Potential Use of Lutalyse to Enhance Libido in Boars with Suppressed Circulating Concentrations of Gonadal Steroids

Livestock Update, November 2003

Mark Estienne, Swine Research Physiologist, Allen Harper, Extension Animal Scientist-Swine, Tidewater AREC, and James Knight, Reproductive Physiologist, Animal and Poultry Sciences

Introduction
In the U.S., consolidation and vertical integration of the swine industry has been associated with dramatic changes in breeding herd management. For example, the proportion of sows bred via artificial insemination increased from approximately 8% in 1991 (Burke, 2000) to nearly 70% in 2000 (Lawrence and Grimes, 2001).

In a typical large-scale swine operation, semen is collected once or twice a week from boars located at off-site "boar studs". Studs usually house 200 or more boars. Collected ejaculates are diluted in an extender, creating multiple insemination doses that are transported for matings at commercial sow units.

In order for a stud to be operated in an efficient manner, boars must consistently and quickly mount an artificial sow and allow semen collection, and produce ejaculates containing large numbers of fertile sperm cells. It is inevitable, however, that some boars will be slow to mount the artificial sow or will completely refuse. For individual boars, this can be a sporadic problem or a more consistent one.

Scientists have not determined why trained boars sometimes become slow or reluctant to mount an artificial sow. A plausible hypothesis is that poor libido is related to low concentrations of testicular steroids such as testosterone and estradiol. Indeed, mature boars lose interest in mounting sows in estrus 30 to 60 days after castration and normal sex drive is restored by treatment with testosterone and estradiol (Levis and Ford, 1989). Moreover, boars consuming a low-protein diet had decreased levels of estradiol in the circulation and consistently refused to mount an artificial sow (Louis et al., 1994).

Management techniques that enhance sex drive and "speed up" the mounting process by lethargic boars would have obvious benefits in regard to the efficiency of the stud. Treatment with a prostaglandin-F2 (PGF2) product restored sex drive in older boars exhibiting low libido (Szurop et al., 1985). Also, work at the Tidewater Agricultural Research and Extension Center's (TAREC) Research Boar Stud in Suffolk demonstrated that PGF2 (Lutalyse; Pharmacia and Upjohn, Kalamazoo, MI) expedited the training of mature boars for semen collection (Estienne and Harper, 2000).

At TAREC, we recently conducted an experiment the objective of which was to test the hypothesis that Lutalyse treatment increases libido in boars with suppressed secretion of gonadal steroid hormones.

Creating boars with low concentrations of gonadal steroids
In order to conduct our experiment, we needed access to boars with suppressed circulating concentrations of testosterone and estradiol. We chose boars implanted with a gonadotropin-releasing hormone (GnRH) agonist as our experimental "model".

In male swine, as in other mammals, GnRH is secreted from the brain and binds to receptors on the anterior pituitary gland, ultimately causing the release of luteinizing hormone (LH). In turn, LH travels through the blood stream and causes the testicles to synthesize and secrete testosterone. An enzyme in the testicles causes the conversion of some of the testosterone to estradiol. As noted above, testosterone and estradiol are necessary for boar sexual behavior.

Chronic treatment of male animals with potent GnRH agonists has been shown to inhibit reproductive function (Vickery, 1986). These GnRH agonists bind to the LH receptors on the pituitary gland but essentially "over-stimulate" them. Rather than causing increased LH secretion like GnRH would, GnRH agonists eventually result in a protracted suppression of LH release. In turn, testosterone and estradiol decrease.

This concept was demonstrated in a preliminary experiment conducted at TAREC. Yorkshire x Landrace barrows (approximately 127 days of age) were used. At week 0, six barrows received an s.c. GnRH agonist implant (Deslorelin; Fort Dodge Animal Health, Fort Dodge, Iowa, USA) in the neck behind the ear. Six barrows were sham-implanted and served as controls. Blood was collected on week 0 (prior to implant insertion), 1, 2, 3, 4, and 5 and serum samples were analyzed for concentrations of LH. The Deslorelin implants were successful in suppressing circulating concentrations of LH (Figure 1).

Effect of Lutalyse on libido in boars with suppressed testosterone and estradiol
Prior to and during the experiment, semen was collected once weekly from terminal line boars (2.3 yr of age) trained to mount an artificial sow and allow semen collection. After semen collection at week 0, boars received a Deslorelin implant or were sham-implanted.

Beginning at week 1, Deslorelin-implanted boars received i.m. treatment with 10 mg Lutalyse (n = 5 boars) or placebo (n = 5 boars) upon entering the collection room. Sham-implanted boars (n = 5 boars) received intramuscular injections of placebo.

Blood samples were collected at weeks 0 and 5. The Deslorelin-implants decreased (P < 0.05) blood concentrations of LH (by 60%), testosterone (by 77%) and estradiol (by 58%). Hormone concentrations for sham-implanted boars were similar for weeks 0 and 5.

Throughout the study, all boars continued to mount the artificial sow and allow semen collection, and most indications of libido such as time from entering the collection room to the first attempt to mount, time from entering to the start of ejaculation, and duration of ejaculation were not affected by treatment. However, between week 0 and 5, the number of false mounts (mounting artificial sow but dismounting prior to semen collection) increased (by 163%; P < 0.05) in Deslorelin-implanted boars injected with placebo, decreased (by 91%; P < 0.05) in Deslorelin-implanted boars injected with Lutalyse and remained similar (P > 0.1) in sham-implanted boars receiving placebo (Figure 2).

Summary and implications
To summarize our study, acutely suppressing concentrations of LH, testosterone and estradiol did not completely abolish libido in boars, but increased the number of false mounts of an artificial sow. The number of false mounts was decreased by Lutalyse treatment. Although more research is needed, Lutalyse has potential for enhancing sex drive in boars with low concentrations of testicular steroids.

References
Burke, P. 2000. Productivity assessment of liquid boar semen usage. Pages 149-152 in Proc. IV Int. Conf. Boar Semen Preservation. Beltsville, MD.

Estienne, M.J., and A.F. Harper. 2000. PGF2 facilitates the training of sexually active boars for semen collection. Therio. 54:1087-1092.

Lawrence, J.D., and G. Grimes. 2001. Production and marketing characteristics of U.S. pork producers, 2000. Staff Paper No. 343, Department of Economics, Iowa State University. Available at: http://www.econ.iastate.edu/research/webpapers/Staffppr343FNL.pdf

Levis, D.G., and J.J. Ford. 1989. The influence of androgenic and estrogenic hormones on sexual behavior in castrated adult male pigs. Horm. Behav. 23:393-411.

Louis, G.F., A.J. Lewis, W.C. Weldon, P.M. Ermer, P.S. Miller, R.J. Kittok, and W.W. Stroup. 1994. The effect of energy and protein intakes on boar libido, semen characteristics, and plasma hormone concentrations. J. Anim. Sci. 72:2051-2060.

Szurop, I., A. Nagy, and W. Jochle. 1985. Stimulation of libido in pubertal and mature boars with prostaglandin F2_ analogs: clinical observations. Zuchthyg. 20:83-86.

Vickery, B.H. 1986. Comparison of the potential for therapeutic utilities with gonadotropin-releasing hormone agonists and antagonists. Endocr. Rev. 7:115-124.

Figure 1. Serum concentrations of LH in barrows receiving an implant containing a GnRH agonist (Deslorelin) and control barrows. A treatment by time interaction (P < 0.01) was detected; Levels of LH decreased in barrows given Deslorelin but not in controls. Values are means (n = 6 barrows per group) and SE was 0.11 ng/mL.

Figure 2. False mounts (mounting artificial sow but dismounting prior to semen collection) in sham-implanted boars injected with placebo, boars receiving an implant containing a GnRH agonist (Deslorelin) and injected with placebo, and Deslorelin-implanted boars receiving Lutalyse. Values are means (n = 5 boars per group).

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