Feeding Behavior of Finishing Pigs under Diurnal Cyclic Heat Stress

**Background:** Pigs in tropical climates are frequently exposed to high ambient temperatures, which reduce voluntary feed intake and alter feeding behavior. Cyclic heat stress (CHS), where temperature rises during the day and falls at night, may allow partial behavioral compensation. Lighting programs also influence circadian feeding rhythms. This study aimed to quantify how CHS and light-on/off events affect meal patterns in finishing pigs.

**Methods:** Ninety crossbred gilts (initial BW 67.7 ± 6.2 kg) were housed in two identical rooms (45 pigs/room) at São Paulo State University, Brazil. One room was maintained at thermoneutrality (TN, 22°C constant), the other under CHS (22°C from 20:00–08:00 h, rising to 35°C from 08:00–20:00 h). Pigs had ad libitum access to feed via 10 Automatic and Intelligent Precision Feeders per room, which recorded each visit (start/end time, feed consumed). The lighting program provided 12 h of light (06:00–18:00 h). After a 7-day adaptation, data were collected over 38 days divided into EP1 (days 0–20) and EP2 (days 21–38). A meal criterion of 49 min was determined using the p(start) method. The day was divided into four periods: PI (06–08 h, light-on), PII (08–18 h, diurnal/HS), PIII (18–20 h, light-off), and PIV (20–06 h, nocturnal/TN). Data were analyzed using the MIXED procedure in SAS with fixed effects of temperature (T), period (P), experimental phase (EP), and their interactions. Preplanned contrasts compared light-on (C1), light-off (C2), and diurnal vs. nocturnal (C3) effects.

**Key Results:** The average AT in TN was 21.9 ± 0.4°C; in CHS it ranged from 22.3 ± 0.2°C (night) to 33.8 ± 0.3°C (day). CHS reduced daily feed intake by 6.13% (2416 g vs. 2268 g). Most feed intake occurred during the light-on period (77.2% TN, 73.2% CHS). A significant T × P interaction was found for feed intake rate (p < 0.01): CHS pigs had the highest rate during PIII (28.6 g/min) and lowest during PI+PII (27.5–27.6 g/min), while TN pigs showed no period differences. Meal size was significantly affected by T × P × EP (p < 0.01). In EP1, TN pigs had the largest meals in PII (686 g/meal); CHS pigs had similar meal sizes across PI, PII, and PIV (460–484 g/meal) but significantly lower in PIII (387 g/meal). In EP2, TN pigs again peaked in PII (816 g/meal), while CHS pigs had larger meals in PIV (559 g/meal) than in PII (496 g/meal) or PIII (420 g/meal). Light-on events increased meal size by 12.9% for CHS pigs (p < 0.01) but only 4.2% (non-significant) for TN pigs. Light-off events reduced meal size by 17.8% (TN) and 19.1% (CHS) (both p < 0.01). The interval between meals was shorter during PII and PIII (08–20 h) for both temperatures. Number of meals per pig was not significantly affected by temperature or period.

**Clinical Implications:** These findings demonstrate that CHS disrupts the normal circadian feeding pattern of finishing pigs. Although pigs attempt to compensate by eating more during cooler nocturnal hours, the compensation is incomplete, resulting in a net reduction in daily feed intake. The lighting program also modulates meal size, with light-on stimulating and light-off suppressing intake, especially under CHS. This suggests that management strategies such as adjusting feeding times to cooler periods or modifying lighting schedules could help mitigate heat stress-induced performance losses. The study provides quantitative data to inform precision feeding algorithms and housing design for pigs in warm climates. However, the compensatory response may be limited by digestive tract capacity and the recurring daily heat challenge.