Limitations of feed efficiency during the growth phase as an indicator of lactation performance in dairy Gyr heifers.

Feed efficiency (FE) is pivotal in dairy production systems. Improving FE reduces feed costs, the largest variable expense in dairy operations, while also decreasing greenhouse gas emissions, particularly enteric methane. In tropical dairy systems based on Bos indicus breeds such as Gyr, understanding the biological consistency of FE indicators across production phases is essential to guide genetic selection strategies. Our study aimed to evaluate the consistency of FE indicators between the growth and lactation phases in dairy Gyr cattle. A total of 171 heifers were evaluated during the growth phase using residual feed intake (RFIg) and residual gain (RG), and 139 cows were assessed during the first lactation using residual feed intake (RFI) and residual milk yield (RMY), considering milk yield observed as well as corrections for 4% fat and for gross energy. A subset of 49 animals had data available for both phases. High phenotypic correlations (0.97 to 1.00) were observed among the different RFI and RMY methods during lactation, confirming their consistency. However, phenotypic correlations between FE indicators across phases were not significant. Additionally, regression coefficients between growth-phase traits (RFIg and RG) and lactation-phase traits (RFI and RMY) were not statistically different from zero, with low R² values. Despite differences in FE indicators between the most and least efficient groups within each phase, animals classified as efficient during growth did not maintain the same profile during lactation. Early-life FE did not predict efficiency during lactation in Gyr cows, indicating the need for phase-specific evaluation in breeding programs.