To distinguish between the direct effects of heat stress (HS) and reduced DMI on metabolic and physiological variables, morphometry, and gene expression of transporters involving in ruminal VFA absorption, 16 Holstein bull calves (178.1 ± 7.55 kg of BW) were used in a randomized complete design using 2 experimental periods. In period 1 (P1), all animals were housed in thermoneutral (TN) conditions (20.7 ± 3.0°C and a temperature–humidity index [THI] of 65.2 ± 3.1) and fed ad libitum for 8 d. In period 2 (P2; 9 d), half of the calves (n = 8) were subjected to HS condition (29.9 to 41.0°C and a THI ≥ 85 for 8 h/d) and the other half (n = 8) were maintained in TN conditions but were pair fed (pair-fed thermoneutral [PFTN]) to the HS calves. Heat stress decreased DMI (20%; P < 0.01) and, by design, PFTN calves had similar reduced intakes. During P1, ADG was similar between groups (1.7 kg/d), but ADG was reduced similarly for both groups (0.14 kg/d) during P2 (P < 0.01). Compared with P1, HS calves had decreased plasma glucose levels (12.5%; P < 0.01) in P2. Basal insulin levels increased in P2 for HS calves (71%; P < 0.03), but there was no difference between periods for PFTN calves. Circulating NEFA concentrations did not differ between periods in the HS calves, but PFTN animals had increased basal NEFA levels (100%; P < 0.01). Compared with P1, HS calves had increased blood urea nitrogen concentration in P2 (P < 0.01) but blood urea nitrogen did not differ between periods in the PFTN calves. Heat stress had no measurable effect on nutrient digestibility. Rumen papillae height increased (51%; P < 0.04) in HS calves whereas papillae top width decreased (40%; P < 0.03), but there was no difference between treatments for other morphometric parameters. Gene expression of VFA transporters in rumen epithelium was not affected by thermal treatments. These results confirmed that short-term HS in the current study had no effect on diet digestibility, rumen morphology, or VFA transporters genes. Therefore, HS-induced decrease in DMI is in large part responsible for gross changes in BW in growing calves (albeit tissue composition of growth is not unknown).

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