Effect of sperm numbers on fertility of frozen bull spermatozoa in skim milk diluent.

Insemination doses of 10, 15, and 20 million progressively motile spermatozoa prior to freezing were compared in a split-ejaculate fertility experiment involving 4,603 first service inseminations to 34 semen samples from six Angus bulls. Based on 60- to 90-day nonreturn rates, fertility was significantly lower for inseminating doses of 10 and 15 times 10-6 than 20 times 10-6 motile sperm (61, 62, and 70%). The 1-ml doses of skim milk diluted semen were frozen in glass ampules. The post-thaw number of motile sperm per ampule after freezing and storage for 3 wk in liquid nitrogen averaged 5.5, 8.4, and 12.4 times 10-6 for corresponding pre-freeze numbers of 10, 15 and 20 times 10-6 motile sperm/ml and ranged from 4.0 to 15.7 times 10-6/ml. Frozen semen packaged in ampules should contain more than 8.4 times 10-6 motile cells post-thaw per ampule.