Evaluation of treatment with a 5-day versus 7-day controlled internal drug-release insert on reproductive outcomes of beef heifers using a modified timed-artificial insemination protocol

The objective of this experiment was to determine the effect of reducing the duration of controlled internal drug-release insert (CIDR) exposure in a CIDR-based, fixed-time-AI protocol on pregnancy per AI (P/AI) and pregnancy rates in beef heifers. The experiment was conducted in 4 consecutive years. British crossbred heifers (yr 1, n = 82; yr 2, n = 68; yr 3, n = 86; yr 4, n = 53) were stratified by BW and age and were assigned randomly to 1 of 2 treatments: (1) CIDR7 heifers (n = 145) received a CIDR insert for 7 d; and (2) CIDR5 heifers (n = 144) received a CIDR insert for 5 d. Heifers in CIDR7 and CIDR5 were given prostaglandin F2α (25 mg) upon CIDR removal followed by gonadotropin-releasing hormone (75 μg) and fixed-time AI 56 to 59 h after CIDR removal. Estrual behavior was monitored following CIDR removal. In all 4 yr, heifers received AI by a single inseminator. Fourteen days after AI, heifers were exposed to bulls for 40 d. Blood samples were collected on the day of CIDR insertion. Pregnancy status was determined by ultrasonography at d 35 and d 67 to 82 after AI. Data were analyzed by logistic regression and ANOVA. Percentage of heifers detected in estrus was different (P < 0.05) between years (69, 44, 51, and 51% for yr 1, 2, 3, and 4, respectively) but not different (P > 0.2) between treatments. At CIDR insertion, mean serum progesterone concentrations were similar between CIDR7 and CIDR5, and there was no effect of treatment-by-year interaction on progesterone concentrations (P > 0.2). There was a treatmentby-year interaction effect on P/AI (P < 0.05). For CIDR7 and CIDR5, in yr 1, P/AI was 39 and 65.8% (P < 0.05), respectively; in yr 2, P/AI was 64.7and 41.1% (P < 0.05), in yr 3, P/AI was 59 and 73.8% (P = 0.09), and in yr 4, P/AI was 50 and 70.4% (P = 0.25). Overall, P/AI tended (P = 0.07) to be greater for CIDR5 compared with CIDR7 (63.8 vs. 53%). Regardless of treatment, heifers detected in estrus had a greater (P < 0.01) P/AI compared with those not detected in estrus (65.9 vs 47.9%). The final pregnancy rates of heifers were unaffected by the treatment protocols or year (92 and 95.5% for CIDR7 and CIDR5, respectively). The P/AI results from this study suggest that reducing the duration of CIDR treatment from 7 to 5 d in a CIDR-based and single-prostaglandin-injection, fixed-time-AI protocol may improve P/AI in beef heifers.