7-Day CO-Synch + CIDR
This protocol involves only two animal handlings prior to fixed-time artificial insemination (TAI), and it offers acceptable results in heifers (Figure 1). Note that the recommended timing of TAI is earlier for heifers (54 hours after CIDR removal) than cows (66 hours after CIDR removal). This protocol works best among heifers that have already reached puberty. Although this protocol can be followed by either split-time AI or fixed-time AI, some studies have not found significant differences between pregnancy rates resulting from split-time AI or fixed-time AI after this specific protocol. Therefore, it is generally suggested to simply perform fixed-time AI for simplicity when using this protocol.
Select Synch + CIDR
This protocol differs from the 7-day CO-Synch because it involves estrous detection and AI rather than TAI (Figure 2). It has similar costs as the 7-day Co-Synch but increases the time and labor commitment for accomplishing AI. A direct comparison of the Select Synch + CIDR protocol (heat detection and AI) versus the 7-day CO-Synch + CIDR protocol (fixed-time AI) has not been reported in beef heifers. However, Bridges et al. (2008) observed no significant difference in pregnancy rates between suckled beef cows synchronized with Select Synch + CIDR (58%) and those synchronized with the 7-day CO-Synch + CIDR protocol (54%), suggesting that fixed-time AI can achieve comparable reproductive performance while eliminating the need for estrus detection.
5-Day CO-Synch + CIDR
This protocol is similar to the 7-Day CO-Synch + CIDR but involves a shorter period of CIDR treatment. As a result, it requires administering two doses of PG approximately 8 hours apart. Research suggests a modest improvement in pregnancy rates in comparison to the 7-Day CO-Synch + CIDR protocol. However, because of the additional handling of animals and the additional pharmaceutical cost, this is a less commonly recommended protocol for beef heifers and more commonly used in dairy operations. Like the 7-Day CO-Synch + CIDR protocol, this protocol works best among heifers that have already reached puberty.
14-Day CIDR-PG
This protocol is considered a long protocol, because it lasts 33 days, but it is very effective in herds that have a mix of prepubertal and pubertal heifers. Longer protocols are highly recommended for prepubertal heifers. It involves treatment with a CIDR for 14 days (Figure 4). It is important to highlight that a very large proportion of heifers will express estrus after removing the CIDR. This is not a fully fertile estrus, however, and AI should not be performed at this time. Instead, PG is administered 16 days after CIDR removal, and a second period of estrus expression will occur after PG administration. Producers may opt to perform TAI, AI based on estrus, or Split-TAI following this protocol. Research done at the University of Missouri (MP739) demonstrated that, when using this protocol and performing split-TAI, pregnancy by AI can increase up to 10% when compared with a single TAI. This approach is especially recommended if using sex-sorted semen. Other advantages of longer protocols are that producers can organize the calendar to perform the pre-breeding vaccinations and examinations at least 1 month before the breeding season, which is ideal to reduce negative effects of vaccines on ovarian function during the protocol.
MGA-PG
This protocol is similar to the 14-day CIDR, but instead of inserting an intravaginal device, heifers are fed melengesterol acetate (MGA) in a feed mixture for a period of 14 days (Figure 5). The biggest advantage is that the cost of feeding the MGA is significantly reduced when compared to the CIDR costs; however, it requires someone to feed the heifers every day and with good attention to detail. Producers may opt to perform TAI, AI based on estrus, or Split-TAI. Research done at the University of Missouri (MP739) demonstrated that when using this protocol and performing split-TAI, pregnancy by AI can increase up to 10% when compared with a single TAI. The main limitations of this protocol are related to feeding logistics, as a very consistent feeding program is needed to ensure every heifer is ingesting an adequate dose of MGA every day. These considerations include the acclimation of heifers to eating the feed carrier, the inclusion rate of MGA in the feed at the recommended rate (0.5 mg per head per day), the total amount of the feed carrier fed to achieve the required daily dose of MGA, the feeding schedule, and the bunk space allotted to each animal. This protocol is usually recommended only for experienced cattle feeders and for situations in which heifers are already being fed daily during the development program. Note that MGA is not label-approved by the U.S. Food and Drug Administration for use in mature cows.
Calendar organization during the breeding season
We understand that selecting the best protocol for your operation can be overwhelming. For this reason, we have created a decision tree to help with your selection criteria (Figure 6). After defining the best protocol for your operation, we recommend you download the estrous synchronization planner from the Beef Reproduction Task Force webpage, which will facilitate organizing your calendar.
Final remarks
Pharmaceutical products used for estrus synchronization should be administered at the label dose. Consult product packaging. Label-approved applications vary from product to product, and pharmaceutical products for synchronization should be used in the context of an ongoing veterinarian-client-patient relationship.
For more information on proper handling and administration of synchronization products, see University of Missouri Extension publication G2022, Guide to Estrus Synchronization Products.
Protocol recommendations are based on peer-reviewed research in the field of hormonal control of the bovine estrous cycle. This is not an exhaustive list of all possible protocols but provides commonly used and effective options.