The morpho-functional and productive characterization of the MN breed is non-existent, since the morphological evaluation is limited to considerations about breeding pattern and is only used in promotional events, fairs and exhibitions organized by breeders’ associations. This study assesses the productive potential of the MN breed through morphometric, zoometric, productive data and carcass characteristics using a systematic approach with successive multivariate techniques for different objectives.
Our main findings were: (i) zoometric analyzes show the capacity for morpho-productive classification of lambs from the two genetic groups of the MN breed; (ii) the zoometric indices and body morphometry are the main factors responsible for differentiation between the genetic groups evaluated in the study; and (iii) the SMMI has potential in the area of selection criteria for conservation programs and genetic improvement of small native sheep breeds.
The morphological indices used in this study were able to characterize the phenotypic and production potential of MN lambs using the TPRI, AI, BCI “1” and BCI “2” variables, enabling the classification of breeding groups in high thoracic development, ability to produce meat, weight gain, feeding efficiency and breathing capacity, infusing greater breed rusticity and adaptability for both genetic groups. These characteristics are in agreement with other studies which evaluated adaptability and productive potential of the MN breed, especially the red variety which has greater potential for meat production (Melo et al., 2018; Rezende et al., 2018; Leite et al., 2020), in addition to breaking paradigms about the low adaptability of white MN to the semi-arid environment, as already reported by Leite et al., (2018; 2020). We emphasize that the breathing capacity of the MN breed is an important adaptive characteristic, since the main mechanisms of heat dissipation during the hottest periods of the day are through transferring sensitive and latent heat through the respiratory tract as observed by (Fonsêca et al., 2019), and in studies of biothermic rhythmicity (Façanha et al., 2020) and caloric management (Ferreira et al., 2020) in native breeds of small ruminants.
The superiority observed in some morphometric and productive characteristics in crossbred lambs are justified by the fact that this crossbreed brought together intrinsic characteristics of the white group, meaning that we still observe the result of the heterosis process and complementarity of characteristics between genetic groups, even within the same breed. This is mainly because the MNW breed is larger when compared to the MNR (Nunes et al., 2020). Therefore, it is inferred that the crossing between the genetic groups of the MN breed is another alternative to increase the carcass weight resulting in greater cuts, especially noble ones (rib, loin and breast), but with similar cut yields indicating proportionality between carcass and non-carcass components within the two genetic groups. This result may be a peculiar characteristic of using the MN breed in crossbreedings, since Landim et al. (2021) observed similarities between the cut yields in pure MN lambs when compared to crossbred Rabo Largo × MN and Santa Inês × MN lambs.
The thoracic body index developed a priori was fundamental for the survey of phenotypic diversity between the genetic groups. This is because there were combinations of morphometric measurements which express high responses with the profile and productive aptitude of a certain animal (BL, WH and TD) for its development. Studies indicate the great importance of the TD measurement due to the close relationship with digestive capacity and rusticity (Santos, 2019).
The direct relationship between ST and CWL (Table 2) is associated with the carcass quality during the final post-slaughter process, since the cover fat has the main objective to act as a thermal insulator in cooling the carcass, thus providing less loss of weight during the maturation process (Nascimento et al., 2018; Landim et al., 2021). However, ST was not sufficient to maintain thermal insulation for the evaluated genetic groups, exceeding the extremity considered adequate (CWL < 7 %; Oliveira et al., 2017).
The discrimination in only one canonical discriminating function (Table 3) for the nine selected groups is explained because the genetic groups (white and red) belong to the same breed, which reduced the discriminatory variance shared between the genetic groups. Even the cut yields do not have an isolated capacity to discriminate genetic groups, which is justified by the similarity between yields (Table 2). The shank yield showed discriminatory power between the genetic groups when all variables were simultaneously analyzed. However, this characteristic acted in a complementary way to the others (TBI, CG and rib weight) to classify 87.0% of the lambs in their group of origin. It is emphasized that adopting the mix of variables is important to identify the main predictor variables for inclusion in genetic breeding programs for native breeds, especially small breeds due to the lack of research.
The higher classification rates observed when the zoometric indices and body morphometry were partially analyzed are justified by the higher values in the characteristics related to the thoracic part and the height of the crossbred lambs (Tables 1 and 2), which infer in the efficiency response from the productive point of view, i.e. the ability to gain weight and intake dry matter, which justifies the higher performance of these animals. In contrast, the lowest rates were observed in carcass performance and indexes, as the effect of crossbreeding between genetic groups did not influence most carcass variables (Melo et al., 2018; Ramos et al., 2019). The discrimination of the thoracic perimeter when evaluating the carcass morphometry is explained because this characteristic is a predictor of live weight at slaughter (Castro et al., 2012), for which both variables were higher in MNF1 lambs.
In this study, the potential for meat production was assessed by factor analysis, in which the characteristics linearly associated with each other were distributed through their respective loads, making it possible to compose factors with biological and economic interpretation. The first factor grouped a larger number of variables including those which indirectly affect the final carcass characteristics, and therefore acted in the economic scope of meat production. This factor also pointed out that CWL is the single factor which most compromises the economic index, since losses due to cooling interfere with the carcass weight and quality, and consequently reduce its price. For (Landim et al., 2017; 2021), an alternative to increase CWL is the use of the Rabo Largo breed in terminal crossbreeding.
The CEI inferred greater explanatory power due to the four factors extracted, in addition to having a high correlation with the AI, which is considered an important indicator of meat production (Sabbioni et al., 2016). Therefore, if there is selection in the MN breed for this morphological index, there will indirectly be a positive influence on the economic aspect. Considering the variables of greatest importance and association within the first factor, it is observed that it is possible to carry out sheep selection through measurements and zoometric indices, indirectly resulting in the carcass traits, which in principle would only be analyzed after slaughter.
The second factor associated characteristics which exert great influence on the productive potential of the MN breed. It is possible to predetermine the digestive capacity and weight gain of a given animal using TPRI (Nunes et al., 2020), while CY can be used in sheep farming in response to the relative percentage of meat to be marketed, as lambs generally have high yields and small variations between biotypes (Oliveira et al., 2018).
The factor associated with commercial characterization, which in turn is directly related to the animal’s production capacity, and consequently greater body and carcass weight, has a direct influence with LegY, as this variable has the greatest quantitative representation of a sheep carcass (McManus et al.,2008; Trindade et al., 2018). The chest height indicates the stature of the animal, and it is recommended to raise small sheep for the Caatinga biome since the availability of food during the dry period of the year is mostly located in herbaceous extract (Nunes et al., 2020). In addition, animals with a compact profile and small to medium body structure should be chosen if the breeder intends to raise animals for meat production with a high capacity for weight gain (Ramos et al., 2019).
The grouping plotted on the heat map clarified the specificities for meat production of the genetic groups. MNF1 lambs presented superiority in the productive, commercial and economic aspects, while the MNR lambs were balanced. Moreover, Issakowicz et al. (2016) report that the lower meat production potential for the MNR breed when compared to other native sheep breeds is related to the small size of this breed group; however, morpho-productive characterizations are useful approaches to circumvent this limitation, as they identify the main characteristics related to the productive potential and use them as a selection criterion. The cluster formed by the morpho-economic and morpho-commercial factors is justified by their direct relationship between factors, while the balance was related to the morpho-productive factor, especially with emphasis on the crossbred lambs due to their greater production when compared to the purebred lambs.
The SMMI suggests that sheep obtained by crossbreeding the white and red genetic groups of the MN breed have a greater potential for meat production when compared with MNR lambs, demonstrating the importance of the white genetic group participating in terminal crossbreeding. Finally, this work identified predictive morphological parameters for meat production with the creation of an aggregate index obtained from the variables with greater participation in the multivariate analysis. This index presents itself as an early and practical selection criterion for meat production in native lambs, and can be used in breeding programs for sheep and by farmers, since biometrics is a simple and easy tool to perform. It is also expected to contribute to the recovery process of the MN white breed variety, as its use has been shown to be favorable in sheep meat production.