Relationships of blood parameters and fertility-related traits with age and feed efficiency in young beef bulls
Feed efficiency and bull fertility are two major factors affecting profitability of the beef industry. However, there is concern of an antagonistic relationship between these two factors, highlighting the need to clarify the relationship between age, feed efficiency and sexual development. Two experiments were undertaken to evaluate associations of blood parameters and fertility-related measures with age and feed efficiency in yearling bulls. Firstly, 32 bulls were studied and among other results, younger and efficient bulls exhibited lower testosterone and triiodothyronine levels, respectively. Secondly, investigation of 158 bulls revealed that younger bulls had smaller scrotal circumference, higher scrotal radiant heat loss and fewer normal sperm while efficient bulls indicated lower scrotal circumference, scrotal radiant heat loss, and a trend towards lower testicular echogenicity and higher sperm head defects. Metabolic differences associated with variation in feed efficiency may impact reproductive function as illustrated by features of delayed sexual development in efficient bulls.