## 4 Discussion
However, a WRF-pretreated corn straw-based diet can provide more dietary protein, thereby counteracting protein utilization by GINs [14]. When WRF are in a nitrogen-limited environment, communal GINs may provide essential nitrogen for communal fungi [31]. As mentioned above, to a certain extent, WRF can offset the damage caused by GINs to protein supply and demand for sheep and further improve meat color, although further research is needed to provide more insights. Plasma iron content is closely related to total muscle iron content and can be used as an indicator of changes in meat color. Heme iron in myoglobin is directly proportional to redness of meat [32]. When heme increases, heme iron in myoglobin also increases, which may finally increase \(a^{\star}\) and decrease \(L^{\star}\) under extreme anemia conditions [33]. Therefore, actors causing plasma iron deficiencyalter meat iron content and meat color. The blood sucking nematode _H. contortus_ is of primary concern, as it can cause anemia. Each _H. contortus_ can suck \(\sim\)0.05 mL blood from its host sheep daily [34], causing iron deficiency anemia and hypoproteinemia. Lambs infected with _H. contortus_ had lower serum iron content than uninfected lambs [35], similar to the previous results, decreases in _L*_ and _b*_ and increases in _a*_ in the present may be due to increased total iron content of muscles. Perhaps WRF-pretreated corn straw alleviated GIN-induced anemia in lambs and thus improved meat quality. In addition, iron is also a cofactor of catalase in the antioxidant system and has an important role in preventing oxidation of lipids and maintaining meat quality [36]. In the present study, the pH\({}_{24}\) of meat from both dietary treatments were all within the acceptable range (5.6-6.4) for lambs [37], implying pH was not affected by consumption of WRF-pretreated corn straw. Similarly, WRF did not affect muscle pH at 24 h in beef cattle [38]. Deposition of IMF is usually slow and mainly affected by nutrition. In the present study, IMF was not significantly affected, perhaps due to the relatively short feeding period (70 d). Tenderness is the most important to consumer's perceptions of acceptability. Compared to pork or chicken, tenderness is even more important for red meats such as beef and lamb, as red meat contains many muscle fibers and much connective tissue. We previously reported that _H. contortus_ infection decreased shear force of lamb from 27.65 N (un-infected lambs) to 18.88 N, with meat softer than the normal. In meat from 6-month-old Ujumqin lambs, shear force ranged from 30.2 to 35.7 N [26; 39]. In the present study, WRF-pretreated corn straw improved tenderness of lamb meat, indicating that feeding WRF-pretreated corn straw is beneficial to improve meat quality. Meat quality should generally have an integrated evaluation, as most parameters are closely related and affected by many physiological processes, especially in ruminants. Distribution and conversion of muscle fiber types is related to meat tenderness. Four distinct single MyHC isoforms were identified, including slow-oxidative MyHC-I, fast-oxidative MyHC-II\(\alpha\), fast-oxidative glycolytic MyHC-II\(\beta\) and fast-glycolytic MyHC-II\(\beta\). Increased content of MyHC-I and MyHC-II\(\alpha\) in muscle fibers increased meat tenderness, whereas MyHC-II\(\beta\) had the opposite effect [40]. Furthermore, muscle fiber transition from MyHC-II\(\beta\) to MyHC-I may cause more tenderness [41]. In the current study, total expression of all four kinds of MyHC was lower in WRF than CON and the proportion of MyHC-I and MyHC-II\(\alpha\) slightly decreased, whereas the proportion of MyHC-II\(\beta\) and MyHC-II\(\alpha\) was greater in the two test muscles from sheep consuming WRF versus CON. The MyHC data partially explained changes in shear force. Meat color is mainly determined by pigments, including myoglobin and hemoglobin. Contents of these two pigments are high in oxidized fiber.