The objective of this study was to compare the pregnancy rate after timed artificial
insemination (P/TAI) in dairy heifers treated with 4- versus 5-day Co-Synch þ controlled
internal drug release (CIDR) protocols. A total of 120 Holstein heifers were randomly
assigned to one of two groups. The heifers received an intravaginal CIDR insert containing
1.38 g of progesterone for 4 days (Monday–Friday 4-day Co-Synch þ CIDR; n ¼ 60) or
5 days (5-day Co-Synch þ CIDR; n ¼ 60). At the time of CIDR removal, 25 mg of PGF2a was
injected intramuscularly, and 72 hours after CIDR removal, the heifers received 100 mgof
GnRH intramuscularly and were artificially inseminated. Artificial insemination was per-
formed by an experienced technician, using commercial frozen–thawed semen from a
single sire. Pregnancy diagnosis was performed by ultrasonography per rectum 32 days
after TAI. Categorical data were analyzed using proc logistic and the chi-square test,
whereas continuous variables were analyzed using the t-test of Statistical Analysis Sys-
tems. Heifers in the 4-day Co-Synch þ CIDR group had an acceptable P/TAI32 (55.0%, 33 of
60), which was not different (P ¼ 0.35) from that observed in the 5-day Co-Synch þ CIDR
group (63.3%, 38 of 60). Progesterone concentration at CIDR insertion or estradiol
concentration at TAI did not influence the pregnancy outcomes. Interestingly, estradiol
concentration at TAI was greater in the 4-day Co-Synch þ CIDR group compared to the
5-day Co-Synch þ CIDR group (P < 0.01). In conclusion, the Monday to Friday 4-day
Co-Synch þ CIDR protocol resulted in adequate P/TAI in dairy heifers, which was similar
to that of the 5-day Co-Synch þ CIDR protocol. This novel protocol might represent a
promising hormonal treatment for TAI in dairy heifers, facilitating their reproductive
management routine, while maintaining an adequate fertility