Prediction of bovine male fertility = Voorspellen van vruchtbaarheid van stieren

Cattle Breeding industry today selects the most valuable dairy bulls based upon the production performance and type classification of their daughters. Once bulls are selected the goal is to disseminate their genes into the population. Therefore it is important to maximize the number of insemination doses per bull without compromising pregnancy rates. AI-industry constantly has to balance the farmers needs, which is to get a cow in calf, with its own needs, which is to perform as many inseminations possible per bull. The goal of AI-industry is to maximize the number of offspring produced by selected genetically superior bulls, thus disseminating genetic improvement to the cattle population as efficiently as possible. This is accomplished by maximizing semen collection and the number of breeding units produced per ejaculate without compromising fertility. The aim of this thesis was to study the possibilities to predict (dose dependent) fertility of bulls by in vitro tests on their semen.The motivation of the research approach taken in this thesis in order to study possibilities of the prediction of dose dependent insemination results is discussed (in chapter 2) against the background of our current knowledge of the biology of reproduction. In order to be able to monitor fertility results after artificial insemination, information on performance of the insemination is retrieved from the field and non-return rates are calculated. Non-return rates are defined as the proportion of cows that were inseminated and did not return for another service within a specified time after insemination, usually 60 to 90 days and in the Netherlands 28 and 56 days. In chapter 3 of this thesis fertility evaluation by non-return rates was studied with the objective to develop a system to adjust non-return rates of bulls and insemination results of technicians for systematic environmental effects. We concluded that under current circumstances in the Netherlands non-return rates should be adjusted for effects of herdseason, season of insemination, age of the inseminated cow, technician and sire, in order to provide unbiased non- return rates for management and research purposes.In order to see whether we could extract more elementary information on reproductive performance of a bull from non-return rate data when knowledge on the biology of conception and gestation is taken into account we studied daily decline of non-return rates in chapter 4. A multiphasic logistic function was developed to model decline in non-return rate by estimating conception rate, calving rate and characteristics of the first two cycles.In chapter 5 the effect of number of spermatozoa inseminated on fertility results is studied. Semen from 20 proven bulls was split sample diluted and each dose contained 2.1 to 17.3 x 10 <sup><font size="-2">6</font></SUP>total spermatozoa. The exponential relationship between dose rate inseminated and non-return rates (chapter 3) or conception rate or calving rate (chapter 4) was evaluated for each bull. For 5 of the 20 bulls in this trial there was no observed dose dependent decline in fertility results. The expected decline was for these bulls outside the tested dose range. The optimal number of spermatozoa per insemination varied between bulls and moreover, ranking of bulls for non-return rate 56 days altered with the insemination dose. It is however, expensive and time consuming to determine the above described relation for each individual bull by performing a field trial. Furthermore, reliable and accurate records are a prerequisite for this type of field trial and therefore this procedure is not feasible for every AI company. Therefore, the aim is to predict the dose dependency of field fertility results using laboratory tests in order to circumvent these field trials.Around 1985 several motility analyzing systems using video taping and tracking of spermatozoa were developed. These systems are objective and repeatable in comparison to the subjective motility estimation by eye which had been performed so far. In chapter 6 the dose dependence of motility characteristics from bull spermatozoa after freeze-thawing and different tests for resistance to incubation stress was evaluated. The information was subsequently used for prediction of the relationship between dose and corrected non-return rate at 56 days, estimated conception rate and estimated calving rate given conception. The motility characteristics had limited predictive value for the relationship between dose inseminated and fertility. The highest correlations found were in the order of -0.5 to -0.6. The correlation between maximal non-return rates and percentage motile cells immediately after thawing, being the most informative predictor, was 0.52. Capacitation and subsequent acrosome reaction was studied by simultaneous determination of viability and acrosomal status of the spermatozoa using microscopical evaluation in chapter 7. Determination of the percentage of intact cells that are competent to acrosome react directly post thaw and the percentage of cells showing acrosome reaction upon addition of lysophosphatidylcholine at 4 hours of incubation can be used to predict the optimal semen dose when no information on field performance is available. In the case where we are able to retrieve field fertility data this test will give us information on the dose range to be tested under field conditions. This is where this test might prove its value for every AI laboratory. In chapter 8 the homologous oocyte penetration was studied for only five of the twenty bulls evaluated in chapter 5. It was quite difficult to design a repeatable test which measured only penetration as there was an influence if the concentration of sperm present altered, while sperm oocyte ratios stayed unchanged. Also the number of oocytes in test had its influence on penetration rates. The variation found in vitro and in vivo insemination results did not relate for the bulls tested. In the final considerations the research performed in this thesis is discussed against the background of the concept of holism and reductionism in science philosophy (chapter 9). It was concluded that the fertility assays evaluated are able to exclude most in- or subfertile males and eliminate wrong processing protocols of semen. Fertility assays are however, not able to point out males with superior fertility. Investment in philosophical thinking might develop our ability to take the total approach in order to get to the right questions. Finally with improvement of real knowledge therapeutic concepts for the in- or subfertile male might be developed as well as new ideas around processing and storage of semen.