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Growth, body composition, plasma leptin concentrations, and reproductive performance of beef heifers receiving subcutaneous injections containing 250 mg of bovine ST (BST; sometribove zinc; n = 25; Posilac, Elanco, Greenfield, IN) or 5 mL of saline (0.9%; control; n = 25)1

 
Item BST Control SEM P-value
Growth variables
    Initial shrunk BW (d 1), kg 207 209 2 0.53
    Final shrunk BW (d 211), kg 307 304 4 0.56
    ADG,2 kg/d 0.51 0.49 0.02 0.27
Body composition3
    Backfat thickness, mm 3.56 3.92 0.08 <0.01
    LM depth, mm 45.9 46.5 0.6 0.46
    Marbling4 456 452 3 0.14
Plasma leptin,5 ng/mL 1.82 2.03 0.07 0.05
Reproductive performance6
    Estrus synchronization rate, % 96 92 5 0.56
    Final pregnancy rate, % 96 100 3 0.32
1Treatments were administered every 14 d beginning on d 14 until d 196 of the experimental period (d 0 to 210).
2Calculated using shrunk BW obtained on d 1 and 211 after 16 h of feed and water deprivation.
3Evaluated via real-time ultrasonography on d 0, 63, 133, and 189 of the experiment. Ultrasound measurements were obtained at the 12th to 13th rib interface using an Aloka 500V (Aloka Co., Ltd, Wallingford, CT) B-mode instrument equipped with a 3.5-MHz, 125 mm general purpose transducer array (UST-5011U-3.5). Images were collected and processed with software from the Cattle Performance Enhancement Company (Oakley, KS) according to Brethour (1994). Least squares means adjusted covariately to values obtained on d 0.
4Marbling score: 300 = traces, 400 = slight, 500 = small, and 600 = modest.
5Evaluated from blood samples collected on d 0, 7, 56, 63, 126, 133, 182, and 189 of the experiment. Least squares means adjusted covariately to values obtained on d 0 and 7.
6Heifers were exposed to an estrus synchronization + timed-AI protocol on d 194 of the experiment (CO-Synch + controlled internal progesterone-release device; Larson et al., 2006) immediately followed by 50-d bull breeding. Estrus synchronization rate was evaluated based on plasma progesterone concentration obtained at AI and 7 d later. Values calculated as pregnant heifers or synchronized heifers divided by total heifers × 100.