[Repeated freezing of bull semen (author's transl)].

Repeated freeze-thaw cycles have been used as a mean to predict the viability and fertility of bull semen. In order to investigate the level of fertility of bull semen that has been frozen, thawed and refrozen again, two split sample field trials were performed. 25 bulls were used in the trial, and inseminations were performed by 30 technicians. The semen was diluted to 27 million spermatozoas per dose in a skimmilk-fructose extender, and filled in the french mini-straw. The straws were coded by use of a batch number system. The one half of the straws was fixed to the freezing rampes. After freezing and transfering of the rampes to liquid nitrogen, the rampes were placed in a water-bath at + 35 degree C in 7 seconds. Immediately after thawing the straws they were transferred to a refrigidaire room at + 5 degree C, dried, remounted on the rampes and frozen again in the ordinary way. The other half of the straws were frozen according to the normal routine. The semen from the two treatments were distributed in equal numbers to the technicians who were not informed of the trial. The motility after refreezing had decreased and the percentage of intravital eosin spermatozoas after refreezing increased by 23, as an average. Fertility results were estimated as 60 days non returns after 1st inseminations. Single frozen semen: 1488 1st ins. 493 ret. 66,86 N.r.-% Refrozen semen: 1511 1st ins. 500 ret. 65,30 N.R.-% The trials indicate that further investigation should be performed to see if semen might be frozen concentrated, rediluted after thawing and refrozen for distribution to the technicians.