The objective of this study was to evaluate the inclusion of a rumen-protected carbohydrate (RUPCA) (US Patent # 8,507,025) on performance, blood and plasma metabolites in growing heifers. Crossbred heifers (n = 135; 136 ± 14kg) were used in an 84-d experiment. Heifers were blocked by initial BW, placed into 15 pens and fed a diet comprised of (DM basis) 38.8% corn silage, 41.5% dry corn, 2% minerals and vitamins mix, and 17.7% supplement or RUPCA, which varied depending on treatments. The supplement and RUPCA consisted of the same ingredients (58.1% soybean meal, 38.9% soluble carbohydrates, 2% urea and 1% mineral salt), differing in the processing of the carbohydrate (i.e., protected or not from ruminal degradation). Heifers within blocks were randomly assigned to 3 treatments: T0) 17.7% supplement (100% unprotected carbohydrate), T1) 8.85% supplement and 8.85% RUPCA, and T2) 17.7% RUPCA (100% protected carbohydrate). Body weight was measured on d 0, 21, 42, 63, and 84. Pen DMI was measured weekly from d 21 to 84. Blood samples were taken on d 0, 42, 63, and 84 from jugular vein prior morning feeding and analyzed for glucose, insulin, urea and NEFA concentrations. Data were analyzed as a randomized complete block design with repeated measures using a mixed model of SAS. Treatment × day interaction were found for DMI (P = 0.02), ADG (P < 0.0001) and G: F (P < 0.0001) and with T1 having the lowest DMI (P < 0.05) and the greatest G:F (P < 0.05). No differences were found in the concentrations of blood glucose (P > 0.91), plasma insulin (P = 0.82), plasma NEFA (P = 0.802) or plasma urea (P = 0.336). Feeding RUPCA to growing heifers improved G:F through lower DMI without altering ADG, blood or plasma metabolites. Results are shown in Table 1. Table 1.

Item	T0	T1	T2	SEM	P-value
Trt	Day	Trt × day
DMI, kg/d	6.9a	5.9b	6.8a	0.06	<0.0001	<0.0001	0.02
ADG, kg	1.18	1.13	1.19	0.027	0.21	<0.0001	<0.0001
G:F	0.161b	0.202a	0.177ab	0.0103	0.0003	<0.0001	<0.0001
Glucose, mg/dL	90.2	91.6	91.2	3.52	0.91	<0.0001	0.92
Insulin, µg/dL	0.26	0.26	0.24	0.053	0.82	0.016	0.72
NEFA, mM	200.8	187.9	181.3	18.74	0.54	0.0025	0.42
Urea, mg/dL	14.9	13.4	14.8	1.03	0.36	<0.0001	0.39

^a,bMeans without common superscript differ (P < 0.05).