Implications

Sex-sorted semen is a revolutionary technology for cattle breeding. Greater utilisation of sexed semen can increase the efficiency of both dairy and beef production, increase farm profitability and improve environmental sustainability of cattle agriculture.

Motivation for sexed semen usage

The primary reasons for mating cattle are to initiate lactation, produce replacements and to produce meat (Hohenboken, Reference Hohenboken1999). Each of these is a direct outcome of a successful pregnancy, highlighting the importance of fertility in dairy and beef production systems. The use of frozen-thawed semen for AI is the most common practice amongst dairy producers worldwide. The use of AI slightly increases the probability of producing a male calf compared to natural mating (Berry and Cromie, Reference Berry and Cromie2007), as does the use of frozen semen (Xu et al., Reference Xu, Johnson and Burton2000). The primary reason for incorporating sex-sorted semen in any dairy or beef system is to impose a desired sex bias in the resulting progeny. In dairy systems, dairy breed heifer calves are required for both replacements and herd expansion, and may also increase the value of calf sales where surplus heifer calves are produced and where there is a good market for such calves (De Vries et al., Reference DeJarnette, Leach, Nebel, Marshall, McCleary and Moreno2008). It has also been reported that gestation of a female calf results in increased milk production, especially if the daughter pregnancy occurs at the first parity (Hinde et al. Reference Hinde, Carpenter, Clay and Bradford2014), but this needs to be verified in different populations of cows managed under diverse production systems. In beef cow systems, female calves with a high maternal index are required to generate replacements, whereas male calves with a high terminal index will achieve greater growth rates and carcass price.

Calf welfare

Inevitably, the use of conventional non-sorted semen leads to surplus calves of the non-desired sex (~50% of all calves), and this is a particularly important issue in the dairy industry. For example, in the United States, 0.1% of the bull calves produce sufficient sires for the dairy industry (De Vries et al., Reference DeJarnette, Leach, Nebel, Marshall, McCleary and Moreno2008), resulting in excess production of low value dairy bull calves. The fate of male dairy calves varies between countries. In some countries, male dairy calves are used for veal production, but most are used in some form of calf-to-beef production system. Any animal that has a small monetary value is a potential welfare concern, because their low value does not incentivise good husbandry. A survey carried out on 242 Brazilian farms reported that 35% of farms killed dairy bull calves on site (Hötzel et al., Reference Hötzel, Longo, Balcão, Cardoso and Costa2014). Utilising sexed semen for breeding replacements would minimise the production of unwanted male dairy breed calves, thus allaying potential welfare issues associated with male dairy calves.

Cow welfare

Sexed semen can reduce the occurrence of dystocia by an estimated 20% (Seidel, Reference Seidel2003; Norman et al., Reference Norman, Wright and Miller2010), as heifer calves are smaller and easier to calve. Moreover, if dystocia does occur, mortality is about 57% greater with male calves than with female calves (Dematawena and Berger, Reference De Vries, Overton, Fetrow, Leslie, Eicker and Rogers1997). Because of the importance of achieving a concentrated calving pattern in seasonal-calving systems (dairy and beef), it is vital that the cow quickly returns to high fertility potential after calving to maintain a 365-day calving interval. It is well established that dystocia is a risk factor for retained foetal membranes, uterine disease, delayed resumption of oestrous cyclicity and conception failure. Hence, reducing the incidence of dystocia has both immediate and subsequent health and welfare benefits for the dairy cow. In addition, utilisation of sexed semen to generate replacements can improve biosecurity, as a farm can more easily generate replacements and expand the herd from within (i.e. not reliant on purchasing stock of unknown disease status). This would facilitate biosecure herd expansion from dams of known genetic merit (Weigel, Reference Weigel2004). Finally, to extract maximum benefits from sex-sorted semen usage, animals must be well managed, which has associated benefits for all aspects of animal welfare.

Accelerating genetic gain

Sire allocation

Up until 2014, the majority of published studies that evaluated sexed semen highlighted that fertility performance was reduced, as sexed semen generally achieved conception rates (SS-CR) that were 70% to 80% of those achieved with conventional semen (reviewed by Butler et al., Reference Butler, Hutchinson, Cromie and Shalloo2014b). In seasonal-calving systems, poor conception rates disrupt calving patterns resulting in financial loss (Dillon et al., Reference Dillon, Crosse, Stakelum and Flynn1995; Shalloo et al., Reference Shalloo, Dillon, Rath and Wallace2004 and Reference Shalloo, Cromie and McHugh2014). Therefore, any reduction in fertility negates some of the benefits that sexed semen offers. Since 2014, a number of publications have reported improved fertility performance with sexed semen (Butler et al. (Reference Butler, Hutchinson and Cromie2014a), SS-CR 87%; Xu (Reference Xu2014), SS-CR 94%; Vishwanath and Moreno (Reference Vishwanath and Moreno2018), SS-CR ~90%). These high SS-CR values are only observed in well-managed herds, and quickly deteriorate in herds with average or poor management. Availability of a sexed semen product with fertility equivalent to conventional semen requires a fundamental change in the strategy employed for herd breeding management. First, decide how many female calves are needed, and breed an appropriate number of the highest genetic merit dams with sexed semen from high genetic merit bulls to generate the required number of female offspring. Second, dams that were not included on the list of highest genetic merit dams should be inseminated with beef semen. Economic modelling of implementing this strategy indicated increased profitability compared with use of conventional dairy semen alone (Murphy et al., Reference Murphy, Shalloo, Hutchinson and Butler2016).

Rearing heifer calves

In order to obtain maximum lifetime milk production, all replacement heifers should be first bred at ~15 months of age (to calve at ~24 months of age). This is particularly important in seasonal pasture-based systems, where it is desirable to have heifer calves (future replacements) born at the start of the calving period. This could be achieved by using the allocated quota of sex-sorted semen in the first 3 weeks of the breeding season. The resulting heifer calves would be born at the start of the subsequent seasonal-calving period, and thus be older at first insemination, which would favourably impact their productivity and longevity in the herd (Archbold et al., Reference Archbold, Shalloo, Kennedy, Pierce and Buckley2012; Butler et al., Reference Butler, Hutchinson, Cromie and Shalloo2014b). In year-round calving systems, heifers could be inseminated 3 weeks before normal age at first breeding (‘the early bird scheme’ Weigel, Reference Weigel2004); breeding earlier would limit the impact of any reduction in fertility due to sexed semen, and birth of heifer calves would offset the occurrence of dystocia from calving at a younger age.

Currently, an estimated 60% of breeding aged dairy cows and heifers are needed to produce sufficient number of replacements (De Vries et al., Reference DeJarnette, Leach, Nebel, Marshall, McCleary and Moreno2008), but this number could be greatly reduced with targeted use of sexed semen in heifers and cows (Hutchinson et al., Reference Hutchinson, Shalloo and Butler2013; McCullock et al., Reference McCullock, Hoag, Parsons, Lacy, Seidel and Wailes2013). Replacement heifers could be generated within the first 3 weeks of the breeding season in seasonal-calving systems, and by selecting only genetically superior animals in year-round calving systems. Crosson (Reference Crosson2008) indicated that beef cow replacements sourced from dairy beef-cross heifers are more profitable than sourcing replacements from within a beef cow herd, partly due to loss of heterosis in the latter. This suggests that there is a market for beef-cross heifers from the dairy herd as future beef cows. Breeding lower genetic merit dairy cows with beef semen means their genes are being removed from the lactating herd, aiding herd genetic gain. McCullock et al. (Reference McCullock, Hoag, Parsons, Lacy, Seidel and Wailes2013) carried out simulations to compare the effect of using sexed semen on heifers and genetically superior cows to generate replacements and breeding the rest of the herd with beef semen versus using conventional dairy semen on all animals. The former strategy was more profitable, resulted in faster genetic gain, and increased the number of heifers born.

Sexed semen in beef production

The uptake and usage of AI is much less in beef production compared with dairy production. Consequently, the utilisation of sexed semen in beef production is also low. Nevertheless, a sexed semen product with high fertility could revolutionise beef production. Successful use of AI, especially with sexed semen, is highly dependent on accurate heat detection. This can be difficult in beef herds, especially in large beef herds managed under extensive rangeland systems. The development of high fertility timed AI protocols, although laborious to implement, facilitate synchronisation and whole-herd insemination at the start of the breeding period. In a simple change to traditional beef production systems that rely on conventional semen for AI, sexed semen could be used to preselect the offspring sex, facilitating generation of female offspring with strong maternal traits and male offspring with strong terminal traits.

Sexed semen could be utilised in a number of different beef production systems. First, with a single sexed heifer breeding system, as described by Taylor et al. (Reference Taylor, Moore, Thiessen and Bailey1985) beef heifers are inseminated with X-chromosome bearing sperm to produce replacements and are then sent for slaughter after her first parturition. In this system the price for a beef cow is not affected by the age of the animal and therefore more valuable. Another system that would benefit from sexed semen is the three breed terminal crossbred. Semen biased for female offspring from a sire with excellent maternal traits would be used to create a maternal crossbred, and this maternal crossbred would then be inseminated with Y-chromosome bearing sperm from a sire with excellent terminal meat production traits. This system would not only increase heterosis, but would also utilise complimentary traits from different breeds for maximum advantage. Beef production systems will only profit from the uptake of sexed semen when the monetary return from producing offspring of a desired sex is greater than the cost of implementing it (Hohenboken, Reference Hohenboken1999).

Assisted reproductive technologies using sex-sorted semen

Embryo transfer currently represents a small proportion of the total commercial market for assisted reproduction, but recent trends indicate that it is increasing. In vitro production of embryos saw a sevenfold increase between 2000 and 2012 (Stroud, Reference Stroud2012), and in 2014 in vitro and in vivo embryo production were roughly equal (Blondin, Reference Blondin2017). Embryo transfer utilising sexed semen for fertilisation increases the chances of producing multiple calves of the desired sex from a specific dam and sire coupling (Hayakawa et al., Reference Hayakawa, Hirai, Takimoto, Ideta and Aoyagi2009). Currently, there are two methods that can be used to achieve a desired offspring sex bias with transferred embryos. In the first, a blastomere is collected by biopsy, PCR with Y-chromosome specific primers used to determine sex of the embryo, and embryos of the desired sex are transferred. In the second, sexed semen is used for the fertilisation step.

During the sorting process, sperm cells bearing an X- or Y-chromosome are differentiated based on their florescence intensity. The process also has the potential to select for other sperm specific physiological characteristics based on fluorescent staining techniques. For example, when sperm is co-incubated with propidium iodide, non-viable cells take up the fluorescent stain and can be removed from the sample during the sorting process through fluorescent gating. This is true for any sperm specific physiological biomarker that can be tagged by a fluorescent marker and identified by the cytometers’ detectors (e.g. acrosome integrity, mitochondrial function).

The addition of a fluorescent marker to the sorting technique would undoubtedly slow down the process, but could be implemented for targeted use in reproductive technologies such as in vitro fertilisation (IVF). High rates of good quality blastocyst development can be achieved through the combined use of ovum pick up and IVF with sexed semen (Matoba et al., Reference Matoba, Yoshioka, Matsuda, Sugimura, Aikawa, Ohtake, Hashiyada, Seta, Nakagawa and Lonergan2014). Technologies such as IVF require far less sperm per oocyte to achieve acceptable fertilisation rates compared with AI. Consequently, a process known as ‘reverse’ sorting has been developed for use in specialised in vitro embryo production facilities. This process involves sex sorting frozen-thawed conventional semen straws for use in IVF (Morotti et al., Reference Morotti, Sanches, Pontes, Basso, Siqueira, Lisboa and Seneda2014). The success of IVF with reverse sorted semen would likely improve if additional sperm physiological markers could be added to remove non-viable, cryo-damaged and sub-fertile sperm during the sorting process.

Future environmental restrictions

To meet projected increases in food demand, Alexandratos and Bruinsma (Reference Alexandratos and Bruinsma2012) forecast that by 2050 global milk production would increase to 1075×10⁶ tonnes (liquid milk equivalent; 62% increase relative to 2005 to 2007) and global beef production would to increase to 112×10⁶ tonnes (76% increase relative to 2005 to 2007). These increases will require marked improvements in the efficiency of production, as arable land resources are not increasing (Bouwman et al., Reference Bouwman, Van der Hoek, Eickhout and Soenario2005). As the global demand for milk increases, dairy herd size is expanding in many countries. However, future environmental regulations may limit milk and meat outputs due to mounting concerns regarding greenhouse gas (GHG) emissions. For example, EU member states have targeted reductions in GHG emissions of 30% below 1990 levels by 2020 and 60% to 80% below 1990 levels by 2050 (Efforts Sharing Decision 406/2009/EC). Across the EU, the dairy and beef sectors account for 70% of total GHG emissions from agriculture (Lesschen et al., Reference Lesschen, Van den Berg, Westhoek, Witzke and Oenema2011). A Norwegian study, employing life cycle assessments, estimated that >45% of CO₂ and CH₄ emissions come directly from cattle, which combined with field emissions from forage production contribute significant burdens on the environment (Roer et al., Reference Roer, Johansen, Bakken, Daugstad, Fystro and Strømman2013).

The opportunity to improve the efficiency of beef production by generating more beef from the dairy herd was investigated in a simulation study. At the outset of the simulation, 100 000 tonnes of beef carcass weight (CW) was being produced, 50% derived from the dairy herd and 50% derived from the beef cow herd. It was assumed that dairy cow numbers would increase by 50% over a 12-year period, but for the purposes of the simulation, total production of beef would be held constant. The purpose of the simulation was to determine the proportion of beef derived from the beef cow herd that could be displaced by beef derived from the dairy herd, and to determine the consequences for the total GHG emissions from beef production.

For the purposes of modelling the potential impact of sexed semen on beef output, the proportions of female dairy calves, male dairy calves and dairy beef-cross calves derived from the dairy herd were assumed to be 0.3, 0.3 and 0.4, respectively. Two scenarios were compared for dairy herd breeding management:

• ∙ Conv-2030: All replacements generated with conventional dairy semen (Holstein-Friesian), beef semen used to sire the remainder (calves born: 0.3 female dairy, 0.3 male dairy, 0.4 beef cross);

• ∙ SS-2030: sexed semen (fertility equal to conventional) used to generate all replacements, beef semen used to sire the remainder (calves born: 0.3 female dairy, 0.03 male dairy, 0.67 beef cross).

The implications of the two alternative future situations (Conv-2030 and SS-2030) on beef GHG emissions are summarised in Table 1. For this analysis, key assumptions were made:

• ∙ Beef output would remain the same as the current levels (100 000 tonnes CW).

• ∙ Annual cow culling rate was assumed to be 22%.

• ∙ 6% mortality was assumed for male dairy calves, 3% for beef-cross calves.

• ∙ The Economic Breeding Index in Ireland (as in other countries) is selecting for smaller cows, and hence cull cow carcase weight was assumed to get lighter by 2030. For the same reason, the carcase weight of male dairy calves at slaughter was also lighter by 2030.

• ∙ Selecting for beef bulls suited for use on the dairy herd could increase carcase weight of dairy beef offspring by 2030.

• ∙ The value of dairy beef was calculated based on current beef prices (Cows €3.25/kg, dairy steer €3.80/kg, beef cross €4.20/kg).

Table 1 Simulation of the effect of utilising conventional (Conv) or sexed semen (SS) for breeding all dairy replacements on the economic value and greenhouse gas (GHG) footprint of beef production assuming an expansion in dairy cow numbers by 50% between 2018 and 2030 when total beef output is kept constant

CW=carcass weight; kg=kilograms; t=tonnes.

In the current status quo, beef was sourced equally from dairy herds and beef cow herds. Expansion of dairy cow numbers reduced the requirement for beef from the beef cow herd with or without sexed semen usage, but the absolute reduction was greater when sexed semen was used to generate dairy female replacements and more beef semen was used. The structural shift from male dairy calves (Conv-2030) to beef-cross calves (SS-2030) resulted in greater beef value (+€20.5 M). Keeping total beef output constant, using sexed semen to generate dairy replacements and increasing the proportion of beef-cross calves reduced the requirement for beef from the beef cow herd from 50% in the status quo to 22% in 2030. Of note, even without sexed semen usage, the requirement for beef from the beef cow herd will decline to 25%. This assumes that a beef market exists (or could be established) for a 50% increase in the number of male dairy calves, and that most of the animals would make it to maturity before slaughter. As outlined in the section on ‘Calf welfare’, this is likely a gross overestimate. Because of the marked reduction in the requirement for beef from the beef cow herd, the carbon footprint of the beef produced also declined. Widespread usage of sexed semen in 2030 reduced the carbon footprint from 17.4 kg CO₂ eq/kg CW currently to 13.1 kg CO₂ eq/kg CW in 2030 (−24.6%). Hence, widespread sexed semen usage could facilitate a pronounced structural shift in the beef industry that would reduce the GHG emissions in the sector, and reduce the carbon footprint of beef.

Although CH₄ and CO₂ emissions are positively and significantly correlated with cow output, the percentage increase in emissions per animal are much lower than the percentage increase in productivity (Gerber et al., Reference Gerber, Vellinga, Opio and Steinfeld2011). Furthermore, many studies have reported between animal variation in feed efficiency and enteric CH₄ emissions (Herd et al., Reference Herd, Arthur, Hegarty and Archer2002; Hegarty et al., Reference Hegarty, Goopy, Herd and McCorkell2007; Yan et al., Reference Yan, Mayne, Gordon, Porter, Agnew, Patterson, Ferris and Kilpatrick2010; De Haas et al., Reference De Haas, Windig, Calus, Dijkstra, De Haan, Bannink and Veerkamp2011). Selection of sires and dams based on feed efficiency has the potential to have a large impact on reducing GHG emissions from livestock systems. Therefore, selective breeding for daughter traits should also take into account animal efficiency in minimising GHG production. Obviously, dispersion of genetics with a favourable environmental footprint could be accelerated with sexed semen.

The use of beef semen in dairy production systems is likely to further increase in the coming years because the requirement for replacements is finite, and the greater value of a beef-cross calf v. a male dairy calf provides some protection from volatile milk prices (McCullock et al., Reference McCullock, Hoag, Parsons, Lacy, Seidel and Wailes2013; Murphy et al., Reference Murphy, Shalloo, Hutchinson and Butler2016). This would obviously diminish the ability to expand herd size, and is most suited to herds that have already stabilised in size. This breeding strategy also has reduced operating costs, lower initial investment requirements and can still generate profit when milk prices are low (€0.22/l; Murphy et al., Reference Murphy, Shalloo, Hutchinson and Butler2016). Similarly, in simulations carried out by McCullock et al. (Reference McCullock, Hoag, Parsons, Lacy, Seidel and Wailes2013) where only heifers and genetically superior cows were inseminated with sexed semen to produce replacements and the rest of the herd was bred with beef semen, profit, genetic gain and the ability to generate heifers were all increased. If production efficiency of milk and beef can be improved, there is large potential to reduce emissions while concurrently improving farm profitability (Place and Mitloehner, Reference Place and Mitloehner2010) and also cater for more efficient land use.