Introduction

Correct feeding management is important to improve animal welfare, growth efficiency and production data of pigs. One possibility to improve feeding systems for pigs is to mix dry feed with water (proportions between 1:1.0 and 1.5)¹. Wet feeding has been shown to reduce stress in the transition from liquid to solid diet of weaned piglets^1,2,3; this may have beneficial effects such as reducing the use of antibiotics in current production systems^4,5. In addition, wet feed improves water and feed intake^3,6, as well as nutrient supply, in growing-finishing pigs⁷, compared to dry feed, potentially favoring the productive performance⁴ without affecting the fat content and carcass quality of the pigs^1,5.

Sensory characteristics, such as tenderness, color, and marbling are important in determining the quality and consumer approval of beef⁸, chicken⁹, rabbit¹⁰, and pork^11,12. These improvements may be greater in regions where the ambient temperature exceeds the pigs' thermoneutral (comfort) zone. However, information on the effect of wet feed on intensive pork production systems in hot areas like northern Mexico is scarce. Therefore, the objective of the present study was to determine the effect of sorghum and soybean meal based wet feed on the growth rate and efficiency, productive behavior, carcass composition and meat quality of finishing pigs. The hypothesis of the present study was that the feed intake and utilization of pigs under these climatic conditions might be improved by a wet diet.

Material and methods

The pigs used in this research were cared for in accordance with the guidelines established in the Mexican Official Animal Care Standard (Norma Oficial Mexicana para el Cuidado de Animales)¹³. The study was conducted at the Swine Experimental Station of the Marín Academic Unit of the Agronomy Department of the Autonomous University of Nuevo León, located in Marín, N.L., Mexico. Sixteen pigs (8 females and 8 castrated males; experimental units) of the York-Landrace x Duroc terminal cross with an initial live weight of 68.4 ± 2.4 kg were used. The animals were housed individually in pens with concrete floors (1.4 m²), equipped with stainless steel drinking troughs and plastic feeders. The pigs were randomly assigned by sex to each of the two treatments: WF, wet feed at a ratio of 1:1 (diet:water), and DF, dry feed. The diet offered to the pigs was based on ground sorghum grain, soybean meal, and a vitamin and mineral premix, formulated with 3,200 kcal ME/kg and 15% crude protein, in order to meet or exceed the nutritional requirements of pigs in the 50 to 120 kg weight range, NRC⁷.

Results

The ambient temperature during the experiment ranged from a minimum of 9.1 °C to a maximum of 35.3 °C, with an average of 27.3 °C during the study. Table 1 shows the results of productive behavior after five experimental weeks. The live weight of wet-fed pigs was higher (P<0.01) at the end of weeks 4 and 5, and during the entire study, compared to dry-fed pigs. Differences in live weight between treatments became more pronounced over the experimental weeks (P<0.01). Although the ADWG was not statistically different in each of the weeks, it was statistically different overall in pigs fed the wet diet (P<0.01; Figure 1). The ADFI was lower at wk 1 and higher at wk 5 when offered wet feed (P<0.01), but it was not different throughout the entire study (P>0.10). The variable FE (P<0.05) was better with wet feed in weeks 1 and 3 (Figure 2), and throughout the study.

^a,b Significant difference (P<0.05) between groups.

Figure 1 Daily feed intake (Mean ± MSE) of pigs in the final phase (68 a 108 kg) fed with wet feed (WF) and with dry feed (DF) 

^a,b Significant difference (P<0.05) between groups.

Figure 2 Feed efficiency (Mean ± MSE) weekly measurements in pigs at the final stage (68 to 108 kg), with wet feed (WF) and with dry feed (DF) 

Discussion

The present experiment was carried out under climatic conditions representative of many dry tropical sites; it is one of the first studies carried out in the northeastern region of Mexico. During the experiment, the pigs were housed in a shed with open sidewalls, so that the animals were under the natural ambient temperature conditions, which ranged from 10 to 35.3 °C. These extremely variable environmental conditions could have affected the feed intake and feed efficiency of the pigs. A significant interaction between experimental week and treatment was obtained for the ADFI and FE, similarly to what was stated in previous reports^22,23.

Exposure to a room temperature of 33 °C has been reported to reduce voluntary feed intake of pigs by 20 to 30 %²². In the present study, the ADFI by wet-fed pigs at wk 1 was 12 % lower; however, it was 18 % higher at wk 5. Although the overall feed intake did not differ, the tendency to increase as the experiment progressed until it became significantly higher at wk 5 indicates that feed moistening may help to recover the voluntary feed intake of animals under climatic conditions of heat stress. Wet feeding also increased the average daily weight gain by 14 %, and the feed efficiency, by 8 %. The higher ADWG of WF pigs, as observed in the present experiment, is in agreement with the results reported in finishing pigs²⁴ and growing-finishing pigs fed a wet diet for 90 d²⁵. Taken together, these results indicate that feeding the pigs a wet diet exposed to conditions of high ambient temperature may improve not only the feed intake but also the feed utilization efficiency. Yang et al⁶ reported a lower ADWG and FE in growing-finishing pigs fed a liquid diet containing by-products of the ethanol industry, compared to pigs fed a wet diet based on corn and soybean meal. This suggests that, in addition to the type of feed (wet or dry), the ingredients used also play an important role in production efficiency. The wet feed offered to pigs at high ambient temperatures may have allowed them to have a better body temperature balance, resulting in higher weight gain and feed efficiency.

The higher HC and CC weights in the WF pigs recorded in the present study agree with other previously published results²⁴, in which the hot carcass weight and yield were higher, but a similar backfat depth were reported in pigs fed a wet diet, indicating the possibility of a positive influence of wet feeding on carcass characteristics.

The depth of the backfat has been reported to be similar in pigs whether they are fed a wet or a dry diet²⁵. However, wet-fed broilers had a higher abdominal fat content²⁶. When the trough is equipped with an integrated water supply, the pigs tend to consume more feed²⁴. The higher feed intake in wet-fed pigs reported in wk 5 of the present work may be reflected in a higher backfat content.

In the present study, the legs were heavier in wet-fed pigs than in dry-fed pigs. This is consistent with previous reports²⁷ in which higher leg weights have been recorded in pigs with higher growth rates.

The term meat quality describes the sum of different properties²⁸ such as pH, color, tenderness, WHC, and chemical composition²⁹, which are particularly important in sensory evaluation³⁰. In this study, differences were observed in favor of pigs fed a wet diet in terms of meat quality and texture characteristics such as shear strength, gumminess, and chewiness³¹.

The WHC and color are important attributes that determine the visual appeal and tenderness of the meat^32,33. The values observed in this experiment were similar between treatments and are in agreement with those previously reported^34,35 in finishing pigs. In the present work, the meat from the wet-fed pigs had lower toughness values than that from dry-fed pigs, which is indicative of a greater tenderness³⁶ ―one of the most important characteristics of the meat quality^32,37. The other results of texture profile analysis, such as adhesiveness and cohesiveness, also used regularly to determine sensory attributes³⁸, did not differ between treatments in the present work.

Color is an important trait of pork quality, which can be affected by various factors, such as the feeding strategy³¹ and the breed of the pigs^11,12. In the present study, no differences were found between treatments in the chromatic variables B*, a*, b*, C, and H, which determine the color. Compared with results from previous research³⁴, the brightness (B*) values in the present work for meat from pigs fed WF and DF (53.6 and 54.0, respectively; Table 4) were indicative of "normal" meat ³⁹. B* values of 58 are indicative of PSE (pale, soft, exudative) meat, while values below 52 indicate the ASD condition (dark, firm, dry)^39,40,41.

The economic analysis performed in this study has been used before⁴², as it allows combining in a single value (difference of economic income minus the feed cost) the effect that the evaluated type of feed has on various characteristics, such as feed intake, daily weight gain, and feed efficiency. In the present study, the difference in the income derived from the growth of the pigs minus the feed cost for five weeks was 27.3 % higher for wet-fed pigs. The largest percentage of this economic benefit originated from the higher growth rate recorded in pigs fed a wet diet. In contrast, there was no difference in the feed cost with respect to the form in which the feed (whether dry or wet) was offered to the pigs. In a study published by Myers et al⁴³, pigs weighing between 80 and 110 kg fed from a wet feed trough had better growth data than pigs fed a dry diet. Based on the weight gain and feed consumption of these pigs⁴³ when fed a wet or a dry diet, as well as the prices of the feed and the pork meat registered by the present experiment, the economic benefit from wet-fed pigs estimated by Myers et al⁴³ would have been 9.1 % better than that obtained from dry-fed pigs; this figure is lower than the economic benefit registered in the present study. In short, based on the results of this experiment, it is concluded that feeding finishing pigs with a wet diet improves the productive variables (daily weight gain, feed intake, feed efficiency), the economic profit, the carcass composition, and the meat quality.