BREEDING PROGRAMS FOR IMPROVING MALE REPRODUCTIVE PERFORMANCE AND EFFICIENCY OF AI DOSE PRODUCTION IN PATERNAL LINES: FEASIBILITY AND LIMITATIONS

This paper aims at reviewing the current genetic knowledge of the issues related to the
efficient use of bucks in artificial insemination (AI). Differences between lines have
been found relevant in semen production and quality traits not necessarily related to
their specialization as maternal or paternal lines. Accurate heritability estimates
indicate that genetic selection for increasing semen production by improving male
libido and reducing the number of rejected ejaculates may not be effective. However,
total sperm produced per ejaculate appears to be as an interesting trait to select for,
despite that genetic correlation between ejaculate volume and sperm concentration has
not been yet accurately estimated. Semen pH has shown low to medium heritability
estimates and a low coefficient of variation, therefore it is not advisable to attempt
improvement by direct selection. In general, sperm motility traits have shown low
heritabilities but, the rate of motile sperms per ejaculate has been considered as
convenient to select for. Morphological characteristics of the spermatozoa have
revealed as medium to highly heritable. There are evidences of high genetic
correlations between sperm traits before and after freezing-thawing. There are few
studies regarding the estimation of heterosis of seminal traits but results indicate
important and favorable direct and maternal heterosis in crosses between maternal
lines. However, this has not been confirmed in a cross between two paternal lines.
Until now, attempts to find parametric or non-parametric functions to predict ejaculate
fertility through seminal characteristics recorded in routinely evaluations have been
very unsatisfactory. Hence, it may be necessary to find other semen quality markers, or
to evaluate some of the currently used ones in a more precise manner or closer to the
AI time in order to improve the ability to predict ejaculate fertility. Several seminal
characteristics phenotypically correlated to male fertility, could be considered as
potential traits to select for in order to genetically improving this trait. However, only
the semen pH has been checked for this purpose, and a negative result has been
obtained. Other traits can be studied in the future but bearing in mind that the required
experiments will need large number of bucks for an accurate estimation of the genetic
correlation of the trait with male fertility. This means that these experiments will be
expensive and difficult to set up. The most common criterion to select paternal lines,
average daily gain, seems not to be genetically correlated to male fertility and seminal
traits. Thus, selection for average daily gain has no detrimental consequences on these
related to an efficient AI semen dose production, is feasible in paternal lines. The male
contribution to fertility after natural mating and after AI with semen doses with high
concentration is negligible, but it has been found that, under more restrictive conditions
of AI, male contributions to fertility and litter size are low but higher in magnitude than
the ones obtained after natural mating. The genetic correlation between the female and
male contributions to fertility has been found to be moderate to high and positive.