Effects of whole-grain paddy rice on growth performance, oxidative stress and morphological alterations of the intestine in broiler chickens exposed to acute and chronic heat stress

We have previously shown that, under thermoneutral conditions, the growth performance of chicks fed a ricebased diet containing 6% soybean oil [metabolizable energy (ME): 2,800 kcal/kg] for 28 days was comparable to that obtained with a corn-based diet containing 6% soybean oil (ME: 3,100 kcal/kg). However, there is no information available concerning how such diets compare with respect to growth performance under heat stress conditions. The present study was therefore conducted to clarify differences in dietary effectiveness between corn-based and wholegrain paddy rice-based diets [formulated to be equivalent in terms of fat (6%), but not iso-caloric] under the two conditions of acute and chronic heat stress (Experiment 1 and Experiment 2, respectively). In Experiment 1, the body weight gain of a corn- and rice-fed group of chickens exposed to acute heat (33℃-12 h) were significantly decreased compared with that of a corn-fed control group (24℃), with similar results obtained for the two heat-treated groups. Malondialdehyde (MDA) levels in the M. pectoralis superficialis were significantly increased in corn-fed birds exposed to acute heat stress, but not in the rice-fed group (33℃-12 h). In Experiment 2, body weight gains in the corn- and rice-fed groups exposed to chronic heat (33℃-6 d) were significantly decreased, and to a similar extent, compared with the corn-fed control group (24℃). Muscle MDA levels in the corn- and rice-fed groups were similarly increased by the chronic heat exposure compared with the control group. These results suggest that, in response to acute heat stress (but not chronic heat stress) conditions, the feeding of whole-grain paddy rice diet to chickens may attenuate skeletal muscle oxidative damage compared with that in corn-fed chickens. The possible involvement of intestinal morphology (such as the villus height: crypt depth ratio observed here) on growth performance is also discussed.