Introduction
The nutrition of pigs during their early life is extremely important because, in addition to affecting the growth performance, it can also affect their later growth (Schinckel et al., 2007). This transition from liquid to solid diet seems to have negative consequences on digestive system of weaning pigs (Balasubramanian et al., 2017). However, the concern of nutrition availability should also be kept in mind while preparing food because the nutrition of food components depends on the food preparation process. Grinding and steam-pelleting are two conventional processes during the processing of feed to improve the nutrient value of diets. Research has shown that pelleting reduces feed wastage and improves body growth and feed efficiency (Ball et al., 2015). But finer grinding and pelleting increase manufacturing expenses. Feeding in a liquid form or high moisture form rather than a dry form can improve pig performance, besides it requires less processing. Weaned piglets are suddenly forced to transition from highly digestible milk to solid diets with complex protein (Ma et al., 2019), which can cause diarrhea or death. It has been shown that transitioning to liquid feeding after weaning can boost feed intake, leading to an increase in growth rate (Jensen and Mikkelsen, 1998). Jensen and Mikkelsen (1998) discovered that the average daily gain of the liquid-fed weaned pigs was higher than that of the dry-fed weaned pigs. But the relation between the presentation of the feed, the processing of the feed, and the particle size is not consistent (Choct et al., 2004). However, whey protein and fish meal are high-quality animal protein sources for weaned piglets. But animal protein sources are limited in weaned piglets' diets due to high cost and limited supply. Some researchers noted that hydrolyzed wheat protein (HWP) is a good alternative with a variety of biologically active wheat peptides that could promote weaned pig growth (Wang et al., 2011; Han et al., 2017).
To the best of our knowledge, the application of a semi-moist diet in weaned pigs’ diet is still inadequate. The purpose of the present study was to examine the effect of replacing whey protein in a semi-moist energy-up diet. It was hypothesized that the different forms of feed supplemented to weaning pigs will show a similar performance as semi-moisture feed had higher digestibility and replacement of whey protein diet had higher energy concentration and similar dietary protein.
Materials and Methods
The experimental protocols of this study describing the management and care of animals were reviewed and approved by the Animal Care and Use Committee of Dankook University, Cheonan, South Korea (DK-2-2107).
Experimental design, animals, and diets
A total of 75 four-week-old crossbred weaning pigs ([Yorkshire × Landrace] × Duroc; 7.17 ± 0.15 kg) were used in a 35-day experiment, and pigs received no creep feed before weaning. All pigs were randomly distributed into three treatment groups in a completely randomized block design. Each treatment has five replicates each with five pigs (mixed sex). The dietary treatment was divided into two phases: Phase 1 (days 0 - 14) and Phase 2 (days 15 - 35). The three treatments were: (1) control (whey protein) by pellet, (2) control (whey protein with energy modify) fed semi-moisture diet by extruder, and (3) no whey protein with energy up fed semi-moisture diets by extruder. All piglets were kept in an environmentally controlled room. At first, the temperature was maintained at 30℃, thereafter it was reduced by 1℃ each week. Each pen was provided with ad libitum access to feed and water.
Preparation of feed
For extruded feed raw ingredients were grounded and blended before placed in the extrusion screw. When the material moves into the extruder, the pressure in the barrel increases due to the restriction of discharge in the barrel. Discharge pressure varied from 30 to 60 atm. This process increases the moisture content of the feed ingredients. The extrusion was done at a controlled temperature of 100℃ with a screw setup of 25 mm. All nutrients in diets were formulated to meet or exceed the minimum requirements that were recommended by the National Research Council (NRC, 2012) (Table 1). All the raw ingredients and feed was provided by Dae-han Feed Co. (Daehan Feed Co., Ltd., Incheon, Korea).
Data collection and sampling
Individual pigs were weighed on d 0, 14, and 35. Average daily gain (ADG), average daily feed intake (ADFI), and the gain/feed (G : F) ratio were recorded. To determine apparent total tract digestibility (ATTD), 2 g of chromium oxide/kg of feed was given to the animals 7 days before fecal collection. On days 14 and 35, fecal samples were obtained from two pigs in each pen (10 piglets per treatment) with identical body weights to obtain a representative composite sample. Fecal and feed samples were frozen until analysis. All ground samples were thawed and dried (72 h; 57℃) before chemical analysis. Association of Official Analytical Chemists (AOAC) techniques were used to assess dry matter, nitrogen, and gross energy in feed and feces (AOAC, 2005). Chromium and N were determined by UV absorption spectrophotometry (Shimadzu, UV-1201, Kyoto, Japan) and the Kjeltec 2300 Analyzer (Foss Tecator AB, Hoeganaes, Sweden) according to Hahn et al. (2006). The GE was measured using a Parr 6100 oxygen bomb calorimeter (Parr Instrument Co., Moline, IL, USA). Formula used to calculate ATTD, ATTD = {1 − [(Nf × Cd)/(Nd × Cf)]}. Herewith, Nf, concentration of nutrient in feces; Nd, concentration of nutrient in diet; Cd, chromium concentration in diet; and Cf, chromium dioxide concentration in feces.
Moisture content
To check the change in moisture content of feed, all the treatment diets were stored in the same storeroom at 30℃. At days 14 and 35 of the experiment, around 100 g of feed samples were randomly collected from different feed bags of the same treatment (eight samples/treatment) to determine dry matter and moisture content. Moisture content was determined by drying the feed in an oven at 105℃ for 24 h, according to AOAC (1995).
Statistical analysis
Data of body weight, feed intake, feed conversion ratio, nutrient digestibility and moisture content of feed were subjected to analysis of variance in a completely randomized block design (CRD) using SAS. Differences among all treatments were separated by Duncan’s multiple comparison tests. In the result p < 0.05 was considered statistically significant.
Result
Effects of reduced whey protein in TRT3 and semi-moisture diets with different energy contents during phase 2 of growth performance in weaning pigs are shown in Table 2. In this study, growth performance measures such as average daily gain, feed intake, and feed conversion ratio did not alter through the supplementation of a semi moisture diet with different levels of whey protein and an energy diet in weaning pigs. Similar overall body weight gain, feed intake, and feed efficiency were found in all of the treatment groups.
The effects of reduced whey protein and/or semi-moisture diets with different energy contents on apparent total tract digestibility in weaning pigs are shown in Table 3. During the second week of the feeding trial, no significant difference was found in nutrient digestibility in the different treatment groups. At the fifth week of feeding trial, nitrogen digestibility was decreased (p<0.05) in energy-controlled and semi-moisture feed compared to the control and no whey protein with modified energy semi-moisture feed. Digestibility in dry matter and energy was not changed by different levels of energy and/or moisture.
Table 4 illustrates the moisture content of different treatment diets. In the second week of the experiment, the control group showed higher (p < 0.05) dry matter with lower moisture content compared to the energy-controlled and semi-moisture-fed groups. Among the semi moisture feed without-whey protein feed contains more (p < 0.05) moisture compared to the energy-controlled semi-moisture feed. At the fifth week, dry matter was significantly higher (p < 0.05) in the control group as in week two. However, energy-controlled semi-moisture feed comprises a higher (p < 0.05) moisture content compared to both control and semi moisture feed without-whey protein.
Discussion
After weaning, pigs suffer from different stresses, such as nutritional, psychological, and environmental stresses (Song et al., 2015). During this period, both their digestive and immune systems are not developed, which causes growth inhibition as well as low feed efficiency (Dong and Pluske, 2007). They are suddenly forced to undergo a transition from highly digestible milk to solid diets including complex proteins (Ma et al., 2019), which can result in diarrhea or even death. Previous work showed that feed conversion efficiency improved when liquid proportions were increased (Geary et al., 1996). But there is a lack of research to confirm the optimum dry matter content in semi moist feed. Additionally, different animal protein sources, including whey protein and fish meal, are considered sources of readily digestible, high-quality protein for weaned piglets. But the high price, limited supply, and unstable quality of animal protein sources have become important reasons for limiting their addition to the diets of weaned piglets. In this study, we supplied feed in different forms of whey protein in TRT1 and TRT2, and replaced whey protein in TRT3 with protein concentrate, wheat, and Gyeji. However, the different diets showed similar body weight gain, feed intake, and feed conversion ratios. Similar results in the whey protein-replaced feed may be due to extrusion. Hancock and Behnke (2001) noted that extrusion results in changes in the physico-chemical properties of feed ingredients because of the temperature, pressure, friction, and attrition of the feedstuffs inside the extruder. But it was found that early weaned piglets' performance is influenced by the physical form of their feed (Kim et al., 2001). However, we used plant base protein source to replace the animal protein source. Previously, for replacing the animal protein source, some researchers tried other alternative protein sources like HWP for weaning piglets and noted that HWP is a good alternative with a variety of biologically active wheat peptides, which could promote the growth performance of weaned piglets (Wang et al., 2011; Han et al., 2017).
This study finds no effect of different treatments on nutrient digestibility except on nitrogen digestibility, where a negative effect on nitrogen digestibility was found in whey protein-containing semi moisture feed. This result agrees with a previous study where they explained that extrusion of whey proteins at a cooking temperature of 75℃ causes denaturation in different products (Onwulata et al., 2003). As the whey proteins denature, they become insoluble and aggregate (Walstra et al., 1999). For other feed ingredients, extrusion alters the physical and chemical characteristics and may increase the nutritional availability, this finding was supported by previous research (Oryschak et al., 2010).
We found lower dry matter and higher moisture content in extruded feed. Many variables influence the moisture content of the final product during extrusion, including extruder screw configuration, screw speed, extrusion temperature, raw material moisture content, and so on. the high water-absorption capacity of the starch content in raw materials makes it have a higher moisture content compared to non-extruded feed (Tovar-Jiménez et al., 2016; Castellanos-Gallo et al., 2019). The starchy components gelatinize, which results in a significant uptake of moisture and a final increase in dough viscosity (Rokey et al., 2010). This may justify the higher moisture content in extruded feed.
Conclusion
In conclusion, when the weaned piglets' diets were formed with different moisture levels and replacing whey protein with protein concentrate, Gyeji and wheat, the growth performance showed similar results in weight gain, feed intake, and feed conversion ratio. These results are very important for commercial applications where whey protein can be replaced by other protein sources in weaned piglets' diets. Though feed extrusion may result in reduced protein digestion of whey protein and lower dry matter content in extruded feed than non-extruded feed, it had no negative effects on pig performance. Therefore, reduced whey protein and semi moisture feed obtained through extrusion may be supplied to the newly weaned piglets.
