Aiming to assess the performance and metabolic effects of heat stress (HS) in dairy ewes, 8 Lacaune ewes (75.5 ± 3.2 kg BW; 165 ± 4 DIM; 2.31 ± 0.04 L/d) were submitted to thermoneutral (TN) and HS conditions in a crossover design (2 × 21 d periods, 6 d washout). Conditions were (day-night, 12–12 h; THI, thermohygrometric index by NRC, 1971): TN (indoors, 20–15°C and 50 ± 5%; THI = 65–59) and HS (chamber; 35–28°C, 45 ± 5%; THI = 83–75), respectively. Ewes were milked × 2-daily and fed ad libitum. Milk was recorded and sampled for composition (MilkoScan FTIR) on the last week of each period. On first period (d 17, 19 and 21), 2 ewes from each treatment, were used for glucose (GLU) tolerance test (GTT, 0.25 g/kg BW), and lipogenic and lipolytic challenges by insulin (INS, 4.6 µg/kg BW) and epinephrine (2 µg/kg BW), respectively. Blood samples were collected at 12- time points (min −30 to 120) in respect to infusions and plasma ovine-INS analyzed by ELISA (Mercodia). Data were analyzed by PROCMIXED of SAS v.9.4 (SAS Inst. Inc., Cary, USA). Results (Table 1) showed that HS reduced 10% DM and increased 29% water intakes, without effects on milk yield and ECM-6.5%. Milk fat and protein contents decreased by 16% each, and SCC increased by 5%. Basal and final INS values in GTT were similar for TN and HS ewes, peaking at min 10, although HS tended to show higher INS levels than TN (min 5 to 30; P = 0.09). No differences between TN and HS ewes were detected in GLU responses to INS challenge, GLU reaching a similar nadir at min 30 (33.9 ± 2.4 mg/dL; P < 0.001). On average, NEFA response to INS challenge was lower in HS vs. TN (0.24 ± 0.03 vs. 0.36 ± 0.03 mmol/L, respectively; P = 0.032), whereas their response to epinephrine was greater in TN vs. HS ewes, the difference being observed from min 5 to 45 (P < 0.001). In conclusion, lactational performances of dairy ewes in late-lactation were moderately impaired by the HS conditions used, although milk fat and protein contents and fat mobilization were severely worsened. Dramatic impacts on cheese yielding and energy balance are expected in dairy ewes exposed to hot weather conditions. Study supported by MINECO Spain (Project AGL2013-44061-R). Table 1.

Item	TN	HS	SEM	Treatment	Period	Trt × Period
Feed, kg DM/d	2.49	2.23	0.08	0.040	0.51	0.62
Water, L/d	7.11	9.20	0.33	0.001	0.004	0.18
Milk, kg/d	1.71	1.87	0.13	0.37	0.14	0.77
Fat, %	6.81	5.74	0.18	0.21	0.70	0.62
Protein, %	6.37	5.51	0.16	0.003	0.69	0.07
SCC, log10	4.40	4.63	0.06	0.015	0.89	0.56