INTRODUCTION

Compared to other domestic species, pigs are more sensitive to high temperatures. The pigs have not functional sweat glands, had double layer of fat and smaller contact area with the environment when the animals reach mature age ¹. In addition, breeding pigs are selected for high deposition of lean tissue and high performance, what ends up selection animals with intense metabolism and greater sensitivity to heat ². These characteristics, when combined with the confined production systems, may result in losses to thermoregulation, with consequent damage to performance, animal welfare and greater emission of odors and ammonia ³.

At the same time, the reduction in performance, heat and social stress can depress the pig’s health ⁴. In this sense, the high ambient temperatures associated with the intensification of pig production lead to the need to develop alternative systems to mitigate the production problems related to heat stress.

The use of pens with shallow pool is one way to mitigate the problems related to high temperatures; it can facilitate heat loss by evaporation from the wet skin, conduction and convection ¹. However, in this system the animals may be more susceptible to etiologic agents due to greater contact with the feces ⁵, which can be higher when the shallow pool pens are interconnected.

A thermally favorable environment makes the animal capable of maintaining its healthiness, consume and absorb nutrients properly ⁶. However, immunological activation cans changes in the requirements and relationships between amino acids and reduction in the growth rate and muscle tissue synthesis ⁷, with a reduction up to 70% of the productive potential ⁸.

A recent study found lower deposition of dust and less accumulation of organic matter in pens with shallow pool, features that can be beneficial in terms of air quality of the facilities ⁹. The presence of excess powder, and gas in installations create problems in cellular defense mechanism because overwhelm the phagocytic activity of macrophages. Besides the fact, many particles when suspended for possessing biological activity (such as endotoxin) can induce ciliostasis or direct toxic effect on the hair cells with damage to the mucociliary defense mechanism ¹⁰. However, the same research group stresses that the presence of shallow pool can be a detrimental factor for air quality, as the highest concentration of gases inside facilities ¹⁰.

Despite the described health challenges, the use of this system meets the natural habits of the species with the habit of wallowing, important behavioral pattern in thermoregulatory processes of swine ¹¹. However, the daily water exchange of the shallow pool can increase the dilution of waste ¹²; which leads to the actual desrecommendation. Although the use of shallow pool pens without water renewal is common in swine production in south of Brazil.

We point out that the number of researches on the subject is small and sometimes do not studied all the variables or diversification that may exist in shallow pool system. One of these aspects is the smallest water renewal of the pool, aiming at reducing the dilution of waste and a consequent improvement in the environmental aspect of the system. Another important variant is adopted stocking rate, aspect that can greatly impact the performance of production systems and scientific exploration does not exist when it comes to the shallow pool systems. Therefore, this study aimed to evaluate the growth performance, carcass traits, thermoregulatory and immunological aspects of pigs kept pens with shallow pool or compact floor pens at different stocking rates.

MATERIAL AND METHODS

Site of study. The study was conducted in an experimental pig farm (23°25›S, 51°57›W at an altitude 542 m). In this study, it was used 72 pigs (36 barrows and 36 gilts), with initial weight of 35.1±4.9 kg, from high-lean commercial line.

Pens (4x2 m) had semi-automatic feeders (feeders with two accesses 0.35 m) in front of the pen (area of 0.4 m²) a drinking (nipple type) at the back. The shallow pool located at the back of pens (0.8 m wide and 0.1 m deep), as described by Moreira et al ¹. To compact floor treatment, shallows pool were covered with wooden pallets, called compact floor treatment. Treatment with shallow pool, the water channels at back pens were kept under running water. Shallow pool area were drained, washed and filled with clean water twice a week (Mondays and Thursdays). In compact floor treatment, area at the back pens was cleaned and dried daily.

Experimental design and analysis. Experiment was carried out in two phases, consisting of three group sizes (2, 4 and 5 pigs/pen representing stocking rates 3.0; 2.0 and 1.5 m² of total area of compact floor available per pig, respectively). Were used, two types of surface (with or without access to shallow pool), and two blocks (according to gender barrow and gilt) by treatment, and initial weight, with 36 animals per step and two steps.

In the first step, growth performance (feed intake, daily gain and feed: gain ratio) was analyzed, in addition to fat thickness at position 2 (P2). In second phase, the same analyzes were repeated and added further analysis as leukogram (held at end of the experiment) and skin temperatures at the end of the growing and finishing phase.

Air temperatures and relative humidity were collected daily in a Weather Station located 50 meters from the experimental facilities (Table 1). To measure the surface skin temperatures, the air temperature in the days of collection, were 24.8 and 21.8ºC, for growing and finishing phase, respectively.

Diets were fed ad libitum, and formulated based on corn and soybean meal with the use of specific basemix for each phase, having the following percent composition: 72, 24 and 4% for corn, soybean and basemix for growing phase, respectively. In the finishing phase, the percent composition was 79, 18 and 3% for corn, soybean meal and basemix, respectively.

Daily intake (DI), daily weight gain (DWG), feed: gain ratio (FGR) and backfat thickness were evaluated at the end of the growing and finishing phase. Backfat thickness (P2) by Sono-Grader (Renco^®) apparatus was performed. Statistical analysis were performed to the DFI, DWG, FGR and P2 with the pens as experimental unit. For the other variables, each pig was considered an experimental unit.

In second step of the study, at the end of the finishing phase was harvest 2 mL of blood by cranial vena cava puncture of two animals per pen, to perform white blood cell count, which was used in Vacutainer^® tubes (EDTA). Leukocyte count was done manually in Neubauer chambers ¹³. Blood smears were prepared and stained by the Romanowski method for evaluation of differential leukocyte.

Statistical analysis. Data were evaluated according to the statistical model:

Y_ijk = µ+T_i+D_j+ e_ijk,

in which: µ = mean, T_i = effect of floor type, D_j = effect of stocking density, and e_ijk = random error.

Polynomials were tested until the first order, and it was used the statistical package SAEG, 2007. For the skin temperature analysis and leukocytes were implemented in the model the effect of sex. Data for white cell count was tested by Shapiro Wilk for normality, and did not present the requirements of parametric analysis, so we used nonparametric approach by Kruscal Wallis, which was considered as a random effect sex, and as fixed effect of floor and stocking density.

RESULTS

Performance and carcass: In this study, there were no differences between the first and second steps (p>0.05). There was no interaction between stocking rate and floor type, and there was no effect on growth performance in the growing phase (p>0.05). In addition, there are no effects of floor type in P2 (p>0.05). We found lower gain and worst feed efficiency of pigs (p<0.05) with the use of shallow pool in the finishing phase (Table 2). Stocking rates did not influence (p>0.05) on the performance of pigs (Table 3).

This experiment showed lower (p<0.05) backfat thickness in pens with more space per pig.

Skin temperature and leukogram: There were no differences (p>0.05) for the skin temperature in P2 and anal region in growing phases. Temperature in the anal region at the end of the finishing phase (Table 3) was lower in shallow pool treatment (p<0.05).

In leukogram analysis was not possible to reject any null hypothesis, so there was no difference (p>0.05) obtained leukocyte count in the blood, so the numbers of neutrophils, lymphocytes, eosinophils, and monocytes were not affected by treatments and stocking rates (Table 4).

DISCUSSION

Performance and carcass: Performance result in this study (Table 2) differ from those obtained by Moreira et al ¹, found best results for pigs housed in pens with shallow pool during the growing. However, in this study the authors report the water of “shallow pool” was renewal daily, different from the present study, in which the renewal was performed twice a week.

Difference in the results of DWG and FGR between the phases of growing and finishing may be associated with the combination of two factors. In the finishing phase is greater demand for shallow pool, a result of increased sensitivity to heat this stage of life ^1,2. Moreover, in the final phase is higher feed intake, and consequent increased production of waste, which increases the challenge of thermoregulation and health for this phase.

Lower performance in the treatment with shallow pool (Table 3), may also be associated with other factors such as shorter shelf/animal space (because pigs do not use the area with shallow pool to sleep, especially in situations of thermal comfort or cold stress), and the most contact with waste and consequently worse health status ⁵. Likewise, Kich et al ¹⁴ reported that increased contact with the waste into pens with shallow pool, is a factor associated with increased serological prevalence of Salmonella in pig farms. Moreover, Oliveira et al ¹⁵ found no relationship between the uses of shallow pool with increased prevalence of Salmonella.

Stocking rates did not influence on the performance of pigs. However, according to NRC ¹⁶ the area for pigs must be at least 1.1 m² for animals over 50 kg body weight, and low space/pigs may cause decrease in productive performance. It should be noted in the experimental pens, the treatment with shortest space/pig (1.5 m²/pig) there are values greater than normally recommended and contributed to the similar results in performance between the different densities.

Backfat thickness for pigs in pens with more space (m²/pig) was lower and can be explained the larger space may have favored exploratory behavior and increased physical activity or less stimulus for consumption because the smaller size of the group and as a result less fat deposition. Similar results were obtained by Lunen ¹⁷ in an experiment with compact floor and slatted floor in different densities for weaning piglets to 25 kg, being observed lower depth of fat in piglets with the largest area available. However, differ of other studies with a pool, in which no differences were found in P2 ^1,18.

Skin temperature and leukogram: Temperature in the anal region at the end of the finishing phase (Table 3) showed lower in shallow pool treatment. Differences of these results between the phases may be related to the factors previously discussed, with more thermosensibility of animals in the finishing phase, which leads to increased demand for water in order to skin wetting for favoring thermoregulation. According to literature, the temperature of the skin can be a physiological indicator of heat stress in pigs ¹⁹, suggesting improved thermal comfort for the pens with water.

Absence of floor effects in the temperature in the P2 region was similar to the results observed by Biazzi et al ⁹ and may be linked to problems with wetting the dorsal region, given the low depth of water in shallow pool, and the lowest blood supply in the region P2 when compared to the anal area.

In Leukogram analysis was not possible to reject any null hypothesis, so there was no difference (p>0.05) obtained leukocyte count in the blood, so the numbers of neutrophils, lymphocytes, eosinophils, and monocytes were not affected by treatments and stocking rates (Table 4). The air quality related to the concentration of gases such as ammonia, H₂S and CO₂, can be worsened in shallow pool pens systems and result in damage to the protective mechanisms of the lower airways and alveoli, mucus cleaning-ciliar and the activity of alveolar macrophages ¹⁰. Moreover, Biazzi et al ⁹ observed less dirt and dust accumulation in the compact part of the pens, with the use of shallow pool, which could have benefited the air quality. Thus, the combination of a lower concentration of gases in compact floor and lower dust in the pens with shallow pool may have contributed to the lack of difference regarding the number of leukocytes.

Finally, it can be concluded pig production in low stocking rates, regardless of the floor type decreased fat thickness. Keeping pigs in pens with shallow pool, washed and filled with clean water just twice a week, reduced growth performance in the finishing phase, although it improve the thermal comfort, evidenced by lower skin temperature values in the anal area.