Characterization of the commercial growth curves of Spanish Merino, Fleischschaf, and crossbred lambs in an associative economy context

We examined commercial growth curves of three sheep genotypes in a Second-Grade cooperative context, with Spanish Merino used as a maternal base, Fleischschaf as a paternal base, and crossbred (Fleischschaf × Merino) lambs as the commercial product. We included weight-age data collected from 2013 to 2016 for 9846 lambs of both sexes belonging to 14 herds across the three genotypes. Five of the most-cited models (Brody, von Bertalanffy, Verhulst, Logistic, and Gompertz) were applied to the data, using the determinative coefficient, mean square error, number of iterations, Akaike information coefficient, and the biological coherence of the estimated parameters as best-fitting criteria. The dataset included lamb weights at different ages and sexes, grouped across nine age levels, for a total of 23,299 wt records. Models were fitted according to non-linear regression. The von Bertalanffy model was found to best fit lambs of both sexes for the Spanish Merino breed, whereas the Verhulst model best fit Fleischschaf and crossbred lambs. Parameters of the best-fit curve and derived parameters (inflection age and weight, asymptotic value, growth rate, maturity degree) were estimated for all sex-genetic groups to examine capacities for use as complementary selection criteria in the breeding program, resulting in promising traits for this purpose. Our findings demonstrate that current crossbreeding strategies implemented at the OVISO cooperative, which seek complementarities and/or heterosis, are not achieving the expected results; therefore, we recommend a new strategy for the breeding program. We also found that artificially conceived Spanish Merino lambs exhibited better performance than those conceived naturally, likely due to superior genetics of carefully selected sires.