PS13-26. Test Length Recommendations for Determining Residual Water Intake.

Abstract As pressure on freshwater resources intensifies, identifying water-efficient livestock is increasingly important for improving the sustainability of beef production systems. Residual water intake (RWI) has been proposed as a selection tool analogous to residual feed intake. Recent advances in precision livestock monitoring have allowed for unprecedented access to high-resolution animal data. However, standardized guidelines for RWI test duration remain undefined despite reassessment of test duration guidelines for feed intake-based traits. Furthermore, existing efficiency testing frameworks are largely adapted from feed intake studies and may not reflect the trajectory of water efficiency over time. We deployed RFID-enabled front-end weighing stations integrated with custom metered waterers alongside Growsafe 8000 feed bunks. These systems facilitated the collection of daily body weight, feed intake, and water intake data at the individual animal level. From 2019-07-08 to 2024-02-16, we evaluated 2,020 animals for individual daily weight, feed intake, and water intake across both research locations. Data collection spanned 1,055 days, covering all months of the year. The dataset includes water intake records from 1,255 bulls, 434 steers, and 331 heifers. Residual water intake was calculated within-test as the residual from a linear regression of average daily water intake on metabolic mid-test body weight (MMWT) and average dry matter intake (DMI). To evaluate test length requirements, RWI was computed iteratively using an expanding window approach, where MMWT, DMI, and water intake were recalculated for each test day. This approach enabled direct characterization of the trajectory of water efficiency within-test. Final RWI was defined as the value obtained on the last day of each test. Stability of RWI rankings was assessed by correlating rolling RWI estimates with final RWI within test. In drylot systems, moderate ranking stability (Spearman ρ ≥ 0.80) was achieved by day 17, with 94.6% of animals in all tests meeting this threshold. High stability (ρ ≥ 0.90) was reached by day 44, at which point all animals were in tests exhibiting strong agreement with final rankings. Very high stability (ρ ≥ 0.95) was observed by day 85 among tests of sufficient duration, indicating diminishing returns in extending test length beyond this point. These results demonstrate that the trajectory of water efficiency-stabilizes rapidly and can be reliably estimated in substantially shorter timeframes than traditionally assumed for intake-based traits. A test duration of approximately 40–45 days appears sufficient to achieve robust ranking of animals for water intake efficiency, while shorter durations (∼17 days) may provide useful early-stage screening. This framework provides a foundation for incorporating water intake efficiency into breeding and management decisions, enabling more targeted selection for reduced water use in beef production systems.