71. Physiological and Performance Responses of Beef Steers Divergently Classified for Residual Water Intake.

Abstract Residual water intake (RWI), a moderately heritable trait, has emerged as a novel metric for evaluating water use efficiency in livestock. However, its association with animal productivity and enteric methane emissions remains poorly understood. This study examined the relationships among RWI, growth performance, feed intake, and gaseous emissions of beef cattle fed a sorghum silage-based diet for 130 d. Forty-eight crossbred beef steers (n = 48; 434 ± 48 kg initial BW; 488 ± 68 d initial age) were blocked by body weight and randomly assigned to one of four pens. Each pen was equipped with two GreenFeed emission monitoring system (C-lock Inc., Rapid City, SD) for measuring gaseous emissions, three electronic feed bunks and one in-pen water intake and weighing system (Vytelle, Lenexa, KS) for recording individual daily feed intake, daily water intake (WI) and daily body weight (BW), respectively. The RWI index was determined by regressing WI against DMI and mid-test metabolic weight and animals were classified as low (n = 15), medium (n = 16), or high (n = 17) RWI. Statistical analyses were conducted in JMP Pro (v.16; SAS Institute Inc., Cary, NC) using a mixed model that included the fixed effect of RWI class and pen as random effect. Significance was declared when P≤ 0.05 and tendencies at P≤ 0.10. Water-efficient steers (low RWI) consumed 22% less water daily than inefficient (high RWI) steers (P < 0.05) and spent less time at the water trough (P < 0.05). Water intake relative to dry matter intake (WI:DMI), average daily gain (WI:G), and metabolic body weight (WI:MBW) was lower in low-RWI steers compared with high-RWI steers (P < 0.03), confirming improved water-use efficiency across functional scaling metrics. Dry matter intake (DMI), initial body weight (BW), final BW, metabolic BW, residual feed intake (RFI), and DMI expressed as a percentage of BW did not differ among RWI classes (P > 0.15). However, low-RWI steers exhibited greater average daily gain (ADG) and improved gain-to-feed ratio (G:F) compared with high-RWI steers (P < 0.04), indicating enhanced growth performance despite similar feed intake. Water-efficient steers tended to produce 13% lower methane intensity (CH4, g/kg ADG; P = 0.09). In contrast, absolute methane production (CH4, g/d), methane yield (g/kg DMI), methane per metabolic BW, carbon dioxide (CO₂), and hydrogen (H₂) emissions were not different among RWI classes (P > 0.46). Residual water intake was strongly and positively correlated with daily water intake (r = 0.70; P < 0.0001), WI:DMI (r = 0.97; P < 0.001), WI:G (r = 0.71; P < 0.001), and WI:MBW (r = 0.80; P < 0.001), supporting the biological consistency of the RWI classification. These findings indicate that water-efficient cattle are also more feed-efficient and likely to emit less methane, highlighting RWI as a valuable trait for improving environmental and resource-use efficiency in beef production.