Somatotropin Concentrations in Plasma and Scrotal Circumference in Bull Calves with Different Dairy Merit

Abstract

Twenty-four Holstein bull calves represented two divergent genetic groups with a 2940-kg difference in milk production in estimated breeding value. These bulls were used to determine the blood plasma concentration of bST under normal feeding management, after a 24-h feed and water deprivation, after stimulation with a bST secretagogue, and to evaluate whether selection for dairy merit affected pubertal testicular development and sperm production. Calves selected for higher estimated breeding value had greater average, but similar stimulated, plasma bST concentrations. Age of calves, but not short-term feed and water deprivation, influenced average plasma bST concentrations both before and after stimulation with growth hormone-releasing factor. Body weight tended to be higher at 350 to 400 d of age in bulls selected for higher estimated breeding value. Scrotal circumference was similar in the two groups at all times, as were seminal characteristics at 1 yr of age. The study implied that plasma bST measurements may be used as an aid in selection of young dairy sires for AI. © 1993, American Dairy Science Association. All rights reserved.