It is widely recognised that bull breeding soundness examination should play a routine role in herd health preparation ahead of the service period. However, routine breeding soundness examination of eligible reproductive females is not commonplace in most UK beef herds. The future health, welfare, productivity and sustainability of the herd is reliant on selecting the most fertile, efficient and suitably sized replacement stock for breeding. Routine practices such as infectious disease monitoring, trace element and metabolic blood profiles, vaccination and nutritional management are key to maintaining a healthy, fertile herd. Breeding decisions are oft en based on important subjective measures such as docility, conformation, external frame, and selection of heifers from the best cows, alongside simple and traditional, objective selection targets such as age and weight at first service. Tools such as reproductive tract examination and pelvic measurements have been developed and adapted, and while they may require more time and skill to implement, they could still have a place in pre-breeding examinations. Reproductive tract scoring (RTS), however, was found to be a better predictor of reproductive performance compared with previously advocated measurements of bodyweight, body condition score (BCS) and the older Kleiber Ratio (Kleiber, 1947; Holm et al, 2009). Improved reproductive tract measurements (Holm et al, 2016) and objective pelvic measurement have been shown to add prognostic value for pregnancy failure and prognostic value for poor performing heifers respectively (Holm et al, 2016).
Johnson et al (1988) found that the external measurements of pelvic angles and slope of rump had low correlations with the calving difficulty score and internal pelvic area. The use of external assessment alone is not enough, and, while internal examination and measurement of the pelvic area is more time consuming, it could increase its value as a better tool to avoid the risk of dystocia and poor performance.
Pubescent heifers have not yet reached sexual maturity however puberty occurs once a certain level of somatic development, quantified by bodyweight, has been achieved (Holm et al, 2009). It is widely reported that heifers should on average achieve 60% of their adult bodyweight by bulling (Murray, 2005). Holm et al's (2016) study demonstrated that the development of the reproductive system is a function of age and bodyweight, but the age and bodyweight when puberty is reached varied between heifers (Holm et al, 2016).
Puberty by definition is the onset of the first oestrus associated with ovulation of a potentially fertile oocyte, followed by a normal luteal phase (Stevenson, 2007). The onset of puberty is shown to be influenced by several factors, including age, bodyweight, genetics, nutritional status, environment and season (Garverick and Smith, 1993; Stevenson, 2007; Holm et al, 2009). The oestradiol negative feedback mechanism on the hypothalamus and pituitary gland is a feature of prepubescence. But, with puberty the concentration of oestradiol receptors in the hypothalamus decreases, reducing the negative feedback effect, and instead allows the pulsatile release of gonadotrophin releasing hormone (GnRH) and luteinising hormone (LH) to increase. It can take 2–4 weeks for this transition into puberty, and before the first, true ovulation occurs (Stevenson, 2007; Holm et al, 2009). Puberty typically occurs at around 12–14 months of age in beef heifers (Gasser, 2013), and they need to reach puberty 60 days prior to service to allow the best chance of conception early in the season (Fontes et al, 2019).
Examination and score systems of the reproductive tract
The RTS system (Anderson et al, 1991) ranges from score 1–5, indicating prepubescent to pubescent reproductive tracts, respectively. The reproductive tracts are palpated per rectum, and a score assigned depending on the features identified in Table 1, which include uterine horn diameter, the length, height and width of the ovaries, the size of follicles present in the ovaries, identification of a corpus luteum (CL) and tone in the uterus.
Table 1. Reproductive tract score (RTS) system (Anderson et al, 1991)
| RTS | Uterine horn | Ovary | ||||
|---|---|---|---|---|---|---|
| Length (mm) | Height (mm) | Width (mm) | Ovarian structures | |||
| 1 | Immature <20 mm diameter, no tone | 15 | 10 | 8 | No palpable structures | |
| 2 | 20–25 mm diameter, no tone | 18 | 12 | 10 | 8 mm follicles | |
| 3 | 25–30 mm diameter, slight tone | 22 | 15 | 10 | 8–10 mm follicles | |
| 4 | 30 mm diameter, good tone | 30 | 16 | 12 | >10 mm follicles, corpus luteum possible | |
| 5 | >30 mm diameter, good tone, erect | >32 | 20 | 15 | >10 mm follicles, corpus luteum present | |
RTS was thought to be a good predictor of heifer fertility, positively associated with pregnancy rate and negatively associated with days to calving (Holm et al, 2009). Holm et al (2009) recognised that RTS has three applications in practice: first, as advocated by Anderson et al (1991) to determine the pubertal status of heifers before service; second, as an indication of nutritional status while monitoring replacement heifers during rearing (Anderson et al, 1991; Holm et al, 2009); and third, as a selection tool for age at puberty (Pence et al, 2007).
Miller et al (2018) and Holm et al (2009) demonstrated that RTS can be used as a selection tool on day 1 of the breeding season. However, it is thought a stronger association with RTS and fertility outcomes is likely when RTS is carried out 30–60 days prior to service (Pence et al, 1999). RTS carried out in advance of the service period allows time for interpretation of the results, later reassessment if required and any freemartins can be identified and removed to limit the economical drain on the system.
The absence of a CL is thought to be a better predictor of reproductive failure when heifers were examined close to service, however, when heifers were examined more than 3 weeks prior to service BCS, uterine horn diameter and relative pelvic area were better predictors of anoestrus and pregnancy failure (Holm et al, 2016).
It is important that all heifers reach puberty before the start of the breeding cycle (Garverick and Smith 1993). Early cycling heifers should be selected, they have the best chance of early conception and therefore calve early, which allows for timely rebreeding and heavier calf weaning weights (Stevenson, 2007). RTS is a good predictor of lifetime production with positive association to calf weaning weights and pregnancy rate of the next breeding season (Holm et al, 2009).
RTS could be used as a selection tool before either the use of a synchronisation protocol, the start of artificial insemination (AI) alongside oestrus detection or before the introduction of a bull. RTS as a method of selection has also been found to significantly correlate to the response to synchronisation and pregnancy rate to synchronised oestrus (Holm et al, 2009). A recent study found that pubescent beef and dairy heifers with reproductive tract scores 4–5 at the time of synchronisation, were significantly more likely to be pregnant, with prepubertal heifers (score 1–3) less likely to get in calf, regardless of the synchronisation protocol used (Miller et al, 2018).
The RTS cut-off at heifer breeding soundness examinations depends on when RTS is carried out, the number of heifer replacements required and the number of heifers that have been found to reach puberty. Holm et al (2009) found that where RTS 2 was the cut-off, the pregnancy rate would not have significantly increased (56–58%), RTS 3 as a cut-off would have improved the pregnancy rate (56–64%), but RTS 4 would have resulted in an increase in pregnancy rate significantly (56–73%). However, the lower proportion of heifers at RTS 4 would have meant an uneconomical cull of 62% of the heifers, so a cut-off of RTS 3, thus selecting 68% of the best heifers, would have been more suitable (Holm et al, 2009). It is also important to consider RTS cut-off alongside pelvic measurements, since pubescent heifers with small pelvic areas may have a better chance of conceiving early, however, would still risk dystocia.
Holm et al (2016) recognised that RTS can predict reproductive outcomes, such as anoestrus and pregnancy failure in beef heifers, however questioned the accuracy as a result of the oestrus cycle stage and the number of true anoestrus heifers at the time of examination. Ultrasound was thought to both improve the accuracy and repeatability of RTS (Holm et al, 2009). However, this is not always the case (Rosenkrans and Hardin, 2003), and further studies have demonstrated discrepancies in their data collection that could be caused by ultrasound measuring errors (Honaramooz et al, 2004; Holm et al, 2016).
RTS was found to be repeatable within and between veterinarians (Rosenkrans and Hardin, 2003), however, a more robust study questioned the repeatability as a result of the complexity of the RTS system (Holm et al, 2016).
The presence of a CL is part of the RTS system criteria for score 4–5 heifers (Anderson et al, 1991); therefore, the presence of a CL can be used to mark the onset of puberty (Holm et al, 2016).
Holm et al (2016) found that transrectal ultrasonography to detect the absence of a CL, absence of a follicle ≥13 mm and uterine horn diameter (the three ultrasound measurements) were better predictors of pregnancy failure than Anderson et al's (1991) RTS system that relied on transrectal palpation. These findings indicate that the RTS system can be improved by using this simpler system, that requires less work and the increased ability to identify a CL with ultrasound. Where these three ultrasound measures were combined with the pelvic area model or the original RTS system was combined with the pelvic area model, predictions of pregnancy failure and anoestrus compared with RTS alone were significantly better.
Pelvic measurement
The Rice pelvimeter must be carefully introduced per rectum alongside the operator's lubed arm, and the levers are depressed to measure the height (Figures 1 and 2) (from the symphysis pubis to the sacral vertebrae) and width (Figures 3 and 4) (at the level of the poas tubercles to measure the widest point) of the pelvis (LeFever, 1997). The pelvic area is calculated by multiplying the width by the height. Several authors describe cut-off thresholds for pelvic areas that are deemed too small, from <130–140 cm² (Larson, 1990; LeFever, 1997). Eliminations can be made when the examination reveals an abnormal shaped pelvis or a small pelvis.
The heifer's bodyweight and age should be recorded on the same day, and using these figures an appropriate conversion factor is obtained (LeFever 1997; Rodning et al, 2015) (Table 2). The pelvic area should be divided by the conversion factor, this produces a figure that gives the estimated bodyweight of a neonatal calf that the heifer should be able to deliver without experiencing feto-pelvic incompatibility. Where the size of this calf is calculated to be unrealistically small, any normal sized calf is unlikely to be delivered without difficulty.
Table 2. Pelvic area/calf birth weight ratio for various heifer weights and ages to estimate deliverable calf weight (LeFever, 1997)
| Heifer weight (lbs) | Age at measurement (months) | |||
|---|---|---|---|---|
| 8–9 | 12–13 | 18–19 | 22–23 | |
| 500 | 1.7 | 2.0 | ||
| 600 | 1.8 | 2.1 | ||
| 700 | 1.9 | 2.2 | 2.6 | |
| 800 | 2.3 | 2.7 | 3.1 | |
| 900 | 2.4 | 2.8 | 3.2 | |
| 1000 | 2.5 | 2.9 | 3.3 | |
| 1100 | 3.4 | |||
The use of the pelvimeter to eliminate heifers with inadequate pelvis size is key. The most common cause of dystocia in heifers is feto-pelvic incompatibility (Mee, 2004), however, pelvic area is not a direct predictor for dystocia (Troxel, 2011). Pelvic area is highly heritable (61%) (Deutscher, 1987), and can be influenced by genetics, breed, level of nutrition and growth stimulating hormones (LeFever, 1997).
Measurements are taken to avoid breeding from heifers with small pelvic area, but should form only part of the consideration, since size and weight of the heifer's calf contributes to feto-pelvic incompatibility too, and attention must still be paid to selecting sires with good direct calving ease estimated breeding values (EBVs). Calf gender (male) can increase the risk of feto-pelvic incompatibility (Mee, 2004), so where AI is used, the use of sex-sorted semen should be considered, with difficult births reduced by 28% in heifers (Norman et al, 2010). Miller (2018) found the use of sex-sorted semen did not have any significant effect on pregnancy rate and was suitable for use with synchronisation protocols.
While pelvic measurements help predict dystocia in heifers we have also looked at how a relatively smaller pelvic area has a tendency to predict anoestrus and pregnancy failure better than RTS alone, with significant value when used in combination with either the three ultrasound measures or RTS (Holm et al, 2016).
Antral follicle count
Antral follicle count (AFC) is the number of follicles ≥3 mm in diameter (Mossa et al, 2012) that can be visualised and counted by ultrasonography (Morotti et al, 2017). AFC is variable between cattle of the same age, but is repeatable within individuals (Mossa et al, 2012) across the lifetime of the animal (Santa Cruz et al, 2018), and cattle can be phenotyped reliably based on one AFC measure (Mossa et al, 2012). AFC is positively associated with fertility (Santa Cruz et al, 2018), with moderate to high heritability reported (Summers et al, 2018). AFC can be considered as another heifer breeding soundness examination (HBSE) tool since it is considered a biological marker of fertility (Morotti et al, 2017). The number of antral follicles present on the ovaries are recorded with the following scores applied: ≤15 = low; 16-24 = moderate; and ≥25 = high (Ireland et al, 2011). Heifers with higher AFC are thought to have healthier follicles and ooctyes, better quality embryos, better pregnancy rate, increased concentrations of progesterone, and are reported to calve earlier in the season (McNeel and Cushman, 2015; Morotti et al, 2017), and therefore have better reproductive potential. Lactating cows with ≤15 antral follicles have lower reproductive performance compared with those with higher AFC (Mossa et al, 2012).
While AFC is typically reported to be used in heifers 13–15 months old, prior to their first breeding season (Summers et al, 2018), studies have investigated use at weaning so less fertile heifers can be removed earlier (Santa Cruz et al, 2018). Santa Cruz (2018) found that maximum AFC and anti-mullerian hormone concentrations may be used to identify and select heifers that attain puberty at an early age.
Discussion
RTS and pelvic measurement, or the three ultrasound measurements of the reproductive tract and pelvic measurement, can be used in conjunction with each other and the results interpreted alongside one another to ensure the most comprehensive decision is made in terms of heifer selection. Table 3 shows an example of how RTS, pelvic area, pelvic shape, BCS and percentage of mature bodyweight could be interpreted together (Larson, 1990). A heifer with a small pelvic size will continue to have a proportionally smaller pelvic area as a cow (Rodning et al, 2015). However, it has been suggested that a prepubertal heifer with a small pelvis may increase in pelvic size with the onset of puberty and should therefore be reassessed if RTS 1–3 (Patterson 2017). But, a heifer with an RTS score of 5 but an inadequately sized pelvis should not be bred from.
Table 3. Evaluation of heifer breeding soundness when put altogether (Larson, 1990)
| Outcome | BCS (1–10) | % of mature body weight | Reproductive tract | Pelvic area | Pelvic shape |
|---|---|---|---|---|---|
| ‘Ready’ (to breed) | ≥5 | 55–65% | Cycling: corpus luteum present and/or >10 mm follicles with good uterine tone | >130 cm² or herd specific cut-off | Normal |
| ‘Intermediate’ (recheck) | ≥5 | 50–60% | Not-cycling, but palpable ovarian structures and slight to good uterine tone | >130 cm² | Normal |
| ‘Stocker’ (cull or sell) | <5 | <50% | Immature uterus with no palpable follicles or follicles <8 mm, freemartin or pregnant | <130 cm² or herd specific cut-off | Abnormal |
To ensure efficiency and productivity heifers must calve at the start of the season. This will ensure reasonably timed cyclicity at the start of their second breeding season, since post-partum anoestrus can last 80–100 days for freshly calved heifers (Larson, 1990) and resumption of cyclicity will take even longer in heifers that are in poor body condition or that have experienced dystocia (Tenhagen et al, 2007). Early heifer fertility impacts on lifetime productivity, and heifers that conceived in the first 21 days had increased longevity and kilograms weaned compared with those that conceived and then calved later (Cushman et al, 2013).
Sustainability of the UK beef industry relies on efficiency of production, but must also focus on responsible applications and improved technologies. Environmental concerns need to be factored into herd health plans. The UK agricultural industry has led the way in terms of reducing agricultural greenhouse gas emissions (NFU, 2019). HBSE are key since selection of the best heifers could help achieve a lower age at first calving and increased calving rates, which are going to have positive impacts on production, profit and reduce greenhouse gas emissions.
Lifetime emissions and emissions/kg of beef produced, calculated from rumen fermentation, feed energy and manure management, are greater in a heifer that is older at first calving (Teagasc 2019). The optimal age at first calving is 24 months (Wathes et al, 2014). Teagasc (2019) reported that greenhouse gas emissions increase by 0.3% for every month that the age of first calving exceeds 24 months. Methods to identify earlier cycling cows is advocated. A reduction in the age at first calving by 1 month alone is thought to generate an additional €50 per cow (Teagasc, 2019).
HBSE helps eliminate unsuitable heifers from the herd which is important since these unproductive animals will still produce the same level of greenhouse gas emissions as a productively sound animal. A prolonged calving season will increase management and time costs too. In contrast, a 40 cow herd with an increased calving rate of 5% will reduce greenhouse gas emissions by 4% and increase herd profitability by €1720 (Teagasc 2019).
Genomic testing is widely recognised as a pre-breeding selection tool for dairy heifers and it could become more widely available, across all breeds, cost-effective and justified for use in commercial beef herds. Both advances in genetics and epigenetics are being made, and it is important to consider the phenotypic changes that can occur through modification of gene expression. It has been widely recognised that epigenetics play a major role in the onset of reproductive development and puberty (Toro et al, 2018). However, the effect nutrition, environment and circadian activity have on the epigenetic pathways to hypothalamic neurons, regulating the onset of puberty in human females, remains to be found (Toro et al, 2018). Management factors, such as improved preweaning nutrition, have been shown to improve hormone production, mature testicular weight, sperm production and improve sexual development in bull calves (Brito et al, 2007). While precocious puberty has been induced in heifers by feeding high concentrate diets (Gasser, 2013), which led to the suggestion that pubescent 10-month-old heifers were not precocious, and instead had shown expression of their genetic potential under the desired environmental conditions. Future studies are required to find epigenetic biomarkers that could serve as detectors of deranged pubertal development (Toro et al, 2018). Five transcription factors that form a regulatory network driving puberty in cattle have been found, but the role they have in relation to GnRH release requires further investigation (Fortes et al, 2011).
Conclusion
Early cyclicity depends on heifers having reached puberty by the start of the breeding season and future genetic and epigenetic research will further our understanding of how this may be influenced by management factors. Many measures are used to make breeding decisions including, physical attributes, age at puberty, weight at service, BCS and genetic analysis, however, examination tools such as the RTS system, the adapted three ultrasound measures of the reproductive tract and AFC could be used to not only better quantify the reproductive potential of replacement heifers, but help make cull decisions. These decisions are best made in conjunction with pelvic measurement to limit the potential economic costs and detrimental welfare effects of dystocia.
KEY POINTS
- To ensure efficiency and productivity we should aim to breed heifers at the start of the service period.
- Heifer breeding soundness examination (HBSE) tools such as reproductive tract scoring (RTS), ultrasound measurements of the reproductive tract and pelvic measurement can be used to improve heifer selection.
- Future HBSE could include antral follicle counts (AFC).
- Further research is required on the use of genomics and the genetic and epigenetic effects on reproductive potential of replacement heifers.
- Better reproductive efficiency will increase herd productivity, profitability and sustainability.
- Better herd fertility can reduce greenhouse gas emissions by 4% where calving rate increases by 5%, however, greenhouse gas emissions increase by 0.3% for every month that the age of first calving exceeds 24 months.