12	3.15	0.03
True Protein, kg/d	1.39	1.38	1.44	1.43	1.40	0.07
Fat, %	4.40	4.40	4.37	4.21	4.26	0.22
Fat, kg/d	1.91	1.91	2.00	1.95	1.93	0.10