Exposure of growing Iberian pigs to heat stress and effects of dietary betaine and zinc on heat tolerance.

The effects of heat stress and dietary betaine and zinc on growth, weight of organs and plasma biochemistry in growing Iberian pigs are of special importance. Forty Iberian barrows (43.8 kg BW) were assigned to five treatments for 28-d: 1.-thermoneutral (20 °C)-ad libitum fed a control diet (TN-CON); 2.-heat stress (30 °C)-ad libitum fed a control diet (HS-CON); 3.-thermoneutral-pair-fed a control diet (TN-CON-PF) on the basis of intake of HS-CON; 4.-heat stress-ad libitum fed a betaine-supplemented diet (HS-BET); 5.-heat stress-ad libitum fed a zinc-supplemented diet (HS-ZN). Heat stress increased rectal temperature and decreased feed intake. As a result, weight gain under TN-CON was greater than under heat stress or TN-CON-PF. Temperature did not affect gain to feed ratio. Heat stress decreased empty BW, kidneys and spleen weights, and tended to decrease total viscera weight compared to thermoneutral counterparts fed ad libitum, but when organ weight was related to empty BW, only spleen tended to decrease. At identical intake, heat stress tended to decrease heart weight. Betaine and zinc had no effect on organs weight under heat stress. Heat stress decreased albumin and the homeostasis model assessment index for estimating ß-cell function (HOMA-%B), increased glucose, and tended to increase urea compared with the TN pair-fed group. Betaine and zinc decreased plasma glucose under heat stress and increased HOMA-%B suggesting improved ß-cell function. Insulin, quantitative insulin sensitivity check index, homeostasis model assessment index for estimating insulin resistance percentage and the rest of biochemical parameters were not affected by treatments. Decreased intake explained the consistent negative effects on performance of pigs after long-term heat stress. Furthermore, it elicited a glucose sparing effect without affecting insulin concentration and increased protein catabolism. Betaine or zinc supplementation did not prevent the negative effect of heat stress on growth performance of Iberian pigs.