Comparison of non-linear models for describing the growth of scrotal circumference in Awassi male lambs

In sheep-like other farm animals-reproductive performance is considered most important in terms of relative economic value. Within these reproductive traits, scrotal circumference (SC) is especially notable and has been suggested as an indicator of seminal performance and male libido as well as the age at puberty. Data on SC of 33 Awassi male lambs, from birth to 330 days of age, were used to obtain a growth pattern for the breed, with the use of four asymptotic and one indeterminate non-linear growth models. The Tanaka model, a four-parameter indeterminate growth model having an initial period of growth and a period of exponential growth followed by an indefinite period of slow growth, described growth in SC remarkably well, with R²=0.999. Among the asymptotic models, the logistic had a good fit (R²=0.967), after the Tanaka. However, as a result of heterogeneity, no curve computed using mean SC values in this study had the ability to uniformly represent every individual (P<0.01). Biological interpretations of the model parameters were discussed.