INTRODUCTION

Heat stress causes alterations in the metabolic system (^{Baumgard y Rhoads, 2013}). These include the decrease in the release of thyroid and growth hormones, which decreases the basal metabolic rate (^{Aggarwal y Upadhyay, 2013}), which affects the expression of genes and proteins involved in the metabolism of energy and nutrients (^{Sanz-Fernandez et al., 2015}). Zinc is a trace mineral with proven importance for the function of more than 300 enzymes (^{Chasapis et al., 2012}). The metabolic action of Zn includes energy metabolism, protein synthesis, nucleic acid metabolism, epithelial tissue integrity, repair and cell division, transport and utilization of vitamin A and vitamin E absorption (^{Borah et al., 2014}).

It has been shown that diets supplemented with Zn improves and prevents the reduction of intestinal integrity during heat stress (^{Sanz-Fernandez et al., 2014}), decreases the intestinal permeability of piglets during weaning (^{Zhang y Guo, 2009}), promotes the restoration of the intestinal epithelium (^{Song et al., 2011}) and improves the metabolism of proteins in pigs (^{Pearce et al., 2015}). Because Zn requirements increase during heat stress (^{Lagana et al., 2007}), it has been suggested that Zn supplementation could be used to attenuate the decrease in serum Zn during periods of high ambient temperatures (^{Li et al., 2015}).

Pig diets are usually supplemented with inorganic Zn (ZnSO₄ or ZnO) to ensure the required demand. ZnSO₄ is the inorganic source with the highest bioavailability (^{NRC, 2012}); however, under normal physiological conditions and with adequate intake, only 5 to 15% of the diet is apparently absorbed (^{McDowell, 2003}).

In recent years, the use of organic sources of Zn has been explored, due to its greater bioavailability (^{Sahin et al., 2005}).

The objective of the present study was to evaluate the influence of organic zinc supplementation, from zinc methionine, on the performance of developing pigs in warm weather conditions.

MATERIAL AND METHODS

The study was carried out from August to October 2016 in the Experimental Unit of Fattening Pigs of the Faculty of Veterinary Medicine and Zootechnics of the Autonomous University of Sinaloa, located in the pig farm "La Huerta", located in the municipality of Culiacán, Sinaloa, Mexico (24° 49' 38" N and 107° 22' 47" W, and 60 meters above sea level); with an average annual ambient temperature of 25 °C and 790 mm of rain (^{INEGI, 2013}).

Experimental design. Ninety-six 84-day-old pigs (48 males and 48 females; 33.8±SD 0.96 kg l.w) were used in a randomized complete block experimental design (DBCA). Pigs were weighed individually; they were grouped into three blocks by initial weight and sex, and in groups of eight (4 males and 4 females); the pigs were placed in 4 pens per block.

The pen was the experimental unit. The treatments were: 1) diet based on meal of corn and soybean (see table 1), with nutritional contribution according to the production stage (control); 2) Control plus 120 ppm of organic Zn/kg of DM (120 ZnM); 3) Control plus 240 ppm of organic Zn/kg of DM (240 ZnM), and 4) Control plus 360 ppm of organic Zn/kg (360 ZnM).

Organic zinc was provided as zinc methionine (ZnMet) of the Zinpro 120 premix (Zinpro® 120, contains 12% of Zn and 27.3% of methionine; Zinc methionine in US Patent Nu. 4,764,633 and 5,430,164; Mexico release office: B00.02.08.02.02.0398/11)

Animal handling Pigs were weighed, identified and grouped into groups of eight in 7x1.5 m (10.5 m2) pens, which includes 1.5 m² of pond; with concrete floor and completely roofed, equipped with plastic feeder type hopper with integrated metal pacifier. Pigs had permanent access to water and food freely available. The animals were weighed at the beginning (day 0) and 42 days after starting the test, to determine the daily weight gain (DWG) in the study period. The food offered to the pigs of each pen was recorded and at the end of the period the daily feed consumption (DFC) was determined. On the basis of DFC and DWG, the feed conversion (FC=DFC/DWG) was calculated.

Measurement of temperature and relative humidity. The temperature (t °C) and relative humidity (RH, %) data were taken with a thermo hygrometer, located inside the experimental unit, and recorded daily during the experimental period (Table 2). The temperature and humidity index (THI) was calculated using the formula THI= [0.8 x room temperature] + [(% RH/100) x (room temperature-14.4)] + 46.4 (^{Mader et al., 2006}).

Statistical analysis. The results were analyzed by ANDEVA (^{Steel y Torrie, 1985}) for a DBCA. The influence of the Zn level in the productive response was explored by orthogonal polynomials. An alpha value of 0.05 was established to accept statistical difference and each pen was considered as the experimental unit. All calculations were performed using version 8 of the Statistix® Statistical, statistical package.

RESULTS

During the experimental period, the average temperature was 30.4 °C; relative humidity 73%, and THI of 82. The results of the influence of the addition of Zn from ZnMet on the productive response of developing pigs are shown in Table 3. Pigs fed diets supplemented with Zn (ZnMet) had a better feed conversion (P=0.05). Quadratic responses to the level of organic Zn supplementation were observed in the final weight (P=0.05), weight gain (P=0.03) and food consumption (P=0.05). The feed conversion tended (P=0.08) to improve linearly as the level of organic Zn increased.

DISCUSSION

High ambient temperatures can negatively affect animals (^{Chauhan et al., 2014}); these, by themselves, can be fatal, but in many areas, high relative humidity also contributes significantly to raising the sensation of heat (^{Parsons, 1995}). The hot and humid conditions during the summer, implies a certain risk that the pigs will be affected by heat stress (HS).

In the present study, experimental animals were exposed to an average temperature and relative humidity of 30.4 °C and 73% RH, respectively, for 42 days; which according to ^{Mader et al. (2006)}, pigs were at risk of HS in a range of physiological emergency danger, with a THI between 74.15 and 90.22 and a daily average of 82.47 (see table 2).

The high ambient temperature decreases the plasma concentrations of Ca, K, Na and Zn in animals undergoing HS (^{Pearce et al., 2013}); because the Zn requirements increase during the HS (^{Lagana et al., 2007}). It has been suggested that Zn supplementation could be used to attenuate the serum decrease of Zn during periods of high ambient temperatures (^{Li et al., 2015}).

In Japanese quails under HS conditions, ZnSO₄ supplementation improved food consumption, egg production, egg quality, feeding efficiency and nutrient digestibility (Sahin y ^{Kucuk, 2003}).

In broilers, supplementation with ZnSO₄ improved weight gain and feed conversion, while reducing oxidative stress (^{Kucuk et al., 2003}). Also, improvement in the function of the intestinal barrier has been observed, when pigs are supplemented with Zn during HS (^{Sanz-Fernandez et al., 2014}).

In the present study, pigs fed a diet supplemented with Zn (ZnMet), at levels of 120, 240 and 360 mg/kg DM, had a better feed conversion; showing a quadratic response in food consumption, daily weight gain and final weight; while food conversion had a linear trend. These results are similar to those observed by ^{Li et al. (2015)} in pigs fed a diet supplemented with 1,500 mg of Zn (ZnSO₄) under HS conditions (40 °C for 5 h daily in a period of 8 consecutive days). Other studies have also suggested that the consumption of diets added with inorganic Zn, at pharmacological levels, improves the productive response of pigs (^{Carlson et al., 1999}; ^{Mavromichalis et al., 2001}).

The diet used in the present study contained 120.28 mg Zn/kg DM, from inorganic Zn, contributed by the mineral premix. The ^{NRC (2012)} recommends a contribution of 60 mg of Zn/kg of feed for pigs of 20-50 kg of live weight, and 50 mg of Zn/kg of feed for pigs of 50-110 kg of live weight. The supplementary source has generally been inorganic Zn from ZnSO₄ or ZnO, the inorganic source of ZnSO4 being the most bioavailable (^{NRC, 2012}). However, under normal physiological conditions and with adequate intake, only 5 to 15% of the diet Zn is apparently absorbed (^{McDowell, 2003}); which suggests that only between 6.01 and 18.04 mg of the Zn contributed per kg of feed is absorbed by the pig under normal conditions. ^{Li et al. (2015)} observed that exposure to caloric stress decreases serum zinc concentration in miniature pigs; it has also been suggested that oxidative stress may contribute to micronutrient deficiency by increasing the demand for antioxidants, including Zn, selenium and vitamins A, C and E (^{Sappey et al., 1994}); what it could explain, the improvement observed in feed conversion in pigs that consumed feed added with organic Zn.

The results of the present study indicate that supplementation with 360 mg of Zn (ZnMet)/kg of DM improves (P=0.5) the nutritional conversion of developing pigs into heat stress.

CONCLUSION

The consumption of diets supplemented with 360 mg of Zn/kg of DM, from zinc methionine, improves the nutritional conversion of the pigs in development, raised in environmental conditions with high heat stress.