ADJUSTING SCROTAL CIRCUMFERENCE OF YEARLING BEEF BULLS IN 5 COMPOSITES

Records from five composite strains of beef cattle were used to estima te age of dam and age of bull adjustment factors for scrotal circumfer ence of yearling bulls. Data were obtained from Beefbooster Cattle Alb erta Ltd., Calgary, AB. There were 5244 scrotal circumference records on yearling bulls born from 1987 to 1992. M1, M2, and M4 were maternal composites selected primarily for weaning weight, M3 was a terminal c omposite selected primarily for low birth weight, and TX was a termina l composite selected primarily for feedlot gain. The prominent foundat ion breeds were Angus for M1, Hereford for M2, various small breeds fo r M3, Limousin and Gelbvieh for M4, and Charolais for TX. Statistical analyses were performed for each composite separately according to the model that included the fixed effects of herd, year, age of dam and l inear (quadratic was not significant) regression on age of bull at mea surement. Average age at measurement was 340, 333, 361, 358, and 375 d for M1, M2, M3, M4, and TX, respectively, and range in age was about 100 d for most composites. Age of dam was significant for all composit es except M4. Linear regressions on age of bull were 0.0376, 0.0379, 0 .0308, 0.0410, and 0.0349 cm d(-1) for M1, M2, M3, M4, and TX, respect ively. Scrotal circumference of composite bulls should be adjusted add itively for age of dam and linearly for age of bull. These adjustments tended to be smaller for age of dam and larger for age of bull than t hose reported for straighbred or purebred bulls.