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Differences in sexual activity between Ronderib Afrikaner and Merino rams

 

P.R. King, A.W. Coetzer* and J.C. Morgental*

Grootfontein College of Agriculture, Middelburg CP, 5900

 

Results on the effect of Ronderib Afrikaner rams on reproduction in Merino ewes over a five-year period indicated that the lambing percentage of Merino ewes mated to Ronderib Afrikaner rams was higher than that of Merino ewes mated to Merino rams. On closer inspection of the results it was found that the elevated lambing percentage was brought about by a higher fecundity (on average 19 %) in the Ronderib Afrikaner mated ewes while the percentage ewes that lambed in each group did not differ significantly (King, Olivier & Wentzel, 1989).

At first it was thought that the Ronderib Afrikaner rams produce more sperm of a better quality than the Merino rams and that this led to the higher fecundity. A macroscopic and microscopic comparison of Ronderib and Merino semen showed no difference in quality. After inseminating two groups of synchronised Merino ewes with fresh Ronderib Afrikaner and Merino semen respectively, it was determined that semen was not involved in this phenomenon as the fecundity of the two groups did not differ at all.

In an attempt to increase the lambing percentage of Merino ewes producing Merino lambs, a group of 100 Merino ewes was joined with three vasectomised Ronderib rams while the control group was joined with three vasectomised Merino rams for a period of 6 weeks. The rams were fitted with raddling harnesses to identify ewes displaying overt oestrus for subsequent artificial insemination with Merino semen. After being inseminated the ewes were again placed in their respective groups. From the reproductive data obtained, it was found that the presence of Ronderib Afrikaner teaser rams increased the fecundity of the ewes with 22 % when compared with that of the control group (King & Wentzel, 1990). In similar experiments it was found that the increased fecundity of ewes mated to Ronderib Afrikaner rams and teaser rams was the result of an increase in the ovulation rate of the Merino ewes (King, Wentzel & Joubert, 1990).

It has not previously been documented that a ram can influence the ovulation rate of the ewe, and the mechanism by which the Ronderib Afrikaner ram accomplishes this is not known. During the experiments it was observed that the Ronderib Afrikaner rams were considerably more active among the ewes than the Merino rams. Because of the fact that the present results indicate that the physical stimulation by the Ronderib Afrikaner ram is responsible for the increased ovulation rate of Merino ewes, a study was done on the sexual activity of Ronderib Afrikaner and Merino rams. Two groups of 15 Merino ewes each were synchronised with progesterone sponges. When all ewes displayed oestrus, a Ronderib Afrikaner ram was placed with one group while a Merino ram was placed with the other for a one-hour period. Ten Ronderib Afrikaner and 10 Merino rams were used in the study and each ram was observed 3 or 4 times. The number of times each ram mated successfully was recorded, as well as the unsuccessful mating attempts. The data obtained for each breed was pooled and statistically analysed.

The Merino rams mated 1,26 ± 1,36 times on average per hour while the Ronderib Afrikaner rams mated 1,80 ± 1,79 times per hour. Although not statistically significant, the Ronderib Afrikaner rams mated 42,8 % more than the Merino rams. The number of unsuccessful mating attempts per hour for the Merino rams was 6,80 ± 5,35 compared to the 15,09 ± 11,62 for the Ronderib Afrikaner rams. This difference of 121 % in the number of unsuccessful mating attempts was statistically significant. In the present study all the observations were made between 08:00 and 16:00. From observations made during previous experiments it would seem that the Ronderib Afrikaner rams were sexually active throughout the night while the Merino rams were totally inactive from approximately 23:00 to 04:00. If sexual activity tests were done during this period the results could be much more favourable to the Ronderib Afrikaner rams.

These results emphasise the importance of determining whether rams are sexually active. It is known that the sexual activity of rams is correlated with the fertility of the ewe flock. Van Wyk, Vander Merwe & Slippers (1984) reported that if no rams in a flock are highly sexually active, the lambing percentage can be decreased by as much as 40 %. Currently rams are selected for economic traits such as wool and carcase quality. Furthermore, the sexual capacity of rams is judged by the semen quality and quantity. If a ram, irrespective of its economic value, is not capable of mating ewes successfully, it has no value and can cause considerable financial loss.

In 1971 Mattner, Braden & George developed a serving capacity test for predicting the sexual performance of rams prior to using them for breeding. The test is very similar to the one used in this experiment except that each ram was placed with two oestrus ewes for a twenty-minute period. It is essential that the test be repeated a number of times for each ram. The ewes can be brought into oestrus by using commercially available estrogens (female sex hormones). Although some controversy exists on the effectiveness of pen-mating tests to increase flock fertility, it is clear that rams that are not sexually active at all can easily be identified. This is important, as the percentage of inactive rams in a flock has been found to be between 9 and 33 % (Anne Perkins, Ph.D. student, University of California, Davis, California USA - personal communication). If the percentage of sexually inactive rams is this high during flock mating it will definitely have a detrimental effect on lambing percentage. Furthermore, by placing selection pressure on mating ability when selecting Merino rams, it is possible to improve their sexual ability. Kilgour, Purvis, Piper & Atkins (1985) reported that the estimate of heritability for serving capacity for sheep was in the vicinity of 0,3 and that of bulls 0,59 ± 0,15. By eliminating rams with a low sexual ability it is possible to increase not only the fertility of the ewe flock, but also their fecundity, as is the case when the ewes are run with Ronderib Afrikaner rams.

 

* Dept of Human Physiology, University of Stellenbosch, Stellenbosch

 

REFERENCE

KING, P.R., OLIVIER, J.J. & WENTZEL. D., 1989. Invloed van Ronderib Afrikanerramme op fekunditeit van Merino-ooie. Proceedings of the 28th Congress of the South African Society of Animal Production. Ermelo. 11-13 April.

KING, P.R. & WENTZEL, D. 1990. Ronderib Afrikaner ram effect on fertility: 1 The effect of Ronderib Afrikaner rams on the fecundity of Merino ewes. Proceedings of the 29th Congress of the South African Society of Animal Production. Stellenbosch. 27- 29 March.

KING. P.R., WENTZEL. D. & JOUBERT, J.P.J. 1990. Ronderib Afrikaner ram effect on fertility: 2 Effect of Ronderib Afrikaner rams on ovulation rate in Merino ewes. Proceedings of the 29th Congress of the South African Society of Animal Production. Stellenbosch, 27-29 March.

KlLGOUR, RJ., PURVIS, L W., PIPER, L.R. & ATKINS, K.D. 1985. Heritabilities of testis size and sexual behaviour in males and their genetic correlations with measures of female reproduction. In: Genetics of reproduction in sheep. Edited by R.B. Land & D. W. Robinson, London: Butterworths.

MATTNER, P.E., BRADEN, A.W.H. & GEORGE, J.M..1971.Studies in flock mating of sheep. 4. The relation of libido tests to subsequent service activity of young rams. Aust. J. Exp. Agr. Anim. Hush. 11: 473.

VAN WYK, J.B., VAN DER MERWE, HJ. & SLIPPERS, S.C.. 1984. Invloed van die dekvemoë van Merinoramme op reproduksiedoeltreffendheid. Goue Vag 14 (2). 31-33.

 

Published

Karoo Agric, Vol. 4, No 4, 1992 (14-15)