The effect of season and relocation upon reproductive competence in Brahman and Hereford bulls

Abstract

This study was designed to determine the effects of relocation and season on growth and reproductive parameters of Brahman and Hereford bulls. Hereford bulls from Montana (n = 15) and Nebraska (n = 15) and Brahman bulls from Texas and Louisiana (n = 18) were randomly assigned to one of three locations: Montana (MT), Nebraska (NE), or Texas (TX). Bulls were relocated in a 4-d period in May, 1984. All bulls were puberal at this time. Body weight, hip height, scrotal circumference, and left and right testis length were recorded for each bull at 28-d intervals after relocation. Paired testes volume was also calculated. Semen was collected within 1 wk of relocation and beginning in November 1984, at 90-d intervals. Semen was evaluated for volume, spermatozoal concentration, gross motility, progressive motility, % motility, % live spermatozoa, % normal heads, % normal tails, % normal acrosomal ridges, and % proximal droplets. Within 2 d of semen collection the bulls were given 200 ug of gonadotropin releasing hormone in (GnRH) and bled at 0, 30, 60, 150 and 300 min after injection. In November 1984 and May and November 1985 the bulls were bled for an 8-h period at 20-min intervals. All blood samples were analyzed for testosterone and luteinizing hormone concentrations by radioimmunoassay. Least squares analysis of variance techniques were utilized to analyze the split-plot design. Breed had an effect on all of the growth traits measured. Relocated Brahman bulls had a lag time of 6 mo in growth traits in comparison to Brahman bulls in TX. Semen quality was lower in relocated Brahman bulls during the winter than during the summer. Hereford bulls from the north were susceptible to summer heat stress in TX as evidenced by decreases in semen quality parameters. Luteinizing hormone and testosterone concentrations were different between the breedtypes. Brahman bulls had lower concentrations of luteinizing hormone and testosterone than Hereford bulls. Relocated Brahman bulls had lower luteinizing hormone and testosterone concentrations than Brahman bulls in TX. Testosterone and luteinizing hormone concentrations were not influenced by relocation in Hereford bulls to the same extent as in the Brahman bulls. There were greater differences between the breedtypes in testosterone than in luteinizing hormone concentration. Any detrimental effects due to relocation were not permanent in either breedtype.