Serum Gonadotropins After Gonadotropin-Releasing Hormone Injection in Bulls Subjected to Spacial Restriction

Abstract

Response patterns of luteinizing hormone, follicle-stimulating hormone, and testosterone after injection of gonadotropin-releasing hormone were investigated in bulls grouped by weight (250 to 459kg body weight) and confined five per pen in 9.2 or 6.4m2 space per bull in two replicates. Blood samples were collected for 1h prior to injection of 100μg gonadotropin releasing hormone and 5h after injection at 15-min intervals. Overall mean luteinizing hormone concentrations were not affected by spacial restriction or replicate. Interaction of treatment by time revealed that luteinizing hormone response curves were not similar. Restricted bulls had a higher response of luteinizing hormone to gonadrotropin-releasing hormone. Follicle-stimulating hormone increased in all groups within 15min and peaked at 219.4ng/ml at 45min. Both gonadotropin responses returned to preinjection concentrations by 4h. Testosterone was affected by treatment, replicate, and time of sampling. Testosterone was higher in restricted bulls and higher in replicate 2. Mean testosterone peak following gonadotropin-releasing hormone was 3.86ng/ml and occurred between 105 and 120min which was approximately 90min after the gonadotropin peaks. It appears that hormone responses to gonadotropin-releasing hormone were not depressed by spacial restriction, and additional spacial restriction of young bulls could be used commercially.