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Components and auxiliary traits of dairy cow fertility: The genetic view
Hermann H. Swalve
Institute of Agricultural and Nutritional Sciences
Martin-Luther University Halle-Wittenberg
Adam-Kuckhoff-Str. 35. 06118 Halle (Saale), Germany
Types of fertility
In dairy farming and breeding, fertility besides milk production is a trait of utmost importance.
When talking about fertility, male and female fertility have to be differentiated. Male fertility, alsocalled paternal fertility, is the ability of a bull to get the cow in calf. For this, as most matings aredone via artificial insemination, the quality of semen has to be such that enough active sperm cellsare contained in each insemination dose. This will be ensured through technical standards on theA.I. stud. However, the ability for fertilization may not entirely be covered by these standards.
Male fertility also includes other traits like libido strength which either for natural service or at theA.I. stud are important criteria.
Since from the perspective of the cattle holder most problems with respect to male fertility aretaken care of by the A.I. stud, female fertility, also called maternal fertility or cow fertility, usuallyis the trait in focus. This is also justified since a cow is expected to stay on farm for several calv-ings and thus should exhibit a repeatable fertility performance which also partly will be passed onto her progeny. In contrast, a different A.I. bull can be picked for each insemination and can bechosen from a worl-wide offer of bulls. Hence, female fertility is much more relevant when tryingto improve sustainable fertility on farm.
Female fertility quite naturally has to be separated into the fertility performance of virgin heifersand the fertility of lactating cows. This differentiation is necessary since a lactating cow has ametabolic condition which is fundamentally different from the status of a virgin heifer. Obviously,the fertility performance of virgin heifers is expected to be higher than the one of lactating cows.
Furthermore, oestrus synchronization programmes are easier carried out in virgin heifers than inlactating cows.
Elements of female fertility
Female fertility, even when differentiated into fertility of virgin heifers and lactating cows, still is ahighly complex trait. Three main elements can be differentiated: The ability to start cycling again(for lactating cows only), the act of fertilisation itself, and early embryonic losses. The first ele-ment, the ability of a cow to exhibit luteal activity after having calved, is difficult to measure.
Fertility: The Genetic View
Prof H. Swalve
Ideally, this can be done by checking hormone (progesterone) profiles. It has been shown that thetrait derived under this approach, commencement of luteal activity (CLA), is very valuable from agenetic point of view as it has a comparatively high heritability. If no progesterone profiles areavailable, an accurate recording of heat can be used to describe the recycling activity. The use ofthe trait “days to first service” (DFS) also points into this direction although quite clearly manage-ment decisions on when a cow should be inseminated will influence the trait. This will not imposeproblems for genetic evaluations as long as the management policy is identical for all cows with-in herd but will create biases if this is not the case.
The act of fertilisation itself can only be measured by the success of an insemination. This willrequire highly accurate insemination records. A big problem is the fact that for a cow that subse-quently will have a calf, very accurate figures can be calculated from insemination records andcalving dates. Cows that leave herd, however, will only contribute data of lesser accuracy.
Early embryonic losses are estimated to be far more frequent than anticipated by most farmers.
Again, accurate figures will depend on highly accurate insemination data, so that deviant cyclescan be detected.
Apart from the three main elements of female fertility, other aspects contribute to reproductivesuccess. One aspect clearly is the behaviour of cows when showing heat as pronounced heatsigns are a prerequisite for successful inseminations. Disorders (Cysts of the ovary, retained pla-centa, etc.) are other topics relevant to female fertility. Genetic approaches to improve heat signsand decrease the incidence of disorders so far have been very limited.
The relationship between milk production and female fertility
A highly debated topic in dairy cattle breeding is the relationship between dairy production andfemale fertility. It has widely been accepted that this relationship is negative, i.e. antagonistic.
Breeding for high yield will have detrimental effects on fertility. However, it is still unclear if thegenetic mechanism behind this truly is a negative genetic correlation arising from pleiotropiceffects of individual genes. Rather than pleiotropic effects of individual genes, metabolic stress ofhigh producing cows may be the reason behind the antagonism. This hypothesis is backed byobservations in many countries which show that phenotypic records of fertility improve in high pro-ducing herds as compared to medium herds. New models may be needed to give further insightinto the genetic relationship between production and fertility.
Fertility: The Genetic View
Prof H. Swalve
Approaches for genetic evaluations
Two main sorts of genetic evaluations for female fertility can be differentiated. The first one isbased on milk recording data only and hence the trait ‘calving interval’ is the central if not the onlytrait in focus. The second approach uses insemination records in addition to milk recording data.
This approach hence requires that insemination records will be made available be A. I. studs. Socalled ‘biological’ records that truly reflect the inseminations performed per cow and service peri-od may be needed to be separated from ‘financial’ records that state which inseminations werecharged for. Insemination records have the intrinsic problem that inseminations done by otherpeople than technicians who are employees of the A.I. stud, may be less accurately recorded. Onall kinds of farms, furthermore the part-use of natural service bulls may create further weakness-es in data quality. In any case, before used in genetic evaluations, insemination records will haveto be subject to rigorous data editing.
Both approaches of genetic evaluations, may insemination records be included, or not, so farhave the drawback that they define traits of low heritability. Hence, chances for genetic improve-ment are limited but not nil.
A fundamental principle of genetic evaluations is that any trait genetically correlated with the tar-get trait can aid in genetic evaluations of the target trait. In recent years, body condition score hasshown that it will function as such a trait. The physiological relationship is quite clear: Cows witha very poor body condition (emaciated, thin) will have poorer reproductive performance. The rea-sons for this have a metabolic background but also hormonal aspects can contribute to this.
Many more traits genetically correlated to fertility can be found: One obviously is milk yield butconformation traits, longevity, and cell scores have been proposed as well. In summary, besidesbody condition score, no trait with a very strong relationship with fertility seems to be available.
This may point to the conclusion that the ideal auxiliary trait is still looked for.
The work ahead
As is true for many functional aspects of the dairy cow, future trait definitions for fertility have tobe more precise, more directly connected with physiology, and more accurately recorded. Quiteclearly, progesterone profiles are an example for this but their collection up to now is costly andthe profiles cover only parts of female fertility. As aids for management purposes, many more
Fertility: The Genetic View
Prof H. Swalve
parameters are already collected on some farms. These include milk conductivity data, pedome-ter data, and milk ingredients other than conventionally used in milk recording schemes. Sincemost of this data is created repeatedly per cow and day, masses of data would be needed to betransferred to evaluation centres per day. This big effort may be ineffective but more research isneeded in this field.
More research is also needed to address the field of early embryonic loss and on the questionhow to distinguish between metabolic stress and genetic relationships based on pleiotropy.
Finally, high hopes exist for the use of genomic selection as an aid in genetic improvement for fer-tility. So far, SNP effects found will mostly be based on associations with the traits recorded con-ventionally. In the long run, SNP associations with new traits will be needed.
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