Last update: August 15, 2011 10:44:14 AM E-mail Print

 

STIMULATION OF SEXUAL ACTIVITY IN MERINO RAMS

 

P.R. King

Grootfontein Agricultural Development Institute, Private Bag X529,

Middelburg CP 5900

 

It is generally accepted that the lambing percentage of Merino ewes mated during spring months, is approximately 20 % lower than when mated during autumn. This phenomenon represents the natural pattern of seasonal breeding activity found in woolled sheep and is due to a large proportion of ewes not displaying a sexual cyclisity during this time of the year. Many scientific papers have dealt with the problem of sexual inactivity in ewes during spring as well as means to overcome it. Basically it involves the isolation of ewes from rams for a period of at least 20, but preferably 30 days prior to mating. Under favourable conditions approximately 85 % of these ewes, when subsequently exposed to rams, will display oestrus and may be successfully mated (Knight 1983). This stimulating effect of the ram on the ewe is referred to as the "ram effect".

In practice, many farmers obtain unacceptably low lambing percentages when practising a spring mating system. Several factors such as feeding conditions, body mass, parasites, ram fertility, sexual activity of rams, etc. influence the reproductive rate of ewes both during and outside the normal breeding season. The sexual activity of the ram seems to be extremely important, especially during the spring mating period, when the majority of ewes are naturally in anoestrus. This effect of male sexual activity on the breeding rate of spring-mated ewes has, however, received relatively little attention from farmers and researchers.

Similar to the reduced sexual activity generally found in Merino ewes during spring and early summer (anoestrous period) rams are also sexually less active during this period (Mattner 1977). A preliminary investigation indicated that the percentage of rams being sexually inactive at the onset of mating during spring could be as high as 90 % (King, P.R.; unpublished results). In addition, Signoret et al. (1982) has demonstrated that males with low libido were less effective in bringing about the "male effect" than those displaying high libido. This phenomenon is probably due to reduced secretion of testosterone, which invariably influences the secretion of pheromones - the latter being mainly responsible for the, so-called "ram effect"(Pearce & Oldham 1984). In accordance with the foregoing, several investigations have demonstrated that ewes mated during the breeding season to rams with a low sexual activity (libido) had a significantly lower breeding rate than those mated to rams exhibiting high sexual activity (Barwick et al., 1989; Perkins et al., 1992; Van Wyk et al. 1984). It is therefore most likely that sexual activity as such will at least have a similar effect during spring. Activity of rams is known to be lower during spring.

It is therefore apparent that the problem of impaired reproduction generally associated with spring mating could, in addition to special treatment of ewes (flushing, teasers etc.), also be addressed by stimulating sexual activity in the ram. The impaired sexual activity of rams or teaser rams during spring can be improved considerably by exposing rams to oestrous ewes for a period of 20 to 30 days prior to the mating period. The rams should be kept in a pen or paddock adjacent to two or three oestrous ewes. It is also advisable to allow oestrous ewes to join the rams for a period of approximately 30 minutes each day to allow copulation to take place.

For the purpose of artificially inducing oestrus in a few ewes, stilboestrol (an artificial female sex hormone) has been used with great success. Although treated ewes display overt oestrus and allow copulation they do not ovulate and conception will therefore not occur. As stilboestrol is no longer available, another artificial female sex hormone, estradiol cypionate (ECP), is used for this purpose, but has proven to be less effective. Preliminary investigations have indicated that the effectivity of ECP to stimulate oestrus could be improved considerably by pre-treatment of ewes with progesterone impregnated sponges. Following sponge withdrawal the ewes are injected with 2 mg ECP intramuscularly (King 1993, unpublished results). Ewes displayed oestrus approximately 48 hours after ECP treatment and remained in oestrus for a period of 24 to 48 hours. Follow-on treatments at 96 hour intervals resulted in the repetition of overt oestrus.

In practice the following programme will ensure the continuous availability of oestrous ewes for the purpose of priming rams to become sexually more active prior to a spring mating period. A first group (Group A) of two or three culled ewes should be sponged 16 days prior to the beginning of the stimulation of rams. A second group (Group B) is to be sponged 48 h later. Fourteen days after the insertion of the sponges, sponges are removed and the ewes treated with 2 mg ECP intramuscularly. Treated ewes should display oestrus approximately 48 hours later and remain in oestrus for 24 to 48 hours. At this stage the oestrous ewes are ready to be joined to the rams. By treating the ewes at intervals of 96 hours one of the two groups should be in oestrus throughout the stimulation period. Following on three or four treatments the ECP treatment may be reduced to half a dose with similar results.

Experience has indicated that after rams have been exposed to the oestrous ewes for a period of 30 days their sexual activity should be very similar to that found during the breeding season. The improved sexual drive of such treated rams has been demonstrated to be instrumental in the successful improvement of reproductive rates, which should be of special significance when spring mating is practised. Further studies to ascertain these inferences are underway at present.

 

REFERENCES

BARWICK, S.A., KILGOUR, R.J., FOWLER, D.G., WILKINS, J.F., & HARVEY, W.R. 1989. Ram mating performance in Border Leicesters and related breed types. 3. Relationships of ram serving capacity, testis diameter, liveweight, breed and age with flock fertility. Australian Journal of Experimental Agriculture 29 (1) 17.

KNIGHT, T. W. 1983. Ram induced stimulation of ovarian and oestrous activity in anoestrous ewes - a review. Proceedings of the New Zealand Society of Animal Production 43, 7.

MATTNER, P.E. 1977. Seasonal variation in libido of intact Merino rams and of testosterone-treated castrates. Theriogenology 8 (4) 149.

PEARCE, D. T. & OLDHAM, C.M. 1984. In: Reproduction in sheep. Edited by D.R. Lindsay and D.T. Pearce.

PERKINS, A., FITZGERALD, J.A. & PRICE, E.O. 1992. Sexual performance of rams in serving capacity tests predicts success in pen breeding. Journal of Animal Science 70 (9) 2722.

SIGNORET,J.P., FULKERSON, W.J. & LINDSAY,D.R.1982. Effectiveness of testosterone treated wethers and ewes as teasers. Applied Animal Ethology 9, 37.

VAN WYK, J.B., VAN DER MERWE, H.J. & SLIPPERS, S.C. 1984. Invloed van die dekvermoë van Merinoramme op reproduksiedoeltreffendheid. Goue vag, Februarie, 30.

 

Published

Karoo Agric, Vol. 6, No 1, 1994 (1-2)