Use of artificial insemination and oestrus synchronisation in beef herds

Traditionally, beef suckler herds have used entirely natural service, with artificial insemination (AI) used only on small numbers of cows in pedigree herds. However, use of AI has been increasing over the last two decades as producers seek to take advantage of its benefits (Table 1). This is especially true when used in conjunction with oestrus synchronisation (OS) programmes, which have been developed to minimise the time needed for oestrus detection and the number of cattle handlings while maximising conception rates.

This article aims to set out the main options available for the use of AI in beef herds, particularly concentrating on the use of OS protocols and how they have been refined over the last 20 years, with increasing knowledge of follicular dynamics.

The management of beef cows and heifers to ensure optimal reproductive performance is beyond the scope of this article and the authors recommend the papers by Caldow et al (2005) and Diskin and Kenny (2016) for comprehensive reviews of this aspect.

It should be noted that there are often differences in success rates attributable to individual farm variables such as herbage and diet, breed, body condition score (BCS), days post partum, age at puberty, climate and geology.

Oestrus detection/synchronisation methods facilitating AI

Heat detection only

Methods include visual observation, use of a vasectomised bull, using heat detection aids such as tail paint and/or activity monitors, or combinations of these. Ideally, animals are inseminated 12 hours after the onset of standing heat; this requires a high labour input to observe and separate cows (Lowman, 2003).

PG and heat detection

The underlying physiological rationale for this protocol is that prostaglandin F 2 alpha (PG) causes the corpus luteum (CL), if present, to regress — allowing for maturation of the next dominant follicle (DF) to produce oestrogen (E2) sufficient to cause oestrus and ovulation.

Cattle must be cycling to respond; for this reason, in the authors' experience, OS protocols with single or double PG are only suited to well-grown cyclic heifers or cows that are in target BCS and calved usually more than 7–8 weeks.

Assuming that the same number of cattle in a group exhibit oestrus each day, 55% should have a CL that would respond to an injection of PG (day 5–16 of the oestrus cycle), 45% either have ‘young CLs’ (day 0–5) or are already undergoing spontaneous CL regression (day 17–20). The heifers or cows that undergo luteolysis due to the PG injection and those that undergo spontaneous CL regression should display oestrus and ovulate a fertile egg within 96 hours of a single PG treatment (75% of cycling cattle in a group).

Other management strategies can be used to increase the percentage of PG-treated beef cattle that respond at a predetermined time that is convenient for the producer. One strategy is to observe for heats for 5 or more days and to AI those detected in heat. At the end of the 5-day observation period, the remaining unserved animals are given a PG injection. In theory, the remaining unserved animals should have a CL that will respond to PG or will be undergoing spontaneous CL regression. Heat detection is continued for at least a further 96 hours. The advantage of this is that use of PG is reduced and only cattle that are likely to respond are treated. The disadvantage is that the length of time committed to oestrus detection and AI is a minimum of 9 days.

Another strategy is to give the first PG treatment and then observe for oestrus and AI for at least 96 hours. Theoretically, 75% of the cattle should be served. Then, 11–14 days after the first PG injection, a second PG treatment is given to those yet unserved. The remaining 25% of animals should be in day 11 to 16 of the oestrus cycle. Observation for heat and AI is carried out as before. Heat detection and AI are reduced to 8 days. The second PG treatment allows another chance to AI those animals that are cycling but failed to respond to the initial PG dose, or were not detected in oestrus after the first PG dose.

Double PG 11–14 days apart and fixed-time AI

This protocol is a modification of the strategy described above and similarly requires animals to be cycling. However, it removes the need for heat detection and replaces it with fixed-time AI (FTAI), either at 72 and 96 hours after the second PG injection, or single FTAI after 84 hours (beef heifers may benefit from being served after 78–80 hours). Additional modifications include giving a gonadotropin-releasing hormone (GnRH) injection to all animals at FTAI, at either 96 hours or 84 hours respectively, or only to animals with no visible signs of heat at FTAI.

In the authors' experience, the strategies described above will provide conception rates of 55–60% to conventional semen in typical beef suckler herds (when the vast majority of the animals are cycling normally, are in adequate BCS and where infectious disease is satisfactorily controlled) in the South-West of England.

Progesterone-releasing devices with PG

Progesterone (P4) released by an intra-vaginal device mimics that produced by a CL so that when P4 decreases on removal of the device, the DF of the next follicular wave can develop and ovulate. PG is given around the time of device removal to luteolyse any existing luteal tissue in cycling animals in order to produce a low P4 environment. A conventional programme usually comprises the P4 device in situ for 7–9 (possibly up to 11) days, with the PG injection given 24 hours before removal and AI being performed to observed oestrus over the 96 hours following P4-device removal with an FTAI at 96 hours in those animals not seen in heat. However, many beef producers will just do FTAI at 48 and 72 hours or one only at 56 hours.

This protocol can be used to induce cycling in cattle in anoestrus but with varying success. It has been superseded by protocols incorporating GnRH and/or equine chorionic gonadotrophin (eCG).

P4-releasing devices with GnRH and PG

In the authors' opinion, the gold standard OS and FTAI protocol in the UK for beef cows and heifers consists of the current ‘on-label’ use of a 7–9 day P4 device with GnRH at insertion, PG 24 hours before removal, a second GnRH 36 hours after removal and single FTAI 16–20 hours after the second GnRH (or, if doing a double FTAI at 48 and 72 hours after P4 removal, the second GnRH given at the first FTAI). This protocol results in consistent conception rates of 55–60% in the authors' hands.

Geary et al (2001) showed that combining the second GnRH injection with the FTAI resulted in no reduction in pregnancy rates but was more practical for beef producers. Thus, current recommendations are made to producers in the USA indicating that FTAI using the 7-d CO-Synch+CIDR/PRID or 5-d CO-Synch+CIDR/PRID are practical and effective systems yielding pregnancy rates that exceed those that rely solely on heat detection (Beef Reproduction Task Force, 2022). This is where the P4 device remains in place for 7 or 5 days, respectively; the first GnRH is given at P4 device insertion, the PG is given at removal and the second GnRH at the same time as FTAI at 60–66 hours and 72 hours, respectively, in beef cows (54–56 hours and 60–64 hours, respectively, in beef heifers).

The addition of a GnRH injection at insertion of the P4 device was shown to induce ovulation or luteinisation of a DF leading to the emergence of a new follicular wave (Lamb et al, 2010). There have been many studies looking at the ideal duration of exogenous P4 supplementation in OS programmes. Lamb et al (2001) noted that when the interval between GnRH and PG was 7 days, a proportion of cows subsequently ovulated follicles of smaller-than-normal diameter, which resulted in decreased fertility. These females may have had decreased preovulatory concentrations of oestradiol (Perry et al, 2005). The suggestion is that the smaller follicles at the time of synchronised ovulation resulted from spontaneous atresia of follicles and initiation of a new follicular wave closer to the PG injection in cows that did not respond to the initial GnRH. Therefore, a reduction in the interval from 7–5 days was proposed to reduce the likelihood that this pattern of follicular growth would occur and result in greater oestradiol concentrations during proestrus (Bridges et al, 2010). However, an increase in the interval from PG to FTAI from 60–66 hours in the 7 d CO-Synch+CIDR protocol may also enhance oestradiol concentrations and improve fertility (Busch et al, 2008).

Development of an effective FTAI protocol for beef heifers has not been as easy as that for cows (Lamb et al, 2006). This is because of the reduced ability to synchronise the emergence of follicular waves in heifers when compared with cows. After an injection of GnRH at random stages of the oestrus cycle, 75–90% of postpartum beef and dairy cows ovulated a DF, whereas only 48–60% of beef and dairy heifers ovulated follicles in response to GnRH (Pursley et al, 1995; Thompson et al, 1999; El-Zarkouny et al, 2004).

Atkins et al (2008) evaluated follicular response to GnRH among pubertal beef heifers on specific days of the oestrus cycle. Response was assessed as ovulation or luteinisation of a DF and subsequent initiation of a new follicular wave in response to GnRH. Heifers receiving GnRH on day 2, 10, and 18 (oestrus = day 0) failed to ovulate more than 70% of the time, whereas 92% of heifers receiving GnRH on day 5 ovulated.

The authors' preference if using a 5-day duration of exogenous P4 is to give two injections of PG at device removal and 12 hours later so as to maximise the chances of full luteolysis of the accessory CL created by the first GnRH injection at device insertion.

P4-releasing devices with, GnRH, PG and eCG

The addition of eCG/human chorionic gonadotrophin (hCG)/pregnant mare's serum gonadotrophin (PMSG) to this breeding protocol at the time of P4-device removal is useful in non-cycling beef cows and cows in poor BCS to stimulate follicular growth and ovulation. For instance, Randi et al (2021) showed that eCG given at the time of P4 device removal in a 7-day CO-Synch+PRID protocol had a positive effect on pregnancy rate for cows lacking a CL at treatment initiation. Similarly, in cows with low BCS (≤2.25), eCG supplementation tended to improve pregnancy rate. In the authors' experience, the typical dose of 400 iu of eCG used does not appear to increase the number of twin pregnancies.

‘Ovsynch’

‘Ovsynch’ protocol development in dairy cattle was the precursor for many of the FTAI protocols used for OS in beef cattle (Pursley et al, 1995). Follicular waves are controlled with an injection of GnRH followed with an injection of PG 7 days later. A second injection of GnRH is administered 48–56 hours later to initiate ovulation of a DF followed by FTAI 16–20 hours later. Giving the second GnRH injection concurrently with FTAI proved to be more practical for beef producers, with no reduction in fertility (Geary et al, 2001 (termed CO-Synch as mentioned above). A disadvantage of this protocol is that approximately 5–15% of suckled beef cows exhibit oestrus before and immediately after the PG injection (Kojima et al, 2000; Lamb et al, 2001). These cows are likely those in the late stages of the oestrus cycle when the first GnRH injection is given (Geary et al, 2000). Unless these cows are detected in heat and inseminated early, they will fail to become pregnant after FTAI as part of the CO-Synch protocol.

‘7 and 7 Synch’ in beef cows

This protocol comprises PG injection at the time of P4-device insertion (day 0), with a GnRH injection 7 days later (day 7) followed by a second PG injection a further 7 days later. P4-device removal takes place on day 14 with FTAI and a second GnRH injection 66 hours after this on day 17. The second GnRH is sometimes only given to cattle not seen in oestrus (Bonacker et al, 2020).

The underlying physiology centres on controlling the stage of follicular development by inducing luteolysis with the first PG (if a responsive CL is present) at day 0. Exogenous P4 is then provided at subluteal concentrations so that follicular atresia is inhibited prior to the GnRH administration at Day 7. This thereby increases the likelihood of an induced ovulation to this GnRH, resulting in a new or an accessory CL, as well as commencement of a new follicular wave. The second PG should induce full luteolysis and the DF should be at a mature stage promoting ovulation at the predicted time.

Field trials involving 1500 postpartum beef cows of varying age, days post partum (DPP) and BCS showed that the ‘7 and 7 Synch’ protocol significantly increased the percentage of cows both expressing oestrus and becoming pregnant to either conventional semen or sex-sorted semen FTAI when compared with a 7-day Co-Synch+CIDR programme (Table 2; Andersen et al, 2021).

It should be noted that this protocol has not been evaluated in maiden beef heifers.

‘Triple synchronisation’

The ‘triple synchronisation’ programme (Table 2) was described by Lowman in 2003 and comprises up to three synchronised oestrus cycles for AI, allowing for dispensation of natural service while improving herd margins. It is, however, labour intensive.

Use of sex-sorted semen

This is an area attracting increasing interest from beef suckler producers in the UK but carries the caveat, as for dairy cows, that lower pregnancy rates are to be anticipated compared with conventional semen (Table 2).

Pre-synchronisation

There are a number of studies citing different pre-synchronisation protocols ahead of OS programmes to try to improve pregnancy rates. These all aim to enhance development, growth rates and synchrony of ovarian follicles (Fortune, 1994), along with reducing the concentration of P4 at the initiation of the OS protocol (Perry et al, 2012; Hill et al, 2014) so that they are more likely to respond favourably to whichever OS programme is being used (Lamb et al, 2001). It is beyond the scope of this article to review them all but a selection of references are listed in Box 1.

Box 1.Reading on pre-synchronisation protocols

• Cerri RL, Rutigliano HM, Chebel RC, Santos JE. Reproduction 2009;137:813–823. Period of dominance of the ovulatory follicle influences embryo quality in lactating dairy cows
• El-Zarkouny et al (2004)
• Grant et al (2011)
• Hill et al (2014)
• Mercadante RG, Kozicki LE, Ciriaco FM et al. Effects of administration of prostaglandin F2α at initiation of the seven-day CO-Synch+controlled internal drug release ovulation synchronization protocol for suckled beef cows and replacement beef heifers. J Anim Sci. 2015;93:5204–5213
• Monn RE, Poole RK, Mackey JC et al. A two-injection prostaglandin F2α presynchronization treatment decreases pregnancy rates of cycling replacement beef heifers. Translat Anim Sci. 2019;3(1):456–463
• Perry et al (2012)

Non-cycling beef cows/late-calving animals

Anoestrus is the major contributor to herd infertility (Short et al, 1990; Lamb et al, 2010). Non-cycling status occurs at the onset of the breeding season, ranging from 16–83% in suckled beef cows and from 48–100% in pubertal heifers (Stevenson et al, 2000; Lamb et al, 2001; Hill et al, 2014; Randi et al, 2021). DPP and BCS at the start of breeding influence cyclic status in beef cows (Vasconcelos et al, 2009). Improved pregnancy rates are seen if cows are more than 50 days post partum (Lamb et al, 2001; Larson et al, 2006), while a single unit increase of BCS was shown to increase pregnancy rate by 23% (Lamb et al, 2001). The picture with maiden beef heifers is less clear as most papers cite very high degrees of BCS adequacy in beef heifers (Stevenson et al, 2000; Lamb et al, 2001; Hill et al, 2014; Randi et al, 2021).

For non-cycling beef cows and heifers, a DF must be developed and ovulated/luteinised. For this reason, the authors' preferred protocol is an 8- or 9-day P4-device with GnRH given at insertion, PG given 24 hours before device removal and 400 iu eCG given at removal. This is then followed by FTAI at 48 and 72 hours post removal or FTAI with GnRH at 56 hours post removal.

Other approaches include a single dose of GnRH between 21 and 31 days post partum in beef cows to initiate oestrus cycles (Twagiramungu et al, 1995) or two doses of GnRH 10 days apart (Webb et al, 1977). ‘Ovsynch’ initiated cyclicity in only 38–49% of beef cows in one study (Stevenson et al, 2000), while P4 provision for 10 days before 30 days postpartum was shown to reduce the calving interval by 13 days compared with untreated controls (Peters, 1982).

Pre-pubertal heifers

Lamb (2013) found that less than 65% of beef heifers had achieved puberty by 15 months of age. Byerley et al (1987) demonstrated a 21% increase in pregnancy rate in beef heifers AI'd on their third oestrus after puberty compared with those AI'd on their first oestrus. Roberts et al (2018), however, found that pregnancy rate was higher in beef heifers that showed oestrus before the start of the breeding season but did not improve from having more than one oestrus cycle. However, the proportion conceiving early in the breeding season was greater if they had two or more cycles before breeding. Thus, pre-pubertal heifers can present a challenge to becoming pregnant in a tight seasonal calving pattern.

Implementation of heifer breeding soundness examinations including reproductive tract scores is suggested for selection of replacement beef heifers (Holm et al, 2009; Gutierrez et al, 2014).

In the USA, the focus has been on implementing ‘presynchronisation’ involving GnRH and/or PG or eCG at varying intervals before the start of OS/FTAI programmes to ensure cyclicity (Grant et al, 2011; Hill et al, 2014; Monn et al, 2019), but with mixed success. More recently, a 14-day period of exogenous P4 in beef heifers, finishing 14–16 days before initiation of OS/FTAI protocols, has shown improved pregnancy rates by overcoming the risk of serving at the first oestrus (C. Penny, personal communication).

Strategy for animals not conceiving to OS/AI/FTAI

Most suckler beef producers in the UK use natural service to sweep up those animals not conceiving to AI or an OS and FTAI protocol. It does allow for a single bull to run with a greater number of females compared with natural service breeding alone; typically 50 cattle per bull compared with 20–30 (Lowman, 2003). If an OS protocol has been used however, the returns to heat will also be relatively synchronised, meaning the number per bull may need to be in between these figures.

KEY POINTS

• Artificial insemination (AI) in beef animals confers a number of advantages over natural service, particularly increased genetic gain, herd performance, e.g. improved weight at weaning, and improved biosecurity.
• Oestrus synchronisation programmes further facilitate the use of AI/FTAI, thereby minimising the need for heat detection and the number of cattle handlings while tightening calving blocks and maximising conception rates.
• The use of eCG/HCG/PMSG at the time of progesterone-releasing device removal is useful in generating a dominant follicle to ovulate in non-cycling beef cows and maiden heifers/beef animals in poor body condition score.
• The so-called ‘7 and 7 synch’ protocol shows promise for further improvement in oestrus expression and conception rates in beef cows.