Effects of Different Protein Energy Ratios on Fattening Performance, Slaughter Perfor-mance and Meat Quality of Pigs

Cui YANG, Rongsheng JIANG, Yongshao WU, Xiying YANG, Haihong LIAO, Qian QIN, Shishan QIN, Ying ZHANG, Jiahuang YANG, Limei QIN, Fulin WANG

Abstract ［Objectives］Protein energy ratio refers to the proportional relationship between protein and energy levels in animal diets， i.e.， the grams of crude protein corresponding to every megacalorie of energy， which is generally expressed as CP： ME or CP： DE. This study was conducted to investigate the effects of different diets on fattening and slaughter performance and meat quality traits for "L （Large Yorkshire） × L （Landrace）" crossbred pigs.

［Methods］Eighteen piglets of L×L crossbred with similar body weights about 51 kg were selected. The piglets were divided into 3 groups randomly and each group was assigned to 3 replicates with 2 piglets in a replicate. Group A was fed diet I （control diet）， group B was fed the same diet of group A in the first month of the trial but fed diet II in second month， and group C consumed diet III. All the pigs were fed in the same feeding condition for two months except the different diets during the trial. One pig in similar body weight from each replicate was slaughtered for the determination of slaughter performance and meat quality traits in the end of the trial.

［Results］ The average daily feed intake （ADFI） of pigs from group B was only （2.32±0.52） kg and significantly lower than those from group A and group C （P<0.05）， but no difference was found in average daily gain （ADG） and feed/gain （P>0.05）. Also， no differences occurred in the carcass length， back fat thickness， longissimus muscle （LM） area and dressing percentage （P>0.05）. In addition， no differences were found in the meat quality traits of shear force， meat color， pH45 min， pH24 h and cooking loss （P>0.05）. However， the water-holding capacity of meat from group C was （2.58±0.02） ms and significantly lower than that of （2.80±0.20） ms from group A （P<0.05）. Although the contents of glutamic acid and cystine in LM from group B was significantly lower than those from group A and group C （P<0.05）， no differences occurred in the contents of other amino acids， the total amino acid and total flavor amino acid among the three groups （P>0.05）. However， the inosine monophosphate content of LM from group C was only （331.80±11.53） mg/100 g and significantly lower than those of （361.00±6.36） and （366.37±4.80） mg/100 g from group A and B （P<0.05）. Even though no differences were found in the contents of DM and CP in LM among the three groups of pigs， the content of intramuscular fat （IMF） in LM from group B and group C was increased by 45.6% and 46.58% respectively from that of group A （P<0.05）， but no difference occurred between group B and C （P>0.05）.

［Conclusions］ Diets II and III in this study caused no differences in fattening and slaughter performance of L×L crossbred pigs， but the effects on some meat traits were still significant， especially on the improvement of intramuscular fat in experimental pigs. Therefore， they could improve the meat quality of crossbred pigs to a certain extent.

Key words Protein energy ratio; L×L crossbred; Fattening; Slaughter performance; Meat quality; IMF

Received： November 15， 2022  Accepted： January 16， 2022

Supported by Guangxi Agricultural Science and Technology Self-financing Project （Z2022114， Z2022111）.

Cui YANG （1979-）， female， P. R. China， assistant research fellow， master， devoted to research about animal nutrition and feed science.

*Corresponding author. Limei QIN （1972 -）， female， senior engineer， devoted to research about ecological breeding of livestock and poultry and development and utilization of local varieties. Fulin WANG （1986-）， male， master of animal nutrition， master of veterinary medicine， devoted to research about animal nutrition and feed research and development.

Pork is the most important meat food for the Chinese people. It is deeply loved by the public because of its thin fiber， less connective tissue， more intramuscular fat， easy chewing and digestion， and delicious taste. How can we continuously improve the fattening performance， slaughter performance and meat quality of pigs has become the most important issue to be concerned about. There are many factors that affect pig fattening and slaughter performance and meat quality traits， such as breed， sex， age， feed nutrition and feeding management conditions［1］， especially feed nutrition， because it is the raw material for forming pig body tissue or meat quality， while the nutritional substances come from the feed diet. Therefore， it is of great significance to study the composition of pig diet and its nutritional regulation on the growth and fattening of pigs and the improvement of their slaughter performance and meat quality. Some studies have shown that when using unconventional feed appropriately， in addition to reducing feed costs， the taste， tenderness and marbling indexes of pork were improved compared with the control group［2］. For example， adding an appropriate amount of mulberry powder to the diet can not only reduce feeding costs， improve the average daily weight gain of experimental pigs， reduce the water loss rate and shear strength of pork， but also significantly increase the content of intramuscular fat， and the relative content of volatile flavor compounds such as hexanal and butyrolactone that have important contributions to flavor in pork［3-4］. Adding an appropriate amount of perilla seed extract to the diet significantly increases lean meat percentage and longissimus muscle （LM） area， reduces back fat thickness， and improves intramuscular fat and inosinic acid content［5］. It can be seen that the quality of pork can be improved to some extent by properly changing the composition of diet.

However， some studies have shown that using different proportions of bean curd residue or fermented corn straw （flour） in the diet can also reduce feed cost， but it does not significantly improve the growth and fattening， slaughter performance and carcass meat quality of pigs［6-7］. Adding arginine to high （digestible） energy diet can increase intramuscular fat content， reduce shear strength and improve meat quality［8］. However， when four groups of Luchuan pigs were fed with four diets with high energy and high protein， high energy and low protein， low energy and high protein， and low energy and low protein at the same time， respectively， no significant differences were found in growth performance， carcass traits and meat quality traits among the four groups of experimental pigs［9］. It can be seen that it is possible to improve the growth， fattening and slaughter performance of pigs and improve their meat quality by changing the composition of the diet or regulating the nutrients therein. However， there are still many difficulties to achieve desired good results. Therefore， more research is needed. In this study， the composition of the conventional corn-soybean meal diet used as a control was changed by changing its protein/energy ratio at the later stage of feeding or adding certain proportions of distillers dried grain with solubles （DDGS） and allicin in the diet to observe the effects on fattening and slaughter performance of "L （Large Yorkshire） × L （Landrace）" crossbred pigs， especially on meat quality traits.

Materials and Methods

Experimental materials and design

L×L crossbred pigs weighing about 51 kg were selected from Kexinyuan pig farm of Guangxi Institute of Animal Science. The piglets were divided into 3 groups randomly and each group was assigned to 3 replicates with 2 piglets in a replicate. Group A was fed diet I （control diet）， group B was fed the same diet of group A in the first month of the trial but fed diet II with reduced protein/energy ratio in second month， and group C consumed diet III. The composition and nutritional levels of the three diets are shown in Table 1. All the pigs were fed in the same feeding condition except the different diets during the trial.

Experimental time and site

The experiment lasted for 2 months and was conducted at the breeding experimental base of Guangxi Institute of Animal Science.

Determination indexes and methods

Fattening performance

At the beginning and end of the experiment， the test pigs were weighed on an empty stomach before early feeding， and the daily feed intake was recorded， and the average daily gain and feed/gain ratio during the period were calculated.

Slaughter performance

At the end of the experiment， one test pig in similar body weight was selected from each replicate for slaughter determination. The animals were fasted （free access to water） for 12 h before slaughter， and the slaughter test was conducted according to the procedure in reference［10］. The left half of the carcass was taken conventionally to measure the carcass length， back fat thickness， LM area， etc.， and the dressing percentage was calculated.

Determination of meat quality traits

After slaughter， the middle part of LM was taken to measure pH （PHS-3C laboratory pH meter： Shanghai Jinmai Instrument Co.， Ltd.）， shear force （digital display muscle tenderness meter： developed by Engineering College of Northeast Agricultural University）， and meat color and water-holding capacity （intelligent meat color detector OPTO-STAR and intelligent conductivity detector LF-STAR： Maites （Germany））. The psoas major muscle was taken to measure cooking loss［7］.

Muscle composition analysis

The middle part of each LM sample was taken to determine the contents of amino acid， inosinic acid， protein， fat， water， etc.［11］. Amino acid and inosinic acid indexes were determined by Guangxi Testing Center.

Data statistics

After the test data were processed with Excel software， SPSS12.0 for Windows statistical software was employed for analysis of variance， and Duncan method was adopted for multiple comparisons. The results were expressed as mean±standard deviation （X±S）.

Results and Analysis

Fattening performance of experimental pigs

The fattening performance of the test pigs is shown in Table 2.

It can be seen from Table 2 that there were no significant differences in the initial weight， final weight and average daily gain among the three groups of experimental pigs （P>0.05）， but the average daily feed intake of group B was significantly lower than those of group A and group C （P<0.05）. Since the average daily weight gain of group B was also slightly lower than those of group A and group C， there were no significant differences in the feed/gain ratio among the three groups （P>0.05）.

Slaughter performance of experimental pigs

From the data in Table 3， it can be seen that although the carcass weight of group B was lower than that of group C （P<0.05）， there were no significant differences in the dressing percentage among the three groups （P>0.05） because the live weight before slaughter was also slightly lower than that of group C. There were no significant differences in carcass length， back fat thickness and eye muscle area among the three groups （P>0.05）. It could be seen that the three different diets had no significant effects on the main slaughter performance of L×L crossbred pigs.

Meat traits of experimental pigs

From the results in Table 4， it can be seen that there were no significant differences in the shear strength， meat color， cooking loss， pH45 min and pH24 h among the three groups of test pigs （P>0.05）， However， the water-holding capacity of group C was significantly lower than that of group A （P<0.05）， while there was no significant difference between group A and group B or between group B and group C （P>0.05）. It showed that except for the water content， the three diets had no significant effects on other meat traits in this study.

Analysis of LM components in experimental pigs

The analysis results of LM components in the experimental pigs are shown in Table 5 and Table 6. The amino acid analysis data in Table 5 showed that although the contents of glutamic acid and cystine in group B were significantly lower than those in group A and group C （P<0.05）， there were no significant differences in the contents of other amino acids， especially total amino acid and total flavor amino acid among the three groups （P>0.05）. However， the inosinic acid content in group C was significantly lower than those in group A and group B （P<0.05）， while inosinic acid is an important flavor substance in pork.

From the results in Table 6， it can be seen that there was no significant differences in the dry matter and crude protein content of LM among the three groups of experimental pigs （P>0.05）. However， the contents of intramuscular fat in groups B and C were significantly higher than that in group A （P<0.05）， while intramuscular fat was a very important prerequisite affecting the flavor of pork products.

Conclusions and Discussion

The control （group） diet used in this study was the corn-soybean meal diet used in pig production or experimental research. Therefore， the average daily gain， feed/gain ratio and dressing percentage of the control group， which were 0.828 kg/d， 3.29 and 75.09%， respectively， and other slaughter performance or meat quality traits were similar to the results of many studies［12-13］. However， when the protein/energy ratio of the diet was reduced from 13.54 g/MJ of the control group to 11.80 g/MJ， that is， corn in the composition of the diet was increased from 67% to 70%， and the soybean meal was reduced from 26% to 20% to form the diet II， even if its digestible energy content reached 13.251 MJ/kg， which was almost the same as 13.372 MJ/kg of the control group diet， the average daily feed intake of the experimental pigs was reduced by 14.7%， and the difference between the two groups was significant. It might be due to the fact that DE in diet II mainly came from corn. Because the proportion of corn in diet II was high， the DE derived from corn was also relatively high in the total DE consumed by experimental pigs; and the proportion of soybean meal in diet I was significantly higher than that in diet II， so the DE from soybean meal was also higher than that in diet II. DE from corn can be directly used by the body after digestion and absorption， but DE from soybean meal can only be used after the bodys protein metabolism process. Therefore， this difference might be the main reason for the reduction or difference of daily feed intake.

Perhaps because the proportion of soybean meal in diet II decreased， the contents of glutamic acid and cystine in LM of experimental pigs in group B fed with diet II in the later period were significantly lower than those of group A and group C. However， there were no significant differences in the total amino acid or the total flavor amino acid of LM from group A or group C. Moreover， except that the intramuscular fat of group B was significantly higher than that of group A， there were no significant differences between group A and group B in other substances related to meat quality determined in this study. It could be seen that diet II had no effects on other meat quality except reducing the contents of glutamic acid and cystine in LM and increasing intramuscular fat.

The water-holding capacity of LM in group C fed with diet III was significantly lower than that in group A， and the content of inosinic acid was also significantly lower than those in group A and group B. The water-holding capacity of pork is one of the important indexes of its meat quality， and is closely related to its juiciness. However， there are many factors that affect the water-holding capacity of pork. In addition to the type， variety， age， growth status of animals， it is also closely related to the structure and composition of muscle tissue， especially the protein and fat contents and the pH of muscle tissue， which directly affect the water-holding capacity of meat［11］.  In this study， pigs in groups A and C were fed in the same condition except for the different diets， and there were no significant differences in the pH and protein content of LM between the two groups. However， the intramuscular fat content of group C was （7.49±0.17）%， which was significantly higher than that of group A， （5.11±0.24） % （P<0.05）. Was the lower water-holding capacity of LM in group C than that in group A related to the higher intramuscular fat content in group C than that in group A？ In other words， was there a certain negative correlation between the intramuscular fat content of muscles and their water-holding capacity？ It needs further study.

Inosine is an important presenting substance for the flavor of livestock and poultry muscles［14］， especially for chicken， the umami taste of which can be significantly presented［15］. Inosine is an important substance that can make muscles taste fresh， especially for chicken flavor［12］. Therefore， the content of inosinic acid in muscles is also an important index of meat quality. There are many factors that affect the content of inosinic acid in muscles， among which the composition of feed diet is one of the important factors［16］. The content of inosinic acid can be increased to a certain extent by adding Chinese herbal medicine additives to the diet， and the content of inosinic acid will also increase with the increase of the amount added［17］. Adding 0.1% betaine to the diet also increased the inosinic acid content of the longissimus dorsi muscle of test pigs by 21.79%［18］. In this study， the content of inosinic acid in LM of test pigs in group C fed with diet III was significantly lower than that of group A， which might be related to the composition of their diet. However， the reason or mechanism of its impact is unknown， and needs further study.

It is worth noting that the intramuscular fat （IMF） of LM in group B and group C fed with diets II and III reached， respectively， （7.44 ± 0.18）% and （7.49 ± 0.17）%， which were 45.6% and 46.58% higher than （5.11 ± 0.24）% in group A （P<0.05）. Studies have shown that IMF is an important index that affects the quality of pork. It is not only the physical factor that makes pork moist and juicy， but also the precursor of flavor substances. Therefore， intramuscular fat is positively correlated with meat quality， affecting the tenderness， flavor and juiciness of meat， especially the tenderness of meat［19］. Many studies have proved that different compositions of diets can improve the IMF content of fattening pigs［20-21］. The results of diet II and diet III used in this study are similar to those of these studies in improving the IMF content. It could be seen that both diet II and diet III could increase the intramuscular fat content of the experimental pigs and improve their meat quality to a certain extent.

Diets II and III used in this study caused no differences in fattening and slaughter performance of L × L crossbred pigs， but the effects on some meat traits were still significant， especially on the improvement of intramuscular fat in experimental pigs. Therefore， they could improve the meat quality of crossbred pigs to a certain extent.

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