In block calving seasonal herds, the 6-week pregnancy rate is a key measure of reproductive efficiency. Maximising the number of heifer calves born to provide replacement animals accelerates the herd's genetic gain. Coinciding the heifer group's calving period with the start of the herd's calving block is advantageous for several reasons. First, the replacement heifer calves born will be uniform in age for the rearing period, and second, they will be a minimum of 24 months old at their first calving. Calving heifers early or before the main herd's calving season also allows these dams to have a minimum of 90 days before the start of the herd's next breeding season.
However, inseminating nulliparous heifers, particularly in pasture-based spring calving systems, can have practical difficulties, with the heifers being at pasture during the breeding season (Figure 1). Synchronisation programmes that allow for fixed time artificial insemination (FTAI) can be very efficient mechanisms of obtaining the aforementioned desired goals, even more so if sexed semen is used, thereby maximising the number of heifer calves born.
Breeding large numbers of heifer replacements using hormonal synchronisation programmes is now widely used particularly in block calving dairy herds. While the use of synchronisation protocols allows a 100% submission rate, the protocols used have variable conception rates (CR) reported in the literature. However, it is generally accepted that FTAI programmes have been shown to be economically viable when compared with inseminating to observed oestrus (Silva et al, 2015; Ribeiro et al, 2018).
New Zealand has large numbers of block calving herds and data published from there using 7-day progesterone protocols (McDougall, 2013; Sahu, 2015) and conventional semen demonstrated CRs of 52.4–54.5% (Sahu, 2015), with the study being carried out in 2008–2010, and 47–57% (McDougall, 2013) with the study being carried out before 2012. There has been much work done subsequent to these studies looking at shorter progesterone protocols in heifers and this is reviewed to support the choice of protocol used.
Considerations as to which protocol to use?
The minimum requirement for success of any synchronisation programme is (Carvalho et al, 2018):
- A short to medium period of high progesterone before prostaglandin injection
- High progesterone at the time of prostaglandin injection
- Low progesterone at the time of the gonadotrophin-releasing hormone (GnRH) treatment prior to insemination
- Ovulation in response to this GnRH treatment
- Low progesterone at the time of insemination.
Nulliparous heifers are usually cycling well (Lima at al, 2011 experiment one for example) and therefore may physiologically achieve these above requirements. However, Lima et al (2013) advised that synchronisation protocols should incorporate such physiology principles to optimise fertility in dairy heifers.
The two main considerations for synchronising large numbers of heifers are: the number of handlings required with the respective costs of hormone treatments, balanced against the achievement of the maximum number of pregnancies.
When considering which protocol to use there are a number of questions to ponder. Assuming a desire to do FTAI with a protocol that fixes the time of ovulation, to avoid having to do oestrus detection with or without the use of prostaglandin F2 alpha (PGF2α) injections, then some of the main considerations/questions are:
1) Do heifers benefit from the use of a progesterone device in the protocol?
PGF2α can be used in heifers, repeated at 11 days with a FTAI at between 72 and 84 hours. To get the best CRs any animals seen in oestrus should be served as this protocol does not fix time of ovulation, and timing to oestrus will vary depending on stage of cycle. McDougall (2013) presented data of CR to artificial insemination (AI) across three protocols in New Zealand heifers. The protocols were double PGF with AI to oestrus within 4 days, an Ovsynch with progesterone device and a Cosynch with a progesterone device with CRs of 31%, 47% and 57% respectively. PGF alone protocols will also have little or no efficacy in peripubertal heifers.
Response of heifers to injectable protocols such as Ovsynch (Pursley et al, 1995) is usually poor in comparison to AI at observed oestrus (Pursley et al, 1997; Rivera et al, 2004). The use of a progesterone device allows better control of the follicular wave, corpus luteum (CL) regression and synchrony of ovulation (Rivera et al, 2005). Addition of progesterone supplementation between GnRH and PGF improves synchronisation without prolonging a FTAI programme (Kasimanickam, 2014). When using sexed semen, the timing of ovulation relative to AI (Saacke, 2008) is even more relevant to CR as the sex sorted semen has shorter viability in the female reproductive tract (Seidel, 2014). Silva et al (2015) considered that improved synchrony of ovulation resulted in an increased CR with sexed semen in synchronised animals compared with control animals served at observed oestrus.
2) How long should the progesterone device be in place for?
Nulliparous heifers are generally considered to be cyclic and do not have the issues of metabolism of steroid hormones that lactating dairy cows do, and therefore possibly have less need of progesterone priming (Sartori, 2004). Colazo and Ambrose (2011) demonstrated that a 5-day progesterone protocol had comparable pregnancy per AI to a 7-day progesterone protocol. The use of 5-day progesterone protocols is now generally considered to have acceptable CR to FTAI (Rabaglino et al, 2010; Lima et al, 2011). There are variations of these protocols with differing reported CR to AI (Lima et al, 2013).
With comparable CRs and the benefit of 2 less days to AI, the shorter progesterone protocols were preferred for this case study.
3) Using a shorter progesterone protocol, what injections and timings of the hormones and FTAI would give the best CR?
Heifers having high levels of progesterone may not ovulate following a GnRH injection and therefore this could reduce the synchrony in the cohort: Dias et al (2010) and Colazo et al (2008) demonstrated a reduction in luteinising hormone (LH) response and ovulation. Stevenson (2008) and Lima et al (2011) also looked at the ovulation rate to first GnRH in FTAI protocols with results between 26.9% and 35.4%. However, a high level of progesterone at this first GnRH has a greater effect on CR than an ovulatory response to the GnRH (Giordano et al, 2012).
Lima (2011) showed the addition of a GnRH to a 5-day progesterone protocol with a single PGF at removal and a Cosynch (giving the GnRH concurrently with FTAI (Geary and Whittier 1998)), increased the ovulation rate from 10.6 to 35.4% (p<0.01), but with no difference between 32 day (52.5 and 54.1%) and 60 day pregnancy rate (49.8 and 50%) respectively with GnRH or without. This same paper confirmed that Cosynch at 72 hours post PGF resulted in statistically significant improvement in 60-day pregnancy rate with pregnancy rate of 51.6 and 55.6 % for Ovsynch and Cosynch respectively (p=0.05); this change is more marked with animals not showing oestrus at AI: 44.7% (Ovsynch) v 53% (CoSynch).
The timing of AI (and GnRH) chosen was a Cosynch at 72 hours post PGF injection.
If using a GnRH at the beginning, the resulting ovulation will result in a young CL formation and this may not regress fully with a single shot of PGF. Silvia et al (1991) suggested that PGF can be secreted in a pulsatile fashion only if all essential components needed to generate a pulsatile pattern of secretion are functioning properly. As progesterone levels at AI have a significant impact on CR a second dose of PGF can be advocated to ensure complete luteolysis and reduction in progesterone levels. Peterson et al (2011) determined that in cross-bred beef heifers the addition of a second dose of PGF 6 hours after the first given at progesterone device removal tended (p=0.06) to higher pregnancy rate 62.1% versus 54.2%.
Lima et al (2013) followed on from their 2011 paper and compared three differing 5-day progesterone protocols in heifers, namely no GnRH at progesterone insertion with either one PGF (NoGPG1) or the addition of a second PGF 24 hours after the PGF injection given on day 5 and time of progesterone device removal (NoGPG2) compared with GnRH being given at the time of insertion with two PGF injections as per protocol NoGPG2 (GPG2). Results are shown in Table 1.
Table 1. Comparison of P/AI with NoGPG1, NoGPG2 and GPG2 in a 5-day progesterone protocol
| NoGPG1 | NoGPG2 | GPG2 | p-value | |
|---|---|---|---|---|
| P/AI day 32 % | 53.0 | 55 | 61.7 | 0.002 |
| P/AI day 60 % | 49 | 51.6 | 59.1 | 0.001 |
| Pregnancy loss % | 7.4 | 6.3 | 4.3 | 0.16 |
P/AI = pregnancy per AI, NoGPG1 = No GnRH at insertion, single injection of PGF, NoGPG2 = No GnRH at insertion, 2 injections of PGF, GPG2 = GnRH at insertion, 2 injections of PGF
Lima et al (2013)The results demonstrated that the combination of GnRH at insertion and double PGF in the 5-day programme improved pregnancy/AI compared with no GnRH and a single or double PGF. While this paper did not have a protocol with a GnRH at insertion and a single PGF, taking the results from both this and the 2011 paper the best CR was achieved with the GPG2 result on a 5-day protocol in dairy heifers. If a simpler (and cheaper) protocol was required, then a 5-day progesterone device with no GnRH at insertion and a single PGF at removal could be used.
Heifers that express oestrus before FTAI can potentially impact the results as the timing of ovulation becomes less synchronous with the FTAI. Santos et al (2017) hypothesised that leaving the progesterone device in until day 6 would reduce the incidence. There was a significant decrease in heifers in oestrus pre FTAI (p<0.01) on the 6-day protocol. The number of heifers displaying oestrus early was 12% and 1% respectively in the 5- and 6-day progesterone groups. For ease of handling and management, preventing early oestrus removes the need for oestrus detection allowing a single handling for FTAI.
4) Would a second insemination increase conception rate to protocols?
There is little published work on this area, possibly because the concept of a FTAI protocol is to fix ovulation and insemination at a specific time point. The most commonly cited study (Stevenson et al, 1990) was in dairy cows and showed no significant improvement with double, compared with single, AI. Walsh et al (2017) showed a non-significant difference between a single FTAI at 56 hours and a double FTAI at 48 and 72 hours after progesterone device removal in nulliparous heifers (pregnancy (P)/AI: 54.6% vs 60.2%), although this protocol had no GnRH on the day of insertion and the progesterone device was in place for 7 days with a single PGF given 1 day before device removal. This is an area in which further work may be useful.
Consequently, the decision was made to use a 6-day progester-one protocol with the first GnRH, given at the time of insertion with two injections of PGF 24 hours apart, with the second GnRH given at the time of a single insemination with sexed semen at 72 hours after the initial PGF for the case study detailed hereafter.
Case study
The use of this 6-day PRID Delta (Ceva Animal Health UK Ltd) synchronisation/two dose prostaglandin (PG) protocol with a single straw of female sexed semen was investigated on two herds. One herd had 200 Friesian/Jersey cross-bred heifers in a spring block-calving, pasture-based dairy herd, and the other was on 103 Holstein heifers in an autumn block-calving, pasture-based dairy herd, both in the southwest of England.
Spring block
200 heifers were selected from a group of 240 based on weight and split randomly into two groups for practicality of handing at AI. The median liveweight of the selected group was 289 kg (target 60% of 450 kg mature weight = 270 kg) and the age variation was 5 weeks (14–15 months old) at the start of mating.
The oestrus synchronisation protocol designed to achieve the five requirements above was started by the attending veterinary surgeon after ultrasound examination of the uterus and ovaries to ensure ovarian cyclicity and no freemartinism. To ensure the best compliance and aim for better CR there was no variation in protocol used.
The chosen protocol used was as discussed and is illustrated in Figure 2.
AI was undertaken by Genus ABS using three technicians on each day of service. All heifers were inseminated with a single straw of Sexcel™ female sexed semen. A sweeper bull was introduced 2 days after the FTAI.
Pregnancy diagnosis was by ultrasound examination at 42 days post AI.
Autumn block
103 Holstein heifers from a cohort of 180 heifers in an autumn block-calving herd were selected on profitable lifetime index (PLI) genomic testing results to be synchronised for AI with Holstein sexed semen. The same 6-day PRID DELTA protocol was used with an initial veterinary surgeon visit as per the spring block and two farm AI trained staff served the animals. Sexed semen was sourced from multiple AI companies.
Results
There was an overall CR of 53.5% in spring calving herd (Table 2) and 62% in autumn calving herd (Table 3).
Table 2. Spring block calving crossbred heifers
| AI group/bull | Bull A | Bull B | Bull C | Bull D | Bull E | Group 1 | Group 2 | Overall |
|---|---|---|---|---|---|---|---|---|
| CR | 72% | 56% | 48% | 58% | 44% | 52.0% | 55.0% | 53.5% |
| Number straws used | 25 | 25 | 50 | 50 | 50 | 100 | 100 | 200 |
Table 3. Autumn block calving Holstein heifers
| Sire | Bull F | Bull G | Bull H | Bull I | Bull J | Overall |
|---|---|---|---|---|---|---|
| CR | 60% | 67% | 57% | 48% | 79% | 62% |
| Number straws used | 20 | 12 | 14 | 29 | 28 | 103 |
PRID Delta had a retention rate of 99.5% (1/200 lost PRID Delta before day 6) in the spring calving herd; the retention rate was not recorded in the autumn block.
Statistical analysis
There were marked differences in CR across bulls. However, a chi-square test did not find an effect of bull on conception rate in either spring or autumn (p=0.18 and 0.21, respectively). This is likely to be because the numbers were too small to detect important differences in CR.
Cost benefit analysis (spring block calving herd)
The costs of medicines, veterinary surgeon/farmer time, semen and extra feed were balanced against benefits of more heifer calves, improved genetics/milk production, earlier calving date, calving ease, reduced bull power requirement.
In order to run a cost benefit analysis, a number of assumptions had to be made, these are shown in Table 4. Using these assumptions, the costs and benefits can then be calculated as seen in Tables 5 and 6.
Table 4. Assumptions made for cost benefit analysis
| Assumptions | |
|---|---|
| Estimated mean calving dates of synch heifers with sweeper bull put in 2 days after FTAI | March 13 |
| Estimated mean calving date with bull only — midpoint of each cycle with an assumed bull CR of 70% to give a mean date after 3 cycles | March 19 |
| Extra days in milk if a single synch programme with sweeper bull compared with bull only | 6 days |
| Total heifer calves from bull only mating's (70% CR) for 3 cycles with F:M ration of 48:52 (=x) | 93 |
| Total heifers produced from synchronised mating sexed semen (53.5% CR) plus 2 cycles of bull mating (70% CR) with F:M ratio of 87:13 for sexed semen and 48:52 for natural bull (= y) | 134 |
| Number of extra heifers produced per 200 heifers synchronised = (y-x) | 41 |
| Number of AI bred heifers produced per 200 heifers bred using synchronised mating with sexed semen (53.5% CR + F:M ratio of 87:13) | 93 |
| Extra milk produced per AI bred heifer in first lactation (10% increase vs homebred) (400l at 30 pence per litre) | £120.00 |
| Value of cross bred bull calf | £10.00 |
| Value of AI cross bred heifer calf | £120.00 |
FTAI - fixed time artificial insemination, CR = conception rates, F:M = female:male, AI = artificial insemination
Table 5. Benefits for synchronised heifers in £ value
| Benefits | Value £ | Number/multiplier | Overall benefit £ |
|---|---|---|---|
| Market value of extra artificial insemination (AI) vs homebred heifers | 50 | 93 | 4650 |
| Market value of net extra heifer calves vs bull calves | 110 | 41 | 4510 |
| Value of extra days in milk (15 litres/day @30ppl) | 4.50 | 200 | 900 |
| Value of earlier calving heifers (benefits to future fertility) less dystocia with easier calving heifer calves | 10.00 | 200 | 2000 |
| Genetic gain giving better milk (+10%), fertility, somatic cell count of AI vs homebred heifer | 150 | 93 | 13 950 |
| Benefits of keeping 50% less bull power (5 instead of 10 bulls) — per bull | 800 | 5 | 4000 |
Table 6. Costs for synchronised heifers in £ value
| Cost | Value £ | Number/multiplier | Overall benefit £ |
|---|---|---|---|
| Sync protocol medicines | 25.00 | 200 | 5000 |
| Vet time for Synch protocol | 7.00 | 200 | 1400 |
| Farmer time for Synch protocol | 3.00 | 200 | 600 |
| Cost of semen per straw | 25.00 | 200 | 5000 |
| Cost of extra feed for milking versus dry per day (6 days per synch heifer @0.50p) | 3.00 | 200 | 600 |
From the assumptions (Table 4), benefits (Table 5) and costs (Table 6).
Total benefits = £30 010 equating to £151.55 per heifer.
Total costs = £12 600 equating to £63 per heifer.
Net cost benefit per heifer using 6-day progesterone FTAI protocol with sexed semen in a spring block-calving herd is £88.55 per heifer (Table 7).
Table 7. Net cost benefit per heifer
| Benefits per heifer | £151.55 |
| Costs per heifer | £63.00 |
| Net benefit of sync program per heifer | £88.55 |
Discussion
The main findings of this field study are outlined below.
Heifer synchronisation using a 6-day PRID Delta/two dose PGF protocol using sexed semen achieved a CR of 53.5% and 62% in spring block-calving and autumn block-calving herds respectively. These results were considered acceptable and in line with expected results from the choice of protocol and sexed semen.
A variation in CR between certain bulls suggests the need to spread the risk of individual bull choice over a group would be recommended. Bull selection was not random but rather targeted for genetic traits, therefore the breed composite of the heifer in-seminated may have influenced the CR.
This potential for significant variation between bulls has recently been demonstrated by Maicas et al (2019). They investigated the rate of P/AI for 10 bulls across 3214 heifers and 5457 lactating cows in Ireland. The ejaculates of the 10 Holstein Friesian bulls were split and processed to provide per straw:
- Fresh conventional semen at 3 x 106 sperm/straw
- Fresh sex sorted (SS) at 1 x 106 sperm/straw
- Fresh SS at 2 x106 sperm/straw
- Frozen SS at 2 x 106 sperm/straw.
This study concluded that fresh semen did not achieve greater P/AI no matter the dose of semen per straw. There was a bull effect for all semen treatments as well as a dispatch to AI interval for fresh semen and this highlights the importance of using a team of bulls for breeding management.
Timing of insemination when using sexed semen has always been an area for discussion and in this study maintaining FTAI at 72 hours post first PGF resulted in similar P/AI for sexed semen, as seen in previous work using conventional semen in FTAI at 72 hours post first PGF of 5-day CoSynch + CIDR (Rabaglino et al, 2010; Lima et al, 2011, 2013).
In this field study, the estimated benefit of using 6-day PRID protocol is £88.55 per heifer (approximately a 150% return on investment). This concurs with the findings of other studies suggesting that FTAI programmes are economically viable (Silva et al, 2015; Ribeiro et al, 2018). Ribeiro (2018) also calculated that in USA farms with detection of oestrus below 60%, the use of either timed AI for first AI followed by detection of oestrus or timed AI alone improve reproductive performance and reduce the cost per pregnancy. Oestrus detection needed to be excellent at greater than 70% for there to be an economic reason not to use FTAI protocols.
It should be noted that managemental, environmental, nutritional and health inputs and factors all need to be optimal to allow such FTAI protocols used as in this study to achieve fertility that is acceptable when compared with AI to standing oestrus. FTAI also has the advantage of removing the need for oestrus detection and reducing the number of handlings required.
Conclusion
The use of sexed semen in dairy heifers has economic and welfare benefits as well as increasing the speed of genetic gain in a herd without having to compromise biosecurity buying in animals. There are many variations of protocols that can be used to allow FTAI in heifers with varying complexity and published conception rates. In this case study a 6-day PRID Delta with an injection of GnRH at insertion and two injections of PGF at days 5 and 6 and FTAI at 48 hours post PRID removal resulted in conception rates comparable to the previously common use of service to observed oestrus.
KEY POINTS
- Importance of reproductive efficiency in block calving herds.
- Differing programmes using progesterone devices for heifer fixed time artificial insemination using sexed semen
- Variation in conception rates to different bulls can be significant
- Cost benefit of £88 per heifer synchronised in this case study.