Last update: August 12, 2011 02:50:42 PM E-mail Print




PR King

Grootfontein Agricultural Development Institute, Middelburg 5900

W A Coetzer*

Dept of Human & Animal Physiology, University of Stellenbosch, Stellenbosch 7600



Ovulation rate of Merino ewes mated to Ronderib Afrikaner rams is on average 19 percentage points higher than that of Merino ewes mated to Merino rams. As ovulation is to a large extent controlled by LH, a study was done to determine the plasma LH concentration during the oestrous period in Merino ewes exposed to either Ronderib or Merino rams. Twenty-six synchronized adult Merino ewes were randomly divided into two groups (A and B). Sixteen days following sponge withdrawal, one of four Ronderib Afrikaner teaser rams was placed with Group A while one of four Merino rams was placed with Group B. To minimize the effect of variation in sexual activity between rams of the same breed, the rams were rotated every six hours. A blood sample was taken every two hours from each ewe following ram introduction. Sampling was terminated 36 hours following the end of oestrus. The number of ovulations per ewe was determined laparoscopically nine days after oestrus. The plasma LH and progesterone concentrations of each sample were determined by means of a RIA. The mean LH concentration for the period between progesterone reaching basal values and the onset of oestrus, did not differ (p>0.05) between the single and multiple ovulators in either group. The mean peak area, was 110.9% larger for the multi-ovulating ewes than that recorded for the ewes displaying a single ovulation in Group A (P=0.13). In Group B the area did not differ (P=0.88) between single and multiple ovulators. Within ovulation Status, no group differences occurred. The period between the onset of basal progesterone levels and the start of the LH peak tended to be longer (P=0.09) for the ewes mated to a Ronderib ram (43.9 ±2.9 hours) than for those mated to a Merino ram (38.0 ±1.8 hours). It would appear that Ronderib rams may increase the ovulation rate of Merino ewes by delaying the onset of the LH surge as it is known that the LH surge occurs later in multi-ovulating ewes within a breed.



Proceedings 34th SASAS congress