Relationships of nutritional plane and feed efficiency with sexual development and fertility related measures in young beef bulls

Sexual development in beef bulls appears to be influenced by nutritional plane and feed efficiency. Yearling bulls fed high (GRAIN = 49) and moderate (ROUGHAGE = 109) nutritional planes, were submitted to a performance test where scrotal circumference (SC), scrotum and testis ultrasonograms and hormones were monitored throughout. Scrotal thermographs, blood cell counts and semen were evaluated at the end of the test. Residual feed intake (RFI) was the measure of feed efficiency, and bulls within each population were characterized as efficient and inefficient. During the test period, the GRAIN group had greater triiodothyronine (T3), leptin and scrotal skin thickness (SST) while having greater T3, leptin, % motile sperm, % normal sperm, scrotal temperature and values for red blood cell variables at the end of the test when compared to the ROUGHAGE group. During the test, the efficient GRAIN group had lesser testis pixel intensity while at the end of the performance test the bulls of this group had greater % normal sperm and lower testis pixel intensity compared to bulls in inefficient GRAIN group. In comparison, the efficient ROUGHAGE group had less T3, SC and SST while at the end of the test these bulls had less T3, leptin, SC, SST and scrotal temperature compared to bulls in the inefficient ROUGHAGE group. Complete blood cell variables, semen quality, scrotal biometry and thermography explained most of the variation in RFI. Results of the present study support the hypothesis that there is an antagonistic relationship between feed efficiency and sexual development in young bulls, which seems partially offset when there is a greater plane of nutrition. Thus, feed efficiency should be factored in the reproductive evaluation of sires.