- Descriptive statistics of traits
From the descriptive statistics (Table 1 and SFig. 1), we find that the three sections (Shoulder cut, Middle cut, Leg cut) are similar in size, and their proportions are around 33%. Among the carcass cutting products, the largest piece is the boneless leg, which has an average proportion (the proportion is the ratio of cutting product in the whole carcass) of 24.73%, and the average weight and standard deviation are 8.77kg and 0.92, ranging from 6.24kg to 12.32kg. The second and third products are picnic shoulder and pork belly, with an average proportion of 18.1% and 13.7%, and the corresponding average weight and standard deviation are 6.42kg and 0.79 and of 4.87kg and 0.73, respectively. The total proportion of the above three cutting products reaches 56.53%, which is higher than the sum of the remaining cutting meats. Furthermore, there are some high economic value products that play a significant role in improving the overall value of the entire carcass cuts, such as boston shoulder, shoulder ribs, loin, ribs, tenderloin, etc. Among them, tenderloin has the smallest proportion, 0.97%, but it is the best quality meat in pork. For details of the weight, proportion, range and standard deviation of cutting products are listed in the Table 1. In carcass traits (Supplementary Table 1), the number of thoracic vertebrae and lumbar vertebrae ranged from 14 to 17 and 4 to 7, respectively. This may be the main reason for the large variation in carcass length, thoracic and lumbar length. In addition, the differences between the thicknesses of the backfat in the four positions are significant, and the order of the average thickness of the four-point backfat is Shoulder > 6th_7th rib > Waist > Hip.
To furthermore investigation into the variation of the cutting products, the coefficient of variation (CV) of weight and proportion of the cutting products were calculated. As expected, CV of weight is relatively large than CV of proportion, most of which is higher than 10%. The highest CV of proportion is backfat, which is 26.9% and the lowest CV of proportion is leg cut with the value of 1.4%. Similarly, the CV of carcass traits (except for the thickness of the backfat) ranges from 3% to 9%, while the CV of the backfat thickness ranges from 18% to 28% (Supplementary Table 1). The CV of the backfat of hip is the largest, with a value of 27.36%. In summary, the cutting meats and carcass traits vary greatly among different individuals, and they are affected by various factors such as carcass weight, sex, genotype, environment and so on.
- The effect of carcass weight
The carcass cuts products are significantly impacted by carcass weight (Table 2; P < 0.01). Unsurprisingly, the weight of all cutting meat increase with the increase of carcass weight. In contrast, carcass weight has a dissimilar effect on the proportion of cutting meat, which has a significant negative effect on most bone products (shoulder ribs, arm bones, shoulder bones, chine bones, leg bones and pelvis, except for ribs), shoulder cut and leg cut, while has a significant positive effect on pork belly, backfat and middle cut (SFig.2). The products with the largest increase in proportion are pork belly and backfat (increasing 1.19% and 0.81%), and the largest declining are the leg bones, ribs and arm bones (decreasing 0.53%, 0.39% and 0.34%). In the three block products, with the increase of carcass weight, the proportion of the middle cut increases significantly (the rise was 1.69%), and the proportion of the shoulder cut and leg cut decrease significantly (the falling-offs were 0.82% and 0.87%). No significant effects are observed for most meat products impacted by carcass weight (boston shoulder, picnic shoulder, loin, boneless leg and tenderloin, except for pork belly and backfat) (P > 0.05). This is consistent with the development and growth of pigs. At the beginning of the growth stage, the pigs are mainly growing the bone and muscles, and the proportion of muscle and bone products increases as the pig grows up. At the late stage of growth, the bone growth is mature and the feeding energy is mainly used for muscle growth and fat deposition. The proportion of muscles and bones is decreasing with the increase of carcass weight. [24,25] The proportion of middle cut significantly increases as it mainly contains fat and muscles. As the middle cut is the most valuable production, it hints us to properly extend the pig growth stage would gain more net economic values.
As can be seen from Table 3 and Supplementary Fig.3, the barrow is significantly heavier than sow on the weight of most cutting meats except for ribs, chine bones and pelvis. In the comparison of proportion of cutting meat between barrow and sow, the results are distinct. Sex has the greatest impact on backfat, and the proportion of backfat in barrow was 0.47% more than that of sow, which is equivalent to an increase of 15.51% of the backfat. Followed by the loin, contrary to the backfat, the proportion of backfat in sow was 0.3% more than that in barrow. Moreover, in the study of [26,27], it was also found that the proportion of loin in sows was significantly higher than that in barrow, while the proportion of backfat was the opposite. As loin and backfat are main parts of middle cut products and there are no significant differences on the proportion of middle cut between barrow and sow. It seems that the barrows are fatter than sows, which is out of our expectations, and is conflicting with the findings of Álvarez-Rodríguez & Teixeira [17]. Recall the fact that the barrow is significantly heavier than sow on the weight of most cutting meats, which means the weight of carcass would have the same result. We herein test the differences of carcass weight between barrows and sows and significant differences were detected (P value = 2.18*e-8). As barrows and sows were under same feeding time, we hypothesis that the higher proportion of fat in barrow than sow is because of the asynchrony of growth and development between the two sexes. It seems that barrows grow faster than sows and reach fattening stage earlier. To confirm this hypothesis, the four traits including picnic shoulder, pork belly, boneless leg and backfat were conducted regression analysis with body weight respectively. It can be seen from Fig. 3 that the weight and proportion of the backfat in the barrow is heavier than that in the sow under the same carcass weight. And as the carcass weight increases, the weight and proportion of backfat in the barrow increases faster than that in the sow. However, as the carcass weight increases, the weight and proportion of the pork belly in the sow grows faster than that in the barrow, while the sow's picnic shoulder and boneless leg grow at a rate similar to that of the barrow, and the trend of the proportion of picnic shoulder and boneless leg in barrows and sows is close to the same level. Results hint that barrows growing faster and reaching fatting stage earlier than sows prompts us that it’s better to breed the pigs separately by sex. The barrow should be slaughtered several days earlier than sows to decrease the fatting time and to provide products with more uniform specifications according to the market demand in different regions. In the three sections of carcass, the proportion of shoulder cut of the sow was higher and the proportion of leg cut was lower than that of the barrow. This means that the main increasing part are the shoulder cut at the growth stage and are the leg cut at the fatting stage during pig development. As different market regions of China have different demands for carcass cutting products, we can adjust the time of pigs come out to the market to fit the requirements.
As shown in Table 4 and Supplementary Fig.4, there are significant differences between genotype on the weight of almost all cutting meats and proportion of cutting meats. And the order of average slaughter weight of the four species is: DLY > YK > LD > LY. DLY is significantly heavier than LD and LY, also YK is significantly heavier than LY. In the weight and proportion of most cutting products, DLY differs greatly from the other three genotypes. Among the three cut products, the largest different section is the middle cuts, with the order of LD > YK = LY > DLY for the four genotypes and the proportion of LD being 1.17% higher than that of DLY. Following the proportion of the shoulder cut are reverse among the four genotypes and DLY is 0.89% higher than that in LD. The proportion of the leg cut is no significant difference among the four genotypes. The proportion differences reflect the differences in body structure between LD and DLY. LD is famous for the body length, so the proportion and weight of middle cut and its derivative products are higher and heavier than other genotypes. LY is a hybrid of LD and YK, and its proportion of cutting meats is often between the two. There are also others cutting products with large differences (such as pork belly, picnic shoulder, backfat and boston shoulder) between LD and DLY which were mainly from the shoulder and middle cut. Of which, pork belly is the product with the largest difference in proportion, with the proportion of LD being 0.8% higher than that of DLY. Their differences of theses cutting products in DLY and LD varieties are consistent with the differences of the section where they are from.
- The effects of slaughter weight, sex and genotype on the carcass performance
Similarly, the sex has an extremely significant effect on most of carcass traits (Supplementary Table 2). We found that barrows were shorter in length (including straight length and oblique length of carcass, thoracic and lumbar length), but were heavier in carcass weight and thicker backfat than sows. This is consistent with the results of [27,30,26]. This also confirmed our hypothesis from the other side that the different growth curves between barrows and sows. The growth rate of barrows is faster while the growing stage of sows is longer. The barrows start to deposit fat earlier than sows in the later stage, which results in heavier carcass weight and more backfat. The sows still keep growing and developing at the time of barrows’ starting of fat deposition, which results of longer body length and lighter carcass weight. This is also in keeping with cutting products results that gilts have higher proportion of ribs and loin and lower proportion of backfat than barrows. Similar to effect of carcass weight, thoracic length explained about 95% of extra increase of straight length in sows. Therefore, we believe that the effect of gender on straight length is mainly concentrated on the effect of the thoracic length. In the backfat thickness, the four-point backfat of the barrows is significantly thicker than that of the sows. This explains why the proportion of backfat in barrows is significantly higher than that in sows, and the proportion of loin, ribs and chine bones in sows are significantly higher than that in barrows.
As for the effect of variety on carcass traits, we can find that the longest carcass straight length is LD, which is significantly longer than the other three groups (Supplementary Table 3). Moreover, the differences in carcass length between LD and the other three groups are mainly caused by the differences of the number and length of thoracic. This is consistent with the result that the proportion of middle segment in LD is significantly higher than that in other genotypes. For the backfat thicknesses traits, the front three points (shoulder, 6th-7th rib and waist) of DLY have the thickest backfat, while the proportion of backfat in DLY is the lowest. The possible reason is that LD is the longest and DLY is the shorter of straight length and proportion of backfat is mainly from the middle cut, the increment of backfat by increase of body length higher than that by increase of backfat thickness.
- Correlation between cutting products and carcass traits.
We conducted a correlation analysis between the proportion of cutting meats and carcass traits. It can be found that the correlations of most carcass cuts and carcass traits were relatively low (r < 0.3, Fig. 4 and Supplementary Table 4). The strong correlation pairs were correlations between picnic shoulder and shoulder cut (r = 0.68), pork belly and middle cut (r = 0.68), boneless leg and leg cut (r = 0.87). This is because these cutting products are the highest proportions from the three preliminary cutting section. Among carcass traits, the correlation between four-point backfat thickness is high (correlation coefficient r > 0.41) while is lowly correlated to other carcass performance traits. The four-point backfat thickness has a medium positive correlation with the segmented product in the middle cut (pork belly, backfat and middle cut), with the correlation coefficient between 0.27 and 0.60. The backfat of hip is moderately negatively correlated with arm bones and leg bones, with correlation coefficients of -0.25 and -0.46, respectively. However, the correlation between other carcass traits with cutting products are low, and the correlation coefficients are less than 0.3.