Finishing steers during the summer can be challenging due to the effects of high temperatures and humidity on DMI. The objective of this study was to evaluate the inclusion of a rumen-protected carbohydrate (RUPCA; US Patent # 8,507,025) on performance of finishing steers during heat stress. Temperature-humidity index average measured every day during the experiment was 72 ± 4.9. Crossbred steers (n = 135; 355 ± 20kg) were used in a 62-d experiment. Steers were blocked by initial BW and placed into 15 pens. Steers within blocks were randomly assigned to 3 treatments: T0 = fed 91.4% of a basal diet (% DM), 22.3% corn silage, 65.9% dry corn, 0.6% sunflower meal, 0.5% urea, 2% minerals and vitamins and 8.6% of a supplement containing (% DM) 58.1% soybean meal, 38.9% soluble carbohydrates, 2% urea and 1% minerals salts; T1 = fed the basal diet plus 4.3% supplement and 4.3% RUPCA; and T2 = fed basal diet plus 8.6% RUPCA. The supplement and RUPCA consisted of the same ingredients, differing on the processing of the carbohydrate (i.e., protected or not from ruminal degradation). Body weight was measured on d 0, 15, 39 and 62 relative to the beginning of treatments feeding (d 0). Pen DMI was measured on d 10, 18, 25, 31, 35, 46, 51, 56, 60 and 62. Back-fat on the 12th rib (BF) and LM area were measured on d 1 and 62. Data were analyzed as a randomized complete block design with repeated measures using a mixed model of SAS. Initial BW was used as a covariate. There were no differences between treatments on final BW, BF or LM area on d 62 (P > 0.10). Treatment × day interactions were observed for G:F, ADG and DMI (P < 0.05) suggesting a different response to treatments during periods of heat stress (Table 1). Feeding RUPCA may be beneficial for finishing steers under heat stress. Table 1.

Item	Treatment			SEM	P-value
T0	T1	T2	Trt	Day	Trt × Day
DMI, kg/d	9.9ab	9.8a	10.0b	0.07	0.04	<0.001	0.001
Initial BW, kg	287	285	285	0.6	0.23	—	—
Final BW, kg	352	357	353	3.1	0.51	—	—
ADG, kg	1.00	1.11	1.07	0.065	0.38	<0.001	0.039
G:F	0.105	0.116	0.109	0.0051	0.32	<0.001	0.004
Backfat 12th rib (62 d), mm	0.58	0.61	0.60	0.017	0.68	—	—
LM area (62 d), cm2	57.6	54.9	56.1	0.97	0.30	—	—

^abMeans without common superscript differ (P < 0.05).