The first aim of this work is to elaborate an in vitro trial which can be used to estimate the fertility of cryopreserved semen. Five different ejaculates from each of 15 bulls are collected to assess the NRR. Three of these ejaculates per bull are individually examined in vitro. The main impact is laid on the assessment of the inducible acrosome reaction (AR) of vital spermatozoa after thawing. Additionally the motion characteristics and the ATP content of the sperm cells are studied. Fourteen of the bulls reveal NRR between 60 and 80% while one bull (No. 15) has a much lower NRR. In the 14 bulls with higher NRR the increase in true AR by stimulation with ionophore after heparin treatment is significantly correlated to the NRR. Moreover, the fertility of bulls is higher if the remaining number of vital spermatozoa with intact acrosome at the end of the trial is lower. On the other hand the total amount of true AR, which also includes the premature AR induced by heparin does not correlate to the NRR and is not predictive for fertility. This supports the thesis that it is not sufficient to have enough spontaneous or induced AR in a sperm sample but that the AR should occur only after an adequate stimulation. This in vitro trial represents a hard challenge for the spermatozoa missing regulating factors of the female genital tract. Bulls with a NRR of between 60% and 69% have only few or no sperm cells which are able to respond to calcium ionophore as is also confirmed by bull No. 15 (NRR = 43%). This functional spermatozoan parameter provides an approach to assess even slight differences in fertility between bulls of high NRR. The low fertility of bull No. 15 in this trial is already revealed by the low percentage of motile spermatozoa in the sample after thawing. Neither the motility parameters nor the ATP amount of the bulls with higher NRR disclose any possibilities to assess their fertility. The second aim of this work was to investigate the possibility of fertility improvement by exchanging seminal plasma. The seminal plasma of 8 ejaculates each of three bulls with low NRR was removed by centrifugation and replaced before cryopreservation by seminal plasma from bulls with higher NRR. This possibility of improving the fertilizing ability of bulls with low NRR cannot be confirmed by the obtained results. Differences between the specimens with and without seminal plasma can be percepted which lead to the conclusion of a protective function of the seminal plasma.
