## 4 Discussion
### Meat Composition and Quality  
Moisture content is the largest component in lamb meat. Researchers reported that different feeding regimes did not affect moisture in mutton [39; 40], similar to this study. Protein is the crucial nutrient in lamb meat, but its content in lamb meat was not affected by grazing or concentrate treatment [41]. The crude protein content in the meat of Tan lambs grazing on pasture was higher than in animals receiving drylot feeding in the present study. This variation might be due to the greater activity of grazing lambs [17], which prompted the decomposition of carbohydrate and fat and enhanced the synthesis of protein. The lambs in the G8, G2, and G0 groups received more concentrate and had relatively higher IMF contents than those in the G12 group. [PERSON] et al. [42] and [PERSON] et al. [43] observed the same situation that drylot feeding with high concentration feeds could increase fat deposition in muscles. Notably, intramuscular fat, as one of the main attributes to evaluate meat quality, was correlated with a tendency to lower water-holding capacity, but better palatability; juiciness, tenderness, and higher flavor precursors with an increase in the fat content [44; 45], and lower IMF indicated an adverse effect on eating satisfaction [46]. Moreover, lambs with proper grazing and concentrate supplementation feeding improved the fatty acid profile and indicated lower saturated fatty acid, higher \(n\)-3 polyunsaturated fatty acids, and ratio of \(n\)-3/\(n\)-6 compared to the indoor feeding [19]. Ash from meat is an important source of mineral for humans. In our study, the levels of ash from different treatments did not change significantly, which might be due to the fact that the lambs fed indoor had free access to the hay, and the ash level of the hay (8.75% of DM) was similar to the levels of some main herbages. Meat quality involves a number of parameters such as palatability, color, nutritional value, and safety, among others. It can be affected by many factors including genetics, feeding systems, and processing conditions [47]. Under different production regimes, diet composition and feed availability can directly or indirectly influence meat quality [48]. Muscle pH\({}_{45\text{min}}\) reflects the pH measured immediately in the meat postmortem [49]. Ultimate pH reflects the pH of muscle excised 24 h post mortem, and it is probably the most important indicator used to measure meat quality at a commercial level [50]. In the present study, the pH\({}_{45\text{min}}\) and pH\({}_{2\text{th}}\) of \(LT\) muscle in Tan lambs recorded did not change as grazing time and the amount of concentrate supplementation changed. The variation in ultimate pH among different feeding systems might be attributed to the variation in muscle glycogen contents, which was lower in lambs finished on pasture than lambs finished on a grain-based feedlot [51]. However, the effects of feeding regimes on muscle pH were inconsistent. Some results demonstrated that being fed with a commercial diet resulted in lower ultimate pH values when compared to grazing animals in reindeer [13] and lambs [31; 41]. Some other studies reported nonsignificant differences in meat pH among different feeding systems [38; 51]. Meat color is one of the main sensory quality parameters and is the most important factor that influences the consumers' initial selection and purchase decision on meat or meat products [52]. Fresh meat of _longissimus thoracis et lumborum_ (meat color was measured after a blooming period of 30-40 min) with a lightness value above 44 was acceptable by 95% of consumers, and below 34 was unacceptably dark for consumers [53]. The lightness value of _LT_ muscle improved with dietary concentrate supplementation in growing Tan lambs in our study. Similarly, a previous study also reported a higher meat lightness in lambs raised on concentrate-based systems compared with grazed pasture counterparts [36].