IN any discussion on fertility, most of the focus falls on areas surrounding female fertility.

A quick look at the various EBVs which address fertility traits, shows almost all of these relate to aspects of female fertility.

For example, Days to Calving, Calving Ease (Dir), or Gestation Length all have a direct impact on fertility within a breeding herd.

This isn’t to say that the bull has no contribution to those traits. Its more that the developments in genetic technologies have been more focused on female aspects rather than males. In terms of genetic selection for fertility in bulls, producers are limited to one trait – scrotal circumference.

In most cases the approach to bull fertility has been to make logistical decisions around management of the animal in order to maximise the opportunity for a bull to achieve both a target pregnancy rate and to pass on superior genetics to the next generation.

Some of these logistical strategies include the requirement to undergo an annual Bull Breeding Soundness Examination; adjusting bull numbers to ensure mating loads are well focused and the management of bulls in the pre-joining period to maximise the ability of a bull to produce healthy semen.

Genetic selection for fertility in bulls

As part of this year’s Animal Genetics Breeding Unit (AGBU) Improving Reproduction Workshop series, the importance of genetic selection for fertility in bulls was discussed.

In a paper prepared by CSIRO & University of Queensland researchers Laercio Porto-Neto, Marina Fortes, Pamela Alexandre, Michael McGowan, Ben Hayes and Antonio Reverter as well as well-known industry consultant John Bertram, the influence bulls have on a breeding system was highlighted.

While bulls are integral to ensuring calves actually are conceived, the paper highlights that within a breeding herd, a bull’s influence is more than ten times that of a breeding cow.

This influence can be determined by looking within a typical breeding herd. Based on a bull serving 25 females a year, over a working life of five years and with a pregnancy rate of 70pc, a single bull will generate around 87 calves in his lifetime, while a good cow will deliver 6-8 calves in total.

Given this influence, it is worth considering whether there are other tools on offer for producers to use to select and improve fertility of their bull teams.

One of the major challenges in the process of identifying genetic indicators for making selection is the complexity of bull fertility. Semen production along with the physical ability to serve can be limited by environmental impacts including temperature and humidity as well as nutrition.

A useful indicator for selection is age at puberty. Puberty and ability to generate sperm are closely linked.  However, puberty is better described as the age when an animal can generate offspring. This accounts for both the production of sperm cells and includes the body condition and sexual behaviour to mate.

Age at puberty has a genetic component, and it can be possible to use this as an indicator for selection.

Early puberty can have some significant flow on benefits for a breeding herd. In bulls, it is positively correlated to early puberty in their female offspring. For producers, this has the effect of both impacting pregnancy rates at a fixed age, for example the number of heifers joined to calve at two years of age.

However, it is important to highlight that selection for decreased age at puberty doesn’t automatically mean heifers will be mated younger. Rather, it is a beneficial trait to have heifers that may ‘cycle’ younger and therefore are more likely to conceive early in the joining period.

In a controlled joining period, this earlier conception allows greater time to recover from calving as well as reducing the spread of calves. The flow-on for weaning and management can result in significant herd productivity improvements.

As a genetic trait, age at puberty can be selected for, although it is important to note that the heritability of the trait will vary across breeds. The heritability for the age at puberty has been reported in research to range between 0.20 and 0.67 across breeds.  This range gives some strong indications that it is a trait that can be improved genetically.

Genomics component

The ongoing focus on genomics will potentially open other opportunities to select for fertility in bulls. It is difficult to determine if a bull’s actual ability to mate is determined by phenotype, or if there is a genetic contribution. However, the AGBU paper suggests that there is potential to utilise DNA based predictions on bull conception rates.

This method may not require the individual DNA to be collected for an entire herd.  Rather the cow herd DNA would be pooled together, and only the individual bulls would have DNA collected.

The method under consideration would be to use pregnancy rate and the pooled cow data could be used to estimate the conception rates from each individual sire. Ideally this could result in developing a ranking of bulls based on their genetic merit for pregnancy rate.

While this method is still some way from commercial use, producers can consider how existing tools such as annual BBSE could be re-focussed to aid in genetic selection decisions.

Conducting Bull Breeding Soundness Examinations earlier than the traditional age of 600 days is one option to commence identifying bulls that have reached puberty earlier than their contemporaries. Earlier assessment offers a chance to make decisions earlier and retain the more fertile animals for breeding than may be the case when done at a later age, when the emphasis may be more on whether a bull can get calves in the season ahead.

It’s also important to recognise that many of the indicators recorded as part of a BBSE are moderately to highly heritable. While scrotal circumference has long been the trait many producers focus on, other fertility indicators can also be used to make decisions on both a bull’s immediate use and his long-term breeding future.

It is suggested that observing traits such as scrotal circumference, volume of ejaculate and sperm motility in younger cattle, particularly in tropical cattle, results in higher heritability estimates and makes greater opportunities for selection decisions.

No doubt as more information emerges from current and earlier industry projects including the Beef CRC and Northern Repronomics, the potential to combine existing tools such as a BBSE with genomic selection will offer breeders and commercial producers an opportunity to select for fertility more accurately and confidently across a broader range of traits.

Alastair Rayner is the Principal of RaynerAg, an agricultural advisory service based in NSW.  RaynerAg is affiliated with BJA Stock & Station Agents.  He regularly lists and sell cattle for clients as well attending bull sales to support client purchases.  Alastair provides pre-sale selections and classifications for seedstock producers in NSW, Qld and Victoria.  He can be contacted here or through his website www.raynerag.com.au