Scrotal circumference measurements in purebred Dorset, Hampshire and Suffolk lamb and yearling rams

Purebred Suffolk, Hampshire and Dorset lamb and yearling rams (n=753), underwent performance testing from 1986 to 1989. Scrotal circumference, birth date and entry weight were recorded for each ram entering the test station. Mean age in days at entry, weight (lbs), and scrotal circumference (SC; cm) were greatest for the Hampshire rams. For all rams combined (Dorset, Hampshire and Suffolk), the mean SC by month of age revealed rapid growth from 2 to 6 mo, with a 3- to 6-mo mean SC significantly (P≤0.05) larger than the mean SC for the previous month. From 6 to 12 mo there was no difference (P>0.05) for mean SC measurement and a less rapid increase in SC diameter. Between 12 and 13 mo a significant increase (P≤0.05) was noted in the mean SC measure. Hampshire rams had a significantly larger SC (P≤0.05) than Dorset rams at 3 mo and at 10 to 12 mo of age; Suffolk rams had a larger SC (P≤0.05) than Dorset rams at 9 to 12 mo. The mean SC for Hampshire and Suffolk rams differed only at 3 mo of age (P≤0.05). Simple regression analysis for SC vs log of age in days described the best fit of the data for predicting SC. For all breeds combined, the polynomial relationship was SC(cm) = -143.05 + 137.71log(age) - 27.073[log(age)]². Most of the variability (r² = 0.738) in SC measurement in younger (2 to 12 mo) purebred Dorset, Hampshire and Suffolk rams was attributable to age of the ram in days. The currently recommended point-score system for evaluating ram breeding soundness could lead to discounting younger rams that have the potential to be satisfactory sires but that do not score high enough due to the influence of age vs SC development. In selecting sires, breed differences must be taken into consideration since the SC of Hampshire rams was different from that of Dorset and Suffolk rams at 3 mo of age and from Dorset rams at 10 to 12 mo of age.