Infrared thermography (IRT) is a noninvasive method that is one of the main indirect tools to optimize livestock measurements, as most changes in physiological conditions affect body temperature. This study aimed to evaluate the use of IRT to estimate residual feed intake (RFI) and residual intake and gain (RIG) of hair lambs, in addition to variations in performance, feed efficiency estimates, and gas emissions. Forty lambs were monitored during two feed efficiency tests to assess performance, feed efficiency estimates, and enteric gas emissions. These metrics were associated with IRT measurements from different body regions (eyes, hooves, rumen, and left flank). Temperatures were measured using infrared images collected with a Fluke camera, employing the non-steady-state rapid detection measurement method. Lambs were grouped into high (RFI-
RIG+), medium (RFI±
RIG±), and low (RFI+
RIG-) classes based on ± 0.5 standard deviations of adjusted RFI and RIG. To assess group differences, Tukey's test (P <
0.05) was applied, along with Pearson correlation and multivariate analyses to determine relationships between variables. In the 2nd test, differences in RIG classification were observed for eye and left flank temperatures. Some performance variables and feed efficiency estimates, such as dry matter intake, feed efficiency, and feed conversion, differed by RFI and RIG classifications. In the 1st test, only N