This trial assessed whether dietary content of CP or supplementation with vigna hay (Vigna sinensis) would improve milk production or efficiency of lactating cows. Thirty-two multiparous Holstein cows were blocked by DIM into 8 squares of 4 cows in a replicated 4 × 4 Latin square with a 2 × 2 arrangement of treatments: 15.5 or 17% CP, and with or without vigna hay (16.6% CP), which replaced 12.5% of dietary DM from sorghum silage (8.4% CP). Soybean meal was fed to adjust dietary CP; diets contained about 50% forage and 50% concentrate, and 38% NDF. Cows within squares were randomly assigned to treatment sequences and fed diets for 3-wk periods; data from the last week were analyzed using the mixed procedures of SAS. Table 1 reports LS-means. Diet did not alter yield of milk or milk components. Higher dietary CP increased DMI but also increased MUN and reduced N efficiency without improving yield. Supplementing dietary sorghum silage with vigna hay maintained production, reduced DMI and MUN, and increased milk/DMI, N efficiency and apparent NDF digestibility. Daily income over feed cost (IOC) averaged $0.85/cow more on the vigna hay diets. Results from this trial indicated that replacing a portion of dietary forage with vigna hay improved nutrient and economic efficiency of milk production. Table 1. Effect of dietary CP (15% and 17%) and vigna hay supplementation (0 and 12.5%) on production

Item	15.5% CP			17% CP		SEM	Probability
0	12.5	0	12.5	CP	Vig	CP × Vig
DMI, kg/d	19.5	19.0		20.4	19.8	0.34	<0.01	<0.01	0.73
Milk yield, kg/d	28.0	29.3		29.1	29.1	0.77	0.35	0.19	0.23
Milk/DMI	1.45	1.55		1.43	1.48	0.047	0.08	0.01	0.33
Fat yield, kg/d	0.96	0.99		0.98	1.02	0.035	0.38	0.17	0.78
Protein yield, kg/d	0.88	0.91		0.92	0.92	0.024	0.17	0.31	0.30
MUN, mg/dL	17.4	16.1		19.6	18.6	0.33	<0.01	<0.01	0.54
Milk N/NI, %	28.2	30.0		25.8	27.4	0.88	<0.01	<0.01	0.81
NDF digestibility, %	42.4	52.1		41.5	52.5	0.68	0.68	<0.01	0.31
IOC, USD/cow/d	9.40	10.47		9.23	10.06	0.438	0.28	<0.01	0.67