Weather Resilience of Hays Converter Beef Heifers with Divergent Residual Feed Intake Across Different Environmental Conditions.

This study examines whether efficiency of feed utilization affects heifer resilience in intensively fed Hays Converter beef heifers in Western Canada from 2019 to 2023. A total of 106 heifers were tested in five yearly contemporary groups; 18-27 per year, each group tested for 75 ± 5 d to determine residual feed intake (RFI) during winter (2019; age 8 mo ± 16 d) and spring (2020-2023; age 12 mo ± 12 d). Performance parameters, including dry matter intake (DMI), and feeding behavior, were recorded daily, and body weight (BW), average daily gain, and gain-to-feed ratio (G:F = ADG/DMI) recorded weekly. Environmental parameters over the years were used to calculate the current effective temperature index (CETI), which was used as a continuous covariate. Resilience was assessed using individual nonlinear models that predicted cumulative feed intake (CFI). Deviations from the modelled CFI curve lasting at least two consecutive days and exceeding 1.5 standard deviations (or 3% of the expected CFI) were classified as perturbations, each labelled as an intake shortfall or an intake increase. Resilience indicators were then analyzed as linear mixed models with RFI class, CETI, perturbation start day (PSD), perturbation direction, and their interactions as fixed effects, and year as a random effect. Environmental conditions affected all performance parameters (P < 0.05). Low-RFI heifers ate less (6.51 vs 7.59 kg DMI/d; P < 0.001) and converted feed more efficiently (0.182 vs 0.144 kg/kg; P < 0.001) while gaining at the same rate (P = 0.98). Of all identified perturbations (383 total), 198 were shortfalls, and 185 were intake increases. Intake increases were predominant during cold risk. No resilience indicator, nor the number of perturbations per heifer, differed among RFI classes; low-RFI heifers were numerically the most stable, with only a tendency for lower deviation variance (P = 0.095). Our data indicate that perturbation number and severity are driven by climatic risk. These findings suggest that extreme climatic risk strongly affects heifer performance and that selecting for efficiency of feed utilization does not reduce animal resilience under climate conditions of Western Canada.