Testing the libido of breeding sires to predict mating performance in the field and the hormonal control of male sexual behaviour.
Crichton, James Spencer.
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The incidence of sexually low-response rams was determined in a group of 31 young Ile de France stud rams by means of three libido tests. Eight rams ( 25,8%) we reclassified as being sexually low-response. The proportion of rams showing normal sexual behaviour did not increase over the three tests. As testing progressed the incidence of mounting behaviour increased significantly. The increase in serving activity was non-significant. Following flock mating two low-response rams had not marked any ewes and showed preference for homosexual company. Five of the eight low response rams had a common sire and it is suggested that sexual behaviour may be influenced genetically. A group of 20 month-old beef bulls (n=54) representing nine breeds were evaluated for libido using five serving capacity (SC) tests (bulls tested in groups) followed by one libido score test (bulls tested as individuals). Bos indicus bulls displayed significantly lower SC than bulls from exotic breeds. Bulls underwent a definite learning process over the five SC tests. The increase in SC was not significant for all breeds. The use of oestrous teaser females held no advantage over the use of properly restrained non-oestrous females. Agonistic behaviour did not depress SC (individual exceptions were noted). Growth rate did not have a consistent effect on SC. Activity on the libido score test was highly significantly correlated to SC. The influence of night or day and the presence or absence of an observer on the SC of 20 month-old Bos indicus bulls (n=18) was evaluated over 12 SC tests. Bulls showed no preference for nocturnal sexual behaviour and were not hindered by personnel scoring SC. The improvement in SC with progression of the tests suggested some form of adaption to pen test conditions and/or learning experience. Serving behaviour was however inconsistent from test to test indicating that restrained females do not represent the ideal stimulus. Agonistic interaction depressed SC. Subsequent pasture mating behaviour was significantly related to SC. Some of the factors influencing pasture behaviour are discussed. High SC bulls achieved a conception rate that was 10,8% higher than that of the low SC bulls (difference non-significant). The relationship between bull libido (libido score test) and conception rate following a 21 day breeding season was investigated over two seasons. The average bulling percentages were 2,9 and 2,5% in the first and second seasons respectively. High libido bulls had no advantage over medium libido bulls in both seasons. Bulls achieved acceptable conception rates (> 70%) in both years and it is suggested that the commonly recommended bulling percentage of 4% is conservative. In an investigation into the hormonal control of sexual behaviour in the ram the technique of active immunization against specific steroid hormones was used in an attempt to highlight the possible central action of dihydrotestosterone (DHT) on mounting behaviour. The central aromatization of testosterone gives rise to DHT and oestradiol (E2). Immunization against E2 and a combination of oestrone (E1) and E2 in testosterone propionate (TP) implanted wethers reduced mounting behaviour to animals (no exogenous hormone). levels seen in control Results suggest that DHT has no or limited central nervous action. The effect of DHT appears to be largely via its sensitization of the penis which enhances mounting behaviour in DHT+E2 implanted wethers to levels higher than those recorded in E2 implanted wethers but similar to the activity recorded for wethers receiving TP implants.