It is commonly believed that ruminants exposed to sunlight, and consequently heat stress (HS), consume more concentrate and lower forage, but there is limited evidence on this issue. The objective of this study was to determine the productivity and voluntary intake of concentrate and alfalfa hay in a cafeteria delivery method in the outdoor (OUT)- versus indoor (IN)-housed growing Afshari lambs during summer. Thirty intact male lambs (30.4 ± 4.6 kg) were randomly housed OUT and subjected to sunlight without any shade or kept IN under shade for a period of 42 days. The average temperature humidity index for IN and OUT environments were 69.8 and 70.7 (over ∼72 for more than 12 h/day), respectively. Feed intake (concentrate and hay, separately), rectal temperature and respiration rate were measured two times a day at 0800 and 1530 h through the experiment. Although hay intake was not different between two groups since morning feeding until 1530 h, OUT lambs had lower concentrate intake (611 vs. 535 g for IN and OUT, respectively; P < 0.02) at the same time. Total concentrate consumption, however, tended to be greater for OUT compared to IN lambs (1 418 vs. 1 513 g/day for IN and OUT, respectively; P < 0.06), without any difference in hay intake. Accordingly, OUT treatment resulted in greater (∼120 g/day; P < 0.04) total dry matter intake (concentrate +hay). The OUT lambs tended (∼1.3 kg; P < 0.07) to outweigh IN lambs at the end of the experiment. Despite the fact that both groups had similar average daily gain (ADG) until the half of the experiment, IN lambs had lower ADG (369 vs. 431 g/day; P < 0.01) until the end and through the study. Feed efficiency was not affected by treatments. The morning rectal temperature was influenced by treatment whereby IN animals were warmer (∼0.2 °C; P < 0.01), whereas the evening rectal temperature was greater in OUT lambs (∼0.2 °C; P < 0.04) and had more evening respiration rate (∼53 more breath per minute; P< 0.01). Among the blood
