| Popis: |
Linear type traits describe biological extremes for a range of visual characteristics of an animal and one of the primary reason for collecting them is to allow breeders to select the most functional and profitable cows. Following this main objective, every year the National Breeders Associations train specific classifiers with the purpose to evaluate linear type traits in cows. Using data from the morphological evaluations on two Italian dual-purpose autochthonous breeds, the scores for each type trait has been investigated aiming to assess their genetic aspects and their correlations with the productive traits. Selection goals among the dual-purpose and indigenous breeds used in the study are very similar and lead to maintain both milk and meat production aptitudes. For this reason, the Associations give great emphasis to detect the correct weights to attribute to traits included in the indexes. The understanding of the genetics of both morphological and productive traits is therefore fundamental. The first two steps of the study have been conducted for two different dual-purpose breeds, but following almost the same analysis and taking into account in the model the same effects: the fixed effect of herd-year-classifier contemporary group, the days in milk and age at calving accounted into different classes and the genetic effect of the animal. For the first study, data regarding 20 different type traits from 10,735 first parity cows of the Italian Rendena breed (mainly raised in Trentino Alto Adige region, Italy) were analysed through single trait animal model for the heritability estimates evaluation and using a multi-trait animal model to assess the genetic and phenotypic correlations between type traits. The most heritable type trait was stature (0.52), whereas the lowest was feet (0.12). The same group of type traits showed strong genetic correlations: ≥0.69 among the individual body size traits, ≥0.87 among the individual muscularity traits, from -0.39 to 0.22 among the individual body shape traits, whereas a wider range were found among the individual udder traits (from –0.39 to 0.91). Furthermore, almost all the genetic correlations between the individual muscularity and individual udder traits resulted negative (from -0.53 between shoulder fore view and rear udder attachment, to -0.15 between back, loins and rump and teat placement side view), with only few exception represented by genetic correlations of udder depth and teat length with all the four individual muscularity traits that showed slightly positive correlations (from 0.07 to 0.31). In this first step, also the genetic trends from 2000 to 2009 of type traits have been analysed. The most important result was that the genetic merit for muscularity traits decreased, whereas the genetic merit of the individual udder traits slowly increased over time, meaning that the dual-purpose characteristics of the Rendena cows are becoming more like specialized in milk production. The second step of the study was carried out with the aim to evaluate the genetic parameters of linear type traits in another dual-purpose indigenous breed: the Valdostana breed (mainly raised in Valle d’Aosta region, Italy). Morphological evaluations on 25,183 cows of the Aosta Red Pied (ARP) strain and 14,701 cows of the Aosta Black Pied and Chestnut (ABP-CHES) strain were used to analyse heritability and genetic correlations of 26 different type traits within strain. Heritability estimates obtained from the single-trait animal model analysis ranged from 0.03 (thinness for both strains) to 0.32 (ARP) and 0.29 (ABP-CHES) of stature. Regarding the genetic correlations between the individual type traits, for ARP strain the strongest correlation was between thigh, buttocks side and rear view (0.97), whereas for the ABP-CHES strain the highest correlations were observed between stature and body length (0.98), meaning that selecting for one trait lead to the simultaneous improvement of the other. Also for the Valdostana breed, most of the genetic correlations between muscularity and udder traits resulted negative, especially those involving udder volume. These results indicate a substantial antagonistic behaviour of type traits related to dairy and beef characteristics of animals. The following step has been addressed to the analysis of a simpler way to manage the large number of information given by the type traits scored on animals and managed by the Breeders Associations. To avoid redundant information in selection indexes, only a limited number of type traits with a known biological relationship with production should be used in the morphological evaluation. Factor analysis resulted to be a useful procedure to group type traits, so that correlated traits could be isolated in the same factor, and therefore each factor could include traits with common biological characteristics. In this study, a factor analysis was applied to 20 individual linear type traits evaluated on 11,399 Rendena cows, and to 22 individual linear type traits evaluated on 36,168 ARP cows. From this procedure, six latent common factors, for each breed, with eigenvalues ≥1 were obtained, explaining 63% (Rendena) and 58% (ARP) of the total variance. For both breeds, factor 1 (F1) included type traits mainly related to muscularity and factor 2 (F2) to body size traits. Factor 3 (F3) and factor 4 (F4) accounted for udder size and udder conformation type traits, respectively. Type traits regarded feet and legs were included in factor 5 (F5), whereas factor 6 (F6) did not show any biological meaning. The greatest heritability estimates trough the REML single-trait animal model for factor scores were 0.52 in the Rendena and 0.37 in the ARP, respectively for F2. Heritability for F1 resulted 0.40 for the Rendena, and 0.32 for the ARP. Rank correlations between Estimated Breeding Value (EBV) of the individual type traits and of factors showed strongly similar coefficients than those observed in factor analysis. From this study, it was appreciable that for both breeds the number of linear type traits could be easily represented by few factors without reducing in accuracy in describing the conformation of animals evaluated from classifiers. Therefore, the use of factor analysis in genetic evaluation could be taken into account for the morphological evaluation aimed at obtaining individual EBVs. The final step of the study was to estimate the relationships between linear type traits and milk production traits. Factor analysis was applied to muscularity and udder individual type traits for 33,206 first and second parity cows belonging to the Aosta Red Pied strain of the Valdostana breed, obtaining three latent factors. The F1 reflected the individual muscularity traits, F2 included dimensional udder traits, and finally, F3 represented a good dairy conformation. Furthermore, data from 169,008 test-day (TD) yield records, regarding milk, fat and protein content (kg/day), belonging to the first 3 lactations of 16,605 cows were analysed. Through a series of AIREML single-trait analysis, genetic parameters of both morphological factors and milk related traits were obtained. The models for the two datasets accounted for different effects: for the morphological information, herd-year-classifier, classes of age at calving and of days in milk as fixed effects, and the random additive effect of cow were taken into account. For milk traits, herd-TD within lactation, classes of gestation, classes of age at parity and of month of parity both within lactation, and permanent environment effect were considered for the repeatability TD model, together with the additive genetic component. All the previous effects were jointly retained for the AIREML bi-trait analysis, to assess the phenotypic and genetic correlations among and between traits. Heritability estimates were moderate for both group of traits. F1, F2 and F3 showed heritability values of 0.31, 0.17 and 0.20, whereas milk, fat and protein content presented values of 0.20, 0.13 and 0.17, respectively. Strong and positive genetic correlations were found between all the three milk production traits and F2 (udder dimension traits; ȓ ≥0.83). On the other hand, negative genetic correlations were obtained between milk yield traits and both F1 (muscularity traits) and F3 (udder conformation traits), ranging from -0.23 to -0.53. Phenotypic correlations resulted lower than the genetic ones in all the four steps analysed. Results from this study could be of great interest in planning the correct weights to give to analysed traits in case of simultaneous selection, as possible for dual purpose breeds like those taken into account. Indeed, in both the Rendena and the Valdostana breeds the selection for purposes play an important role, and represent the main selection goal in both Breeders Associations. Therefore, a deep understanding of the relationships between the antagonistic muscularity and udder type traits and also between them and the productive traits should be of the primary interest for future breeding plans in the two breeds. |
