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• Fertility differences still exist among A.I. bulls.
• Non-return rates (the rate of cows not returning to estrus after being inseminated and therefore assumed to be pregnant) from bulls in the same AI organization may differ by 20-25 percent.
• An accurate laboratory test of fertility would identify those less fertile bulls prior to using their semen for artificial insemination.

INTRODUCTION

It is often relatively easy to identify completely infertile bulls because their sperm usually are considerably less motile or have abnormal shapes. However, identifying bulls with somewhat below average fertility (subfertile bulls) or individual subfertile semen collections is typically more difficult because the defects are not always obvious. Sperm from subfertile bulls may appear normal, but insemination with this semen may produce conception rates 25 percent lower than the most fertile bulls. Identification of these subfertile bulls prior to being used heavily for AI would allow producers to reduce the number of cows bred to subfertile bulls and would increase reproductive efficiency. More accurate and sensitive measures of fertility are needed to identify these subfertile bulls. An accurate laboratory test of fertility would be very valuable because it would give immediate results and would eliminate the need to inseminate cows to measure fertility accurately.

One approach is to develop a laboratory test that exposes sperm to conditions that resemble those in the female reproductive tract and then determine if these sperm can fertilize an egg. Several scientists have used in vitro fertilization rate as an indicator of fertility, but the value of this test has been variable, probably because of technical problems that produced variable test results. When sperm bind to eggs during fertilization, they first bind to the egg coat, known as the zona pellucida. There are one or more proteins on sperm that tether sperm to the zona pellucida. One possible explanation for differences in fertility is that more fertile sperm have more of a binding protein allowing them to bind eggs more effectively than subfertile sperm. We measured amount of one sperm protein that binds to the zona pellucida and the abundance of that protein was not related to fertility, although it was related to fertility of human sperm. We are currently attempting to identify other egg binding proteins in sperm. It is possible that the amount of other sperm proteins that bind to the egg is related to fertility.

CURRENT STUDIES

A current research project is aimed at determining if the ability of sperm to bind eggs is related to fertility. We use a competitive test in which sperm from two males compete with each other to bind to eggs. We have devised a method to stain live sperm with fluorescent dyes. Importantly, these dyes do not affect the ability of sperm to fertilize eggs. We stain sperm from two males with different fluorescent dyes, then mix sperm from the males together and add eggs to the mixture. We count the number of sperm from each bull that bind to the eggs to see which sperm win the "race to the egg". The donor of the sperm that bound to the egg can be identified by the fluorescent stain. If we make many comparisons between pairs of bulls, we can rank a group of bulls based on their egg binding ability. These test results resemble a football "Power Ranking", where many teams are ranked based upon how they competed against each other in head-to-head competition. Since we are using a competitive test, this test is very accurate and repeatable. We also use fixed eggs that we can store indefinitely, so that this test is easy to perform frequently.

Our first goal in this research is to be able to estimate the actual fertility of bulls by a laboratory test. First, we must find out whether the laboratory test is really related to fertility. We need to use semen whose fertility is already known. There are two good, although expensive, estimates of actual fertility in cattle. The first is nonreturn rate. If adequate numbers of inseminations are used, this is a reasonably good estimate of relative fertility. We are determining if nonreturn rate is related to egg binding ability.

A second approach is to mix semen from pairs of bulls and inseminate cows. The sire of the offspring can be identified and a competitive insemination fertility ranking can be established, based upon the frequency that a particular sire fertilizes the egg and sires the offspring. This provides a very sensitive test of male fertility. We are determining if the ability of sperm to bind eggs is related to fertility, as estimated by competitive insemination. If an animal has sperm with below average fertility, as determined by a laboratory test, an AI company may be able to correct that deficiency by putting more sperm in a straw of semen. If this corrects the fertility problem, the semen could be used routinely without a loss in reduced reproductive efficiency. If the subfertility can not be corrected, at least the bull can be labeled properly as a lower fertility bull before his semen is used to inseminate cows.

An accurate laboratory test could also be used to determine if some semen collections of bulls are lower fertility than the bull=s typical semen. If this was true, the semen could be discarded. If the fertility is only slightly lower, the company may be able to correct the subfertility by packaging that semen collection at a higher sperm concentration in straws. It is also possible that this test may be able to indicate if the lower fertility is temporary, caused by temporary environmental problems such as animal immaturity, injury, or summer heat stress, or is permanent. In the latter case, the animal could be immediately culled from the herd to save additional housing costs, rather than waiting for him to recuperate

CONCLUSION

Research aimed at understanding how fertilization occurs should result in the development of more accurate laboratory estimates of fertility. These research studies could then lead to more accurate selection of bulls and other farm animals for fertility prior to processing the semen and using it in breeding programs. The end result is a lower cost of producing milk.