Genetic gain in dairy cattle is a crucial tool to improve milk production efficiency [1]. Heifers should be the highest genetic merit animals in any dairy herd, so breeding heifer replacements from maiden heifers using artificial insemination (AI) is a useful tool to increase the speed of genetic gain [2].

In extensive pasture based dairy systems, such as those found in New Zealand, heifers are often managed at separate properties distant from the main dairy herd, which poses logistical challenges to the implementation of AI programs. Therefore, reproductive programs that enable synchronization of ovulation for fixed-time artificial insemination (FTAI) or AI to detected estrus during a short period of heat detection are necessary to enable the use of AI for breeding replacement calves from maiden heifers.

Ovsynch programs are based on a gonadotrophin-releasing hormone (GnRH) injection and a prostaglandin (PG) injection administered 7 days apart; these programs aim to synchronize emergence of a new follicular wave then induce luteolysis, leading to a synchronized ovulation [3,4]. Ovulatory response to the first GnRH injection of an Ovsynch program is poor in dairy heifers [5], and insertion of a progesterone (P4)-releasing device can be used to prevent new ovulations between the GnRH and PG injections [[6], [7], [8], [9]].

Following restrictions on the use of estradiol imposed by the European Union, a randomised controlled trial evaluated heifer reproductive performance following three different synchrony programs in seasonally calving grazing dairy heifers [10]. Heifer reproductive performance and economic benefits were highest following a Cosynch + P4 program, so use of this program (also known as a 7-day Cosynch or 7dC program) became widespread for estrus synchronization of dairy heifers.

Dairy heifers synchronized with the 7dC program in a seasonal pasture-based dairy system conceive more quickly and are more likely to be pregnant at the end of the mating season compared to unsynchronized heifers [11]. However, conception rate (CR) to FTAI following the 7dC program has been reported to be lower than CR for subsequent AI to detected estrus [12], indicating potential to further improve reproductive performance at FTAI. Although slow genetic gain was recently identified as a concern for the New Zealand dairy industry [13], the number of maiden heifers submitted to AI has decreased by over 35,000 animals between 2018 and 2023 [14]. Improving CR to FTAI for dairy heifers may help to reverse this decline, but the only other synchrony program evaluated in dairy heifers within a seasonal pasture-based dairy system resulted in similar reproductive performance to the 7dC program [15].

Researchers evaluating heifer reproductive management in non-seasonal housed dairy farming systems aimed to achieve CR to FTAI similar to the CR achieved from AI to detected estrus when evaluating heifer synchrony programs, rather than only comparing reproductive performance between programs. Differences identified in ovarian physiology between maiden heifers and cows were used as the basis for proposed alterations in synchrony programs. Heifers are more likely to have 3 follicular waves in their estrous cycle than cows, and on average the time to emergence of the second follicular wave is two days shorter in heifers compared to cows [16]. Shortening the interval between the first GnRH injection and PG injection from 7 days to 5 days, and decreasing the duration of P4 supplementation, was therefore hypothesized to improve follicle quality. However, although this alteration resulted in an improved CR to FTAI, it was still not equivalent to the CR from AI to detected estrus [12].

Further work to improve CR to FTAI from a 5-day Cosynch (5dC) program then focused on establishing the most suitable proestrus duration and optimising control over the follicular wave. Ovulatory response to GnRH treatment for heifers treated on a random day of the estrous cycle is lower than that for cows, with estimates of heifer ovulatory response ranging from 19 % to 40 % [[17], [18], [19], [20], [21]]. Follicular development is therefore less tightly controlled by the synchrony program in heifers, with poorer CR to FTAI reported for the heifers that did not ovulate in response to the first GnRH treatment of a Cosynch program [22]. However, ovulation in response to the GnRH treatment at the start of a 5dC program produces a young CL (under 5 days old) with poor responsiveness to PG [23], resulting in a higher likelihood of a high P4 concentration (>0.5 ng/ml) at the time of FTAI [5]. Luteal regression and CR to FTAI were optimised by addition of a second PG injection on day 6 of a 5dC program in combination with GnRH treatment at program initiation [17]. Extending the interval between removal of the P4-releasing device (with concurrent PG treatment) and the second GnRH treatment (concurrent with FTAI) to 72 h improved CR to FTAI [5,18,24].

Stage of the estrous cycle and P4 concentration both influence the probability of ovulation following GnRH treatment in heifers. Treatment to reduce plasma P4 concentration increased ovulatory response to GnRH from 19 % to 48.4 % [17], which suggested that estrus synchronization and resultant CR to FTAI could be improved by timing GnRH treatment to occur when plasma P4 concentration was low. Administration of PG two days prior to the initiation of a 5dC program increased the ovulatory response to GnRH treatment from 19 % to 86.3 %, reduced the proportion of premature ovulations from 23.8 % to 0 %, and increased CR to FTAI from 49.6 % to 61.6 % in heifers inseminated with conventional semen at FTAI [19].

The aim of this study was to compare reproductive performance (primarily CR to FTAI) and economic effects of synchronizing heifers in a seasonal pasture-based dairy system for FTAI with a modified 5-day Cosynch (m5dC) program, incorporating a PG pre-synchrony treatment, compared to a 7dC program. It was hypothesized that the m5dC program would result in superior reproductive performance, economically justifying the additional costs compared to the 7dC program. A secondary hypothesis was that the pre-synchrony PG treatment in the m5dC program would result in lower serum P4 concentration at the time of GnRH treatment and insertion of the P4-releasing intravaginal device.