Abstract The nutritional demands of beef cows fluctuate across physiological and production stages, yet changes in efficiency as females transition from pregnancy to lactation and the effects of consuming endophyte-infected (EI) rations on metrics like residual feed intake (RFI) remain unclear. Therefore, this two-year study evaluated feed intake (FI) and physiological parameters of cow-calf pairs across pregnancy and lactation. In year 1, purebred Angus pregnant heifers (452.6±10.2 kg; 17-19 mo of age; n = 16) were fed an endophyte-free (EF) ration for 56 days. Individual FI was recorded using the Vytelle® Sense system, and weekly measurements were collected to monitor growth and physiological changes. In year 2, the same females (444.4±12.5 kg; 29-31 mo of age; n = 16) were reintroduced to the Vytelle® Sense system with their calves (166.4±8.5 kg; 5-7 mo of age; n = 16) during late lactation and were fed an EI ration. The same physiological measurements as in year 1 were recorded biweekly. Additionally, the Weigh-Suckle-Weigh (WSW) method was used to estimate individual energy-corrected milk production in cows. In both years, biweekly ration samples were collected to assess nutritional content and ergot alkaloid concentrations in the EI ration. A post hoc analysis of FI data estimated RFI for each year and assessed RFI category repeatability across years. Physiological data from cows and calves were analyzed using the MIXED procedure with repeated measures in SAS®. Effects of RFI category, time, and their interactions were evaluated for weight (BW), ADG, Body Condition Score (BCS), Hair Shedding Score (HSS), and FI. In year 2, these effects were also evaluated for milk (MILK) production in cows. For calves, effects of sex, dam RFI, time, and their interactions were analyzed for BW, ADG, BCS, HSS, and FI. In year 1, no differences were observed in BW, ADG, and BCS, while low-RFI (LRFI) heifers had lower HSS (2.3±0.2 vs 3.1±0.2; P = 0.0204) and tended to have lower FI (74.1±2.7 vs 80.9±2.4 kg DM; P = 0.0671) than high-RFI (HRFI) heifers. In year 2, 57% of dams retained the LRFI category and 78% retained HRFI. Based on this classification, no differences were observed in BW, ADG, BCS, and MILK, whereas LRFI cows had lower FI (73.6±3.4 vs 83.8±2.5 kg DM; P = 0.0159) and tended to have lower HSS (2.6±0.5 vs 3.7±0.4; P = 0.0987) than HRFI cows. For the calves, no differences were observed in BW, ADG, BCS, HSS, and FI. These findings indicate that the RFI category remains moderately consistent across pregnancy and lactation, and that a greater hair shedding ability may support feed efficiency in beef cattle.
Rajo-Gomez et al. (Wed,) studied this question.