The study was designed to test the effects of dry period lengths on follicular dynamics parameters during the subsequent lactation in Holstein cows. Cows (n = 42) were randomly assigned in 1 of 3 treatments: 1) traditional 60 d dry period (DP) (n = 14), 2) 35 d DP (n = 15) and 3) 20 d DP. Holstein cows were blocked in pairs based on their previous 305 d milk, parity, and expected calving dates. All cows were fed by routine ration of farm (total mixed diet) twice a day and had at all time free access to water. To monitor follicular parameters, ultrasound measurements of follicular activity were made on alternate days from d 10 to 35 postpartum to determine the characteristics and fate of the 1st follicular wave. The data were analyzed using the General Linear Model (GLM) for a completely randomized design. Number of follicles (≥5 mm in diameter) on d 10 and 14 postpartum were similar among the groups. Length of dry period had no effect on days postpartum to first ovulation (P = 0.91; 24.69 ± 2.2, 24.07 ± 2.2, and 23.27 ± 2.4 d, respectively), number of days until detection of a follicle ≥10 mm in diameter (P = 0.42; 11.53 ± 1.0, 11.64 ± 0.9, and 13.23 ± 1.0 d, respectively), diameter of the largest follicle at first ultrasound (P = 0.92; 10.8 ± 0.91, 10.46 ± 0.8, and 13.11 ± 0.9 mm, respectively), and also days open (P = 0.88; 170.25 ± 25.8, 159.10 ± 28.3, and 178.72 ± 27.0 d, respectively). Diameter of the first dominant follicle (P < 0.05; 18.76 ± 1.1, 19.80 ± 1.1, and 14.46 ± 1.1 mm, respectively) and diameter of the dominant follicle on d 14 (P < 0.05; 13.65 ± 0.8, 12.96 ± 0.7, and 9.95 ± 0.8 mm, respectively) were different among the groups. Results from this experiment demonstrated that dry period length had no apparent effect on most follicular dynamics parameters and days open during the subsequent lactation. Meanwhile, a short dry period reduced the diameter of dominant follicle.

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