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Simple Summary: The selection of highly efficient animals will support meeting the world's future demand for products and food of animal origin. Thus, the identification of efficient animals and an understanding of the mechanisms inherent to this efficiency is fundamental for the progress of breeding systems. In the present study, we identify highly efficient animals for residual feed intake in dairy heifers. This animal category is unexplored in relation to this index. We utilized the classical parameters evaluated in cattle of different ages to carry out the study on these animals. The objectives of this study were: (1) to identify and rank phenotypically divergent animals for residual feed intake (RFI) regarding their efficiency (high: HE or low: LE); (2) to evaluate their relationships with ingestive behavior, ruminal and blood parameters, performance, and infrared thermography; and (3) to determine if such measurements can be used as feed efficiency markers in rearing dairy heifers. Thirty-eight heifers, 143 d ± 4 (Mean ± SD) of age and 108.7 kg ± 17.9 of body weight were used. The animals were fed with a total mixed ration during the 91 d of the trial. A phenotypic divergence of DMI for RFI was observed between −0.358 and 0.337 kg/d for HE and LE, respectively. Dry matter intake (DMI) was lower in the HE (2.5 kg DMI/d vs. 3.1 kg DMI/d), as was the number of visits to the feed bin with consumption (59 vs. 71). Feed intake was the best predictor of said divergence. Water intake and number of visits to the feed bin were presented moderate correlations with RFI. The ruminal fermentation variables, blood metabolites, blood hormones (such as the other ingestive behavior variables), and infrared thermography were not able to accurately predict HE or LE animals. [ABSTRACT FROM AUTHOR]

Highlights • Hr-Tag rumination monitoring system was evaluated for calves at the weaning period. • There is a high correlation between Hr-Tag and real rumination time values. • Hr-Tag system can identify differences in rumination time among weaning periods. • Adjustments in Hr-Tag system are needed for lower rumination values. Abstract Rumination time is a relevant marker for health evaluation. The use of rumination monitoring systems can decrease labor costs when measuring rumination time. The objectives of this study were to: i) validate the Hr-Tag rumination monitoring system for estimating rumination time in Zebu calves during the weaning period and ii) to evaluate the Hr-Tag capacity to identify differences in daily rumination patterns during the weaning period. Thirty-two Gyr calves averaging 69 ± 7.6 kg body weight and 77.0 ± 4.0 days were used. Calves were fed 4.7 ± 0.5 L/day of whole milk and ad libitum commercial textured starter ration up to 83 days old (pre-weaning period). During the weaning period (84–86 days old) the same starter was used, but corn silage was also available and only half the milk dose was offered. After weaning (87–92 days old, post-weaning period) only corn silage and starter were used. For the Hr-Tag system validation, the rumination time obtained by Hr-Tag was compared with visual observations at four 2 h intervals (0600 to 0800, 0800 to 1000, 1400 to 1600, and 1600 to 1800 h) at pre- and post-weaning. Two observers were used in all visual evaluations. To compare daily rumination pattern at pre-weaning, weaning and post-weaning, only the Hr-Tag data was used. Rumination time values between observers were highly correlated (ρ = 0.956; P < 0.001). Visual and Hr-Tag rumination time did not follow a similar probability distribution (P < 0.001), but they were correlated (ρ = 0.702; P < 0.001). The Hr-Tag system overestimated rumination time, mainly when the calves had visual rumination activity close to or equal to zero. The Hr-Tag detected differences in rumination time between weaning periods (P < 0.05). In conclusion, the Hr-Tag system overestimates rumination time in weaning calves, mainly by recording nonexistent rumination. However, the Hr-Tag system recorded rumination values correlated to visual observations and detected differences in rumination activity among weaning periods. [ABSTRACT FROM AUTHOR]