Studies on the artificial insemination of Merino sheep. III. The effect of frequent ejaculation on semen characteristics and fertilizing capacity

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Abstract
(1) Fifty-five ejaculates were collected from each of two Merino rams over a period of 5 days (11 per day per ram at intervals of 20 min). Ejaculates were examined for semen characteristics and for fertilizing capacity following artificial insemination, a total of 300 ewes being used. (2) Volume, density, and number of sperm declined on successive days, and in successive ejaculates within days (P <0.001). Motility and percentage of abnormal sperm were not affected by frequent collection. (3) Fertility following insemination with a standard volume of diluted semen showed a significant linear decline with successive ejaculates, both within and between days of collection ( P <0.01, <0.001). (4) The decline in fertility could be accounted for entirely on the basis of sperm numbers. There was no evidence of a decline in the fertilizing capacity of spermatozoa with frequent ejaculation. (5) There was a highly significant linear relationship between sperm numbers and lambing percentage, over the range of 28 to 128 X 106 sperm (27–118 x 106 normal; b = 0.27; r = 0.69; P <0.01). (6) It is concluded that a dose of approximately 120–125 x l06 normal sperm is necessary for maximum fertility to artificial insemination; and that each 25 x 106 reduction down to a dose of 25 x 106 may be associated with a drop in lambing rate of about 13%.