The Effects of Different Levels of Citric Acid on Growth Performance, Nutrient Digestibility and Gastrointestinal pH of Weaned Piglets

The immune system, digestive system and intestinal flora of weaned piglets are not completely developed, showing low immunity, insufficient gastric acid secretion and poor digestive function. The change of external environment, physical properties and dietary composition of diet after weaning may very easily cause weaning stress syndrome in piglets, which is mainly manifested as increasing diarrhea rate and decreasing growth performance of piglet (Giannenas et al., 2014; Salisbury et al., 2002; WU et al., 2015).

Studies have shown that CA is a type of high-quality natural acidifier which can improve feed flavor, growth performance and nutrient digestibility, decrease the pH value of gastrointestinal tract, reduce the number of harmful bacteria, increase the number of beneficial bacteria such as lactobacillus and improve the anti-stress ability and immunity of animals (Boling et al., 2000; Luo et al., 2019; Xu et al., 2018; Zheng et al., 2020; Zhou and Chen et al., 2010). However, in the practice of pig breeding, there are different reports on the amount and effect of CA in the diet of piglets (Huang et al., 2002; Krause et al., 1994; Krishnakumar et al., 2008; Li et al., 2009; Tsiloyiannis et al., 2001), this may be due to differences in basic diet composition, breeding environment and animal species. Therefore, the present study evaluated the effects of different levels of CA on the growth performance, apparent nutrient digestibility and gastrointestinal pH of weaned piglets in order to give a more theoretical basis for the rational use of CA in production practices.

This study was conducted in the Yunong livestock breeding farm located in Shijiazhuang and College of Animal Science and Technology, Hebei Agricultural University. The CA monohydrate, the main ingredient is CA, belongs to food additive, the content is 99%, manufactured by Weifang Yingxuan Industry Co., LTD.

A total of 120 piglets which were weaned at 21±2D days, with weight of approximately 7.15±0.01 kg, were supplied from the Yunong Livestock Breeding Farm located in Shijiazhuang and randomly divided into four groups including control treatment each with three replicates of 10 piglets. Each group was supplemented with 0.5% (control group), 1%, 1.5% and 2% CA in the basal diet, respectively. The experiment was carried out for 45 days, including 3 days of pre-feeding trial. The basal diet formula, a corn-soybean basal diet was prepared according to the guiding feeding level of the NY/T 65-2004 of China (Xiong and Zhang, 2010) and NRC (2012), as detailed in Table 1.



The ambient temperature of the piggery was controlled at 25-27^oC and the humidity is maintained at 55-65%. The piggery was cleaned and disinfected regularly. The piglets were fed three times at 7:00, 14:00 and 18:00 every day, with unrestricted intake of food and water. Daily feed consumption and body weight at the beginning and end of the trial were determined to calculate for the average daily feed intake per day (ADFI), average daily gain (ADG) and feed to gain ratio. The fecal samples from the pigs were collected every day from day 39 to 40 for the determination of digestibility. 10 ml 10% hydrochloric acid per 100 grams of feces was added to fix the nitrogen. The feeds and feces were oven-dried at 65^oC to obtain a constant weight and then ground to pass a 1-mm screen and store at 4^oC for chemical analysis according to the method of AOAC (2012). The CP were estimated by the Kjeldahl method, ether extract (EE) by the Soxhlet ether extraction apparatus, Calcium(Ca) by Potassium permanganate titration, phosphorus(P) by Molybdate yellow colorimetric method and  acid insoluble ash(AIA) was described by the GB/T23742(2009).The digestibility of nutrient were calculated as follows:
 

 
(Zhou and Chen, 2010)
 
At the end of the trial,3 weaned piglets with similar body weight in each group were killed under anesthesia. The stomach, duodenum, jejunum, ileum and cecum were separated with sterile cotton thread and the pH of gastrointestinal contents were measured with a portable pH meter. SPSS 22.0 software was used for processing and statistical analysis of the experiment of single factor test data (ANOVA) and Duncan’s method was used for multiple comparisons. The test results were expressed as “Mean ± SE”. Statements of statistical significance were tested at the level P<0.05 and extremely significant when P<0.01.

Growth performances
 
The growth performances are presented in Table 2. The ADFI and ADG of test group 3 were significantly higher (p<0.05) in comparison with the control. However, there was no significantly difference in ADFI in group 1, group 2 and group 3 and no significantly difference in ADG in group 1 and group 3. There was no significant difference in feed/gain ratio among all groups (p>0.05).



A previous study has shown that 2% (Giesting and Easter, 1985), 2% ~3% (Cronmwell, 1987), 2.5% (Krause et al., 1994). CA diets increased the ADG of weaned piglets. However, the current study testified that 1% CA diet had the best performance. But there was no significant difference in ADFI 1%, 1.5%. 2% CA groups. Similar to these findings were the studies by (Huang et al., 2002), Wang (2011) and (Zheng et al., 2005). The reasons for the inconsistent results among studies are not known but might be the weaning age of piglets, the buffering capacity of the diets, dietary components and the feeding environment.
 
Apparent nutrient digestibility
 
The apparent digestibility values were shown in Table 3. The digestibility of CP increased with the increase in dietary CA levels and there were significant differences among all groups (p<0.05). The digestibility of EE was reduced with the increase in dietary CA level and there was significant lower (p<0.05) in the 2% CA group in comparison with control and 1% CA groups. The digestibility of Ca and P were significantly increased with the increase in dietary CA levels (p<0.05), but there were no significantly differences in Ca and P digestibility between the 1% CA and the 1.5% CA groups (p>0.05), P digestibility between the 1.5% CA and the 2% CA groups (p>0.05).

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Similar to this result, (Debi et al., 2010) indicated that supplementation of 0.5%, 1.0%, 1.5%, 2.0% and 2.5% CA in growing rabbit diet improved CP digestibility. (Broz et al., 1987) found that adding 0.5%, 1.0% or 2.0% CA in the basal diet of weaned piglets could significantly improve feed utilization rate. (Cao et al., 2015) reported that adding 0.5% CA and compound acid (0.25% benzoic acid and 0.25% CA) significantly increased Ca and P digestibility. (Diao et al., 2013) determined that dietary 0.5% CA significantly increased the digestibility of crude protein, ether extract, calcium and phosphorus of weaned piglets.But this study showed that 2% CA group had the highest digestibility of ether extract, which was inconsistent with the report by (Diao et al., 2013).
 
Gastrointestinal pH
 
As shown in Table 4, The gastrointestinal pH were significantly reduced with the increase in dietary CA levels (p<0.05), but there were no significantly differences in pH of stomach contents between the 1% CA and the 1.5% CA groups (p>0.05), pH of duodenum contents between the 0.5% CA and the 1% CA groups and between the 1% CA and the 1.5% CA groups (p>0.05), PH of jejunum contents  between the 0.5% CA and the 1% CA groups, pH of ileum contents between the 0.5% CA and the 1.5% CA groups.

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(Klieviiut et al., 2016) showed that organic acids had no significant effect on pH in different parts of the stomach and intestines of rabbits. There were no significant differences in stomach and small intestine pH of weaned piglets which also supported by (Piva et al., 2007) who reported that no effect on gastrointestinal pH after inclusion of sorbic acid in swine diets. However, (Esmaeilipour et al., 2011) reported that adding CA to a low-phosphorus diet in broilers significantly reduced gastrointestinal pH (Risley et al., 1992) found that adding 1.5% CA to corn soybean meal diet of weaned piglets reduced the pH of ileum contents. The results of this study showed that additions of CA could significantly reduce the pH levels of gastric, ileal and jejunal contents. The results of different effects of acidifiers on the pH of gastrointestinal tract contents may be related to the treatment technology and the inclusion level of acidifiers as well as the differences of animal species and individuals.

In the present study, it was found that under the experimental conditions, the addition of 1% CA in the diets of weaned piglets in accordance with various indicators and production benefits had effectively improved intestinal development, significantly reduced the pH level of the gastrointestinal contents and increased the apparent nutrient digestibility in the feed, thereby enhancing the performance of the examined weaned piglets.

Thanks to the Hebei Provincial Department of Agriculture (HBCT2018110203) for providing financial support for publishing this article.