Does feed restriction mimic the effects of increased ambient temperature in lactating sows?

We evaluated the effects of high ambient temperature and feed restriction in primiparous lactating sows. Females were exposed to either a constant thermoneutral (20 degrees C) or hot environment (30 degrees C). Lactating sows at 30 degrees C were given free access to feed (30AL; n = 12), and sows at 20 degrees C were restricted according to the feed intake recorded at 30 degrees C (20RF; n = 6) or were given free access to feed (20AL; n = 6). Jugular vein catheters were surgically inserted at 100 +/- 1 d postcoitum. During lactation, 30AL sows exhibited higher rectal temperatures (P < .05) than 20AL and 20RF sows. Feed intake was reduced by 43% for 30AL compared with 20AL sows. Daily body weight loss was lower (P < .05) in the 30AL than in the 20RF group, and mean litter daily gain over the whole lactation was 18% lower in 30AL than in 20AL sows (P < .05) and was intermediate in 20RF females. Plasma concentrations of thyroid hormones (triiodothyronine [T3] and thyroxine [T4]) were lower at 30 degrees C than at 20 degrees C at d 4 postpartum and d 8 after weaning for T4 (P < .001) and at d 4 postpartum (P < .001) and at d 1 and d 8 after weaning for T3 (P < .01) but were not influenced by feed restriction at 20 degrees C. Mean concentrations of cortisol measured on d 4 and 19 postpartum and on d 1 after weaning were lower in the 30AL than in the 20AL group (P < .05), and neither was different from that in 20RF sows. Ambient temperature and feed intake had no influence on prolactin concentrations on d 19 postpartum and d 1 after weaning. In the 30AL group, concentrations of T3, cortisol, and prolactin measured at d 19 postpartum were positively correlated with the litter gain observed during the 2nd and 3rd wk of lactation (P < .05). The return to estrus was slightly delayed in 20RF compared with 20AL sows (P < .05) and was quite variable in the 30AL group. These results demonstrate that high ambient temperature has negative consequences on litter growth and return to estrus and induces plasma hormonal variations, that cannot be fully mimicked by feed restriction in primiparous sows.