For modern dairy farms, the largest contributors to the costs of production are the feed costs for the lactating herd, which is the major one, followed by the costs of producing replacement heifers, which impacts as much as 20-25% of the total costs of a dairy enterprise (Gabler et al., 2000). Of these costs, feed typically comprises about 50% of the total expense (Akins, 2016).
Recent research performing a complete economical evaluation on these “unproductive animals” on farms, has demonstrated that each day beyond an optimal age at first calving increases the total costs for rearing the heifer by 3.3€ (Boulton et al., 2017).
- Reduce the age at first calving,
- Apply strategies which will enhance (and not diminish) the future productive capacities of heifers,
- Minimize the costs and the nutrient excretion involved with heifer rearing.
Age at first calving is the most important parameter which needs to be evaluated for farm rearing policies. The decision to introduce heifers into the breeding group should be based not exclusively on age, but needs also include an evaluation of biometrics measurements, such as weight and height (Heinrichs et al., 2017).
The return on the investment from birth to first calving for replacement heifers is only recovered by the time the heifer reaches the peak of the second lactation, on average 530 days of productive life (Boulton et al., 2017), for this reason correct management should be applied in order to ensure the longevity of first calvers.
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The key role of the correct age at first calving:
The decision on when to introduce heifers into the breeding group is predominantly made on the basis of heifer age (Nor et al., 2014), but to be accurate it should take into account also the speed of growth of the heifer during the rearing period.
As gestation length is a relatively constant within breed, approximately 278 days for the Holstein breed (Norman et al., 2009), average age at first calving depends on two parameters; age at first breeding combined with the reproductive efficiency of the heifers expressed as conception rate (pregnancy*AI %).
According to various reports heifer conception rate can vary highly, between 57 to 67% according to studies collating data from different countries (Kuhn et al., 2006, Brickell et al., 2009). Commonly, the target age at first calving (AFC) for Holstein animals is set around 22 to 24 months with about 620 kg of BW (before calving) (Bach et al., 2021).
Ettema et al. (2004) established that heifers calving at a younger age than 22 months compromised first lactation yields and had impaired reproductive performance during their first lactation. However, extending the age at first calving beyond 25 months did not improve lactation, reproduction, or health of primiparous cows it actually diminished it.
Calving age has frequently been considered a critical factor associated with the incidence of dystocia in replacement heifers; impaired skeletal growth is often referred as worrying factor from cattle producers (Hickson et al., 2012).
Many studies have highlighted instead that older age at calving has more detrimental effects on future reproductive perspectives on these animals.
For the Holstein breed, which has precocious puberty, heifers calving at >27 months of age have higher incidence of dystocia (Ghavi Hossein-Zadeh, 2016) and a consequent higher risk of culling soon after parturition. It has also been demonstrated that Holstein heifers calving at 3 years of age have higher risk of retention of foetal membranes, metritis and clinical endometritis when compared to heifers calving at 2 years of age (Cutullic et al., 2012).
Breeding management of Holstein heifers:
Very often, dairy heifers are maintained in “non-convenient locations” which can impair breeding success in these animals, even if they represent the most fertile component of the dairy herd.
The challenges associated with their reproductive management can lead to unsatisfactory results. Observation for signs of oestrus is very often the exclusive method used for breeding of dairy heifers.
Standing oestrus expression in cattle is influenced by many factors. In heifers, environmental factors are particularly important, such as underfoot surface type, stocking density and nutrition (Roelofs et al., 2010).
Stevenson et al. (1996) studied the efficiency of oestrus detection by visual observation of heifers compared to a radiotelemetric pressure sensitive system applied to the rump of the animals, communicating all the standing events which the heifers displayed in real-time. Observation for signs of heat failed to detect 37% of the animals in oestrus, and the rate of failure was the highest in those animals having short oestrus duration (Stevenson et al., 1996).
Often when auditing farm breeding policies, it is reported that oestrus observation of heifers is performed continuously during the day and heat detection aids are frequently not applied because they are perceived as unnecessary. Recently, it was demonstrated that farms adopting this type of policy had heifers on average 1.5 months older at first service, which calved on average 1 month later and had the lowest likelihood of being pregnant by 20 months of age, when compared to farms performing 2 or 3 times daily observations for oestrus of ≈40 minutes duration combined with simple heat detection aids such as tail paint (Fodor et al., 2018). This is likely explained by higher level of commitment to this activity when performed in an organised manner.
The possibility to form sexually active groups is higher when the size of the herd is increased. The degree of oestrus expression and, therefore, the possibility to detect an animal in oestrus can be dramatically increased as the number of heifers that are in oestrus or approaching oestrus at the same time increases.
The simultaneous presence of other animals in heat allows heifers the opportunity to share oestrus behaviour so that oestrus expression may also be the result of sexual stimulation by other animals in heat, which creates a higher level of standing and mounting events, and allows for an increased submission for AI, maintaining the same observation policy (Roelofs et al., 2005).
Proper reproductive management plays a major role in heifer raising because it determines when they become pregnant and, therefore, the feed costs necessary to bring this group of animals into the lactating herd. Programs for breeding control many different strategies have been designed to manipulate fertility in breeding heifers in order to maximise the number of animals conceiving in the above mentioned ideal narrow window of time.
The use of prostaglandins for Holstein heifers:
The use of prostaglandins (PGF₂α) is a very common practice for heifers breeding. The aim of this treatment is to induce luteolysis of a competent corpus luteum. An injection of PGF₂α is only effective when cycling heifers are approximately between day 6 to 17 of their oestrous cycle (day 0 = oestrus).
It is generally accepted that fertility, expressed in terms of conception rate, of cyclic heifers bred after oestrus induced with a prostaglandin treatment is comparable to untreated control animals inseminated after natural oestrus (Lucy et al., 2001, Stevenson et al., 2008). Oestrus observation must be implemented after a prostaglandin injection and must be performed efficiently for at least 7 days; all animals that did not receive AI after the first injection should receive a second prostaglandin administration 11 to 14 days after the first, followed again by 7 days of observation for signs of oestrus. This approach has proven to boost the fertility when using prostaglandins as a breeding strategy (Diskin et al., 2002). One drawback of this type of program is that it requires intensive oestrus observation for a minimum of 14 days to maximize the proportion of bred heifers in the timeframe.
When 2 injections of prostaglandins are administered to all the animals at an 11 or 14 days interval and oestrus observation is implemented exclusively after the second injection, overall fertility is reduced (Murugavel et al., 2003), likely linked with the circulating concentrations of progesterone during follicular development. Heifers injected with prostaglandin at the late stage of the luteal phase have both a greater oestrous response and a higher conception rate than animals treated in the early and/or mid-luteal stages (Tanabe and Hann,1984). The effect of stage of the oestrous cycle at the second prostaglandin injection may be partly explained by the likely lower peripheral concentrations of progesterone in the early and or mid stages of the luteal phase, meaning that animals which have responded to the first injection will be in an early-mid luteal phase when they receive the second prostaglandin administration.
A number of synchronisation studies have established a direct correlation between the concentrations of progesterone at the time of prostaglandin administration and subsequent conception rate (Pursley and Martins, 2011, Diskin et al., 2006).
The use of Progesterone based Timed AI (TAI) programs for Holstein heifers:
The most consistent results for timed AI in dairy heifers are found using the 5-day progesterone-based programs.
Several studies performed over a number of years by the University of Florida have looked at the importance of the use of GnRH and the number and the timing of the PGF₂α administration (Rabaglino et al., 2010b, Rabaglino et al., 2010a, Lima et al., 2011, Lima et al., 2013). Increasing follicle turnover at initiation of 5-d progesterone-based timed AI program by using GnRH combined with 2 doses of PGF₂α administered on d 5 and 6 to optimise luteolysis is the most successful strategy to achieve highest P/AI synchronising dairy heifers with a 5-day progesterone based Co-Synch program. The conception rates per AI in these protocols has been demonstrated to be very acceptable, varying between 58 to 62% (Lima et al., 2013).
More recently, this type of approach has also been compared to a more conventional PGF₂α + oestrus detection program for dairy heifers (Silva et al., 2015). The timed AI program utilised at introduction of the heifers into the breeding group (400 days on average), allowed for the same level of pregnancy per AI compared to the animals detected in oestrus after the PGF₂α treatment, but on average heifers exposed to the 5-Day Co-Synch+P4 conceived 12 days earlier. In addition, by the end of the allowed breeding period of 84 days in this study, there were 6% more heifers pregnant which mean a lower level of economical loss for the dairy farms.
In a very recent study conducted by Ceva in collaboration with TEAGASC and UCD in Ireland, involving 823 Holstein heifers, the removal of the Progesterone Release Intravaginal Device (PRID) was postponed for 24 h using a 6-day protocol and the delay of 8h of artificial insemination when using sexed sorted semen for the breeding of Holstein heifers was tested outline (Fig. 1) (Moore et al., 2023).
This practice was evaluated because one of the criticisms for the utilisation of the 5-Day Co- Synch+P4 protocol is the requirement for oestrus observation from the second injection of PGF₂α to the timed AI, because early heat onset in a proportion of animals has been reported, which requires anticipated insemination of up to ≈20-30% of the heifers (Silva et al., 2015).
The use of sexed sorted semen requires the semen deposition to happen closer to the timing of ovulation as the sexed semen has a reduced fertility and lifespan inside the reproductive tract of the female due to the sexing treatment that the sperm should receive. The delayed removal of the PRID, significantly decreases the incidence of oestrus before TAI without affecting overall P/AI, removing the need for oestrus detection in synchronized Holstein dairy heifers.
The delay of 8h of the AI when using sexed semen in a 6-day PRID synch protocol increased pregnancy pr AI of 9%, moving from a 50% of P/AI in heifers bred 48h after PRID removal, to a very remarkable 59% of the heifers which received the delayed AI at 56h.
Conclusions:
Replacement heifer rearing programs should facilitate the reaching of optimal weight at breeding to achieve first calving at 2 years of age. High submission rates for AI are crucial to concentrate breeding at the correct time. Oestrus detection is often impractical in replacement heifers; several strategies can be adopted to solve the inconvenience derived by ineffective continuous research for signs of heat, from the maximisation of oestrus expression, using prostaglandins, as well as the adoption of the modified 6 Day-PRID Synch protocols for TAI which do not require heat detection and provides satisfactory pregnancy rates particularly critical when using sexed semen.
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