Establishment of Prediction Equations for Tissue Weight and Chemical Composition of Gangba Sheep Based on Carcasses Weight

Gangba sheep, a pivotal breed in Qinghai-Tibet Plateau sheep production, are well-adapted to the harsh plateau environment, contributing to local ecological balance and serving as an important income source for farmers (Ding et al., 2024). Accurate prediction of body and carcass traits is crucial for improving farming efficiency and meat production. Researchers worldwide continue to pursue this, studying breeds like Ouled Djellal and Hamra sheep in Algeria (Houssou et al., 2024), Harnali sheep (Kumar et al., 2017) and Hassan sheep (Kumar et al., 2022) in India.

Various studies have explored various methods such as body measurements (e.g., height at withers, body length) and ultrasound to predict sheep body composition. While body measurements are non-invasive and cost-effective, their accuracy can vary with breed and age (Salazar-Cuytúnet_al2022). Ultrasound measurements can accurately assess fat and muscle depth but is limited by equipment and operator expertise (Van Der Merwe et al., 2022). The most accurate method for predicting carcass composition is complete carcass separation into lean meat, bone and fat, but it is labor-intensive and impractical for commercial use. Recent studies suggest that tissue measurements (Hopkins et al., 2008), joint dissection (Santos et al., 2017) and neck traits Rivera-Alegria et al. (2022) can reliably indicate carcass composition. Thus, using carcass traits has become a more practical approach. Carcass traits such as slaughter body weight (SBW) and empty body weight (EBW) directly and accurately predict body composition (Barcelos et al., 2021). SBW is the weight before slaughter, while EBW is the weight after removing gastrointestinal contents and non-carcass components, offering practical measurements less affected by age and breed (Owens et al., 1995). Additionally, fleece-free empty body weight (FF-EBW) serves as a common baseline for evaluating carcass or organ conditions and was included as a key indicator in our regression equations (Aziz et al., 1993).

Thus, this study aims to utilize carcass traits, including SBW, EBW and FF-EBW to precisely forecast the tissue weight and chemical composition of Gangba sheep carcasses. This approach has not been extensively explored in existing literature, particularly within the context of Gangba sheep, thereby offering new insights and potential applications in the field of animal nutrition and production.

Ethics statement
 
This experiment was conducted from April to June 2024 at the Mende sheep raising professional cooperative (Tibet, China). All experimental procedures mentioned in the manuscript comply with the ethical guidelines and regulations for animal trials of Tibet Academy of Agricultural and Animal Husbandry Sciences (Approval code: TLRI-SEC-2024-031).
 
Animal management and diet
 
Eighteen healthy male Gangba lambs (average initial body weight: 15.4±1.2 kg) were selected for the experiment. After a 10-day adaptation period, the lambs were randomly assigned to six groups (three lambs per group) with varying feeding levels to achieve different body weight gains. Groups I, II and III targeted nutritional levels at 0.8, 0.9 and 1.0 times the recommended average daily gain (ADG) of 100 g/d for a 20 kg sheep, respectively. Groups IV, V and VI targeted 0.8, 0.9 and 1.0 times the ADG for a 20 kg sheep gaining 200 g/d. Diets varied in metabolizable energy (ME) and crude protein (CP) levels, designed to meet the specific growth targets (Table 1). We intentionally varied ME and CP levels to examine their effects on tissue weight and chemical composition in Gangba sheep.

 

Slaughter and sample collection
 
Lambs in Groups I -III were slaughtered at an average weight of 28.6 kg, while those in Groups IV-VI were slaughtered at 33.3 kg. After a 17-hour fasting period, the slaughter body weight (SBW) was recorded and lambs were slaughtered using standard commercial procedures after CO2 stunning. The empty body weight (EBW) was calculated by subtracting the gastrointestinal contents from the SBW and the fleece-free empty body weight (FF-EBW) was obtained by further subtracting the wool weight from the EBW. Carcasses were split along the dorsal midline and the right half, including hooves and head, was dissected to separate muscle, fat, bone and hide (Pereira et al., 2014). Representative samples of bone, muscle, fat, blood plus viscera, hide and fleece were collected for further analysis.
 
Chemical analysis
 
Samples were analysed for dry matter (method 934.01), crude protein (method 920.87), crude fat (method 920.85) and ash (method 924.05) according to Association of Official Analytical Chemists (AOAC, 2000). Calcium and total phosphorus contents were determined using coupled plasma emission spectroscope and total energy was measured using an oxygen bomb calorimeter (Mitong Electromechanical Technology Co. Ltd., Shanghai, China).
 
Establishment of  prediction equations
 
Simple linear regression models were employed to develop prediction equations for tissue weight and chemical composition, using SBW, EBW and FF-EBW as independent variables. The selection of these variables was based on their significant correlations with tissue traits. The models were evaluated by the coefficient of determination (r²) and root mean square prediction error (RMSE).
 
Statistical analysis
 
Data were analysed using IBM SPSS Statistics (version 26.0; Chicago, IL, USA). Normality and variance homogeneity were assessed before conducting parametric tests. One-way ANOVA was used to identify significant differences among groups, while Pearson’s correlation and t-tests assessed relationships between variables. Regression models were evaluated by r² and RMSE values, with coefficients greater than 0.70 indicating strong associations and those from 0.30 to 0.70 indicating moderate correlations Gomes et al. (2021). The significance level was set at P<0.05.

Carcass measurements
 
Table 2 shows that carcass characteristics, including SBW, EBW, FF-BW and FF-EBW, increased linearly with feeding levels. Notably, the EBW/SBW ratio in Group I was lower than in the other groups (P<0.05). Similarly, carcass composition parameters, such as DM, protein, fat, ash and gross energy, also increased linearly with feeding levels (P<0.05). Table 3 reveals that the hide DM/body DM ratio in Group V was higher than in Groups II and IV and in Group VI, it was higher than in Group II (P<0.05). Both tissue fresh weight and DM weight increased linearly with feeding levels across all tissues, with the lowest values in Group I and the highest in Group VI (P<0.05).




As anticipated, carcass characteristics and components increased linearly with increasing feeding levels. This can be attributed to the elevated levels of crude protein and ME in the diets, resulting in optimal synchronization of carbohydrate and protein in the rumen and subsequently improved production performance (Costa et al., 2013). However, the fat content, being the most variable component, raises question of whether its changes are primarily influenced by breed, sex, age, or body weight (Tedeschi et al., 2013). Our results revealed that body fat content is mainly related to carcass weight, making body weight the key parameter. To further analyze changes in carcass components, we divided the sheep’s carcass into five parts: bone, muscle, fat, blood and viscera (BV) and hide. As expected, the fresh weight and DM weight of different tissues in Gangba sheep carcasses were consistent with the aforementioned carcass characteristics. Specifically, they uniformly displayed the lowest values in Group I and the highest values in Group VI, indicating the accumulation of major body tissues during growth. It is noteworthy that carcass characteristics and components in Group III were consistent with those in Groups IV and V, despite Group III’s lower crude protein content in the diet compared to the other two groups (12.88% versus 15.29% and 16.95%, respectively). This finding aligns with a previous experiment where the growth rate of young sheep was not affected, despite different crude protein levels being provided (110 g versus 160 g CP/kg of DM) (Hajji et al., 2015). Furtherly Considering the consistent metabolic energy across the three groups, this finding underscores the pivotal role of dietary energy concentration in the growth process of Gangba sheep. The beneficial effects of higher energy density in the diet on sheep’s growth performance and meat production have been well-documented (Jaborek et al., 2018). Thus, our findings indicate that in Gangba sheep production, a diet with higher energy density and relatively lower crude protein content could sustain favorable growth performance, thereby reducing the consumption of protein feeds.
 
Correlation coefficients and prediction equations of carcass measurements
 
Based on Fig 1, except for carcass blood and viscera (CBV), all tissue traits were highly correlated with SBW, EBW and FF-EBW (0.73≤ r≤0.92; P<0.01). Regression equations for estimating tissue weight were developed (Table 4). While blood and viscera (BV) had a low correlation with the EBW (r²adj = 0.23; P = 0.03), other tissues, including bone, muscle, fat and hide, showed significant correlations with EBW (r²adj ≥0.59; P<0.01). Table 5 presents the regression relationship between nutritional contents and FF-EBW, with significant correlations found for moisture, DM, protein, fat, ash (r²adj ≥0.55; P<0.01) and gross energy (r²adj = 0.92; P<0.01).





Many studies have demonstrated the feasibility of forecasting tissue weight and body chemical composition using carcass traits, which aids in managing feed strategy precisely (Santos et al., 2017; Barcelos et al., 2021). For instance, a previous study accurately predicted the carcass composition of Santa Inês sheep using cold carcass weight (CCW) as the predictor: muscle (kg) = 0.23671+0.5829 CCW (r² = 0.9768, P < 0.0001); adipose (kg) = – 0.76543 + 0.20132 CCW (r² = 0.8615, P<0.0001) (Gomes et al., 2021). Additionally, according to Morais et al., (2016), fasting body weight and withers height were the variables with the best results in predicting carcass fat and muscle content. Therefore, in the present experiment, we selected empty body weight (EBW) to predict the tissue weight of Gangba male lambs. While the blood and viscera (BV) had a low correlation with EBW (r²adj = 0.23; P = 0.03), the other tissues, including bone, muscle, fat and hide, were significantly correlated with EBW (r²adj ≥ 0.59; P<0.01). Despite a lower r² compared to a previously mentioned muscle prediction equation (0.83 versus 0.98), our prediction equation showed promise for application in Gangba sheep production. To predict carcass composition more accurately, we chose fleece-free empty body weight (FF-EBW) as the indicator instead of EBW. Our results revealed that moisture, DM, protein and fat were highly correlated with FF-EBW (r²adj ≥0.77; P< 0.01). However, ash showed a moderate correlation (r²adj = 0.55; P <0.01), possibly due to the stable concentration of major elements in tissues across different feeding levels (Bellof et al., 2006). Based on our results, both EBW and FF-EBW can serve as appropriate indicators to predict tissue weight and chemical composition of Gangba sheep carcasses. Notably, both EBW and FF-EBW can be estimated with high accuracy using the slaughter body weight (SBW) in the present experiment (r²adj ≥ 0.86; P< 0.01). Therefore, our findings offer a new approach for estimating tissue weight and chemical composition in Gangba sheep. However, relying solely on weight parameters may lead to inaccuracies in predicting actual carcass composition. Future studies should incorporate additional biometric measurements for more accurate predictions (Gomes et al., 2021). While our preliminary results are promising, we acknowledge the limitation posed by the small sample size. Further research with larger samples is needed to validate the reliability and robustness of these prediction equations.

Our findings indicate that Gangba lambs weighing between 23.7 and 33.3 kg are suitable candidates for moderate-intensity fattening. Significant correlations were observed between EBW and tissue weights (r²adj ≥0.59; P<0.01) for bone, muscle, fat and hide. Additionally, FF-EBW showed significant correlations with nutritional contents such as moisture, DM, protein, fat and ash (r²adj ≥0.55; P<0.01). These findings underscore the potential of EBW and FF-EBW as reliable indicators for estimating tissue weight and chemical composition of Gangba sheep carcasses.Our findings offer valuable insights into Gangba sheep nutrition and production, despite some limitations. 
 

This work received funding from the Tibetan Major Science and Technology Projects (XZ2021ZD0001N); the National Key Research and Development Program of China (2022YFD1302102); and the National Wool Sheep Industry Technology System (CARS-39-33). The authors would like to thank the Mende sheep raising professional cooperative for animals handling.
 
Data availability statement
 
Data will be made available on request.

Informed consent
 
All experimental procedures mentioned in the manuscript in compliance with the ethical guidelines and regulations for animal trials of Tibet Academy of Agricultural and Animal Husbandry Sciences (Approval code: TLRI-SEC-2024-031).

The authors declare no conflicts of interest.