Relationships between light exposure, milk yield, and reproductive traits in Norwegian Red cattle were studied using data from 1538 farms in southern Norway during the darkest part of the year (November 1 to February 28). Production level was expressed as milk yield at first artificial insemination (AI) and 305-d milk yield. Reproductive performance was calculated using the following dependent variables: age at first AI, age at first calving, days open, calving interval, number of AI per cow, and nonreturn rate at 60 d. Days open and calving interval were both 4.0 d shorter, number of AI per cow was 0.07 less, non-return rate was 3.1% greater, and daily milk yield at first AI was 0.5 kg greater in herds that used dim illumination at night compared with herds that used no illumination at night. Milk yield at first AI was 0.5 kg greater in herds with photoperiods > 12 h. Photoperiods > 12 h were associated with a reduction in age at first AI by 4.8 d and age at first calving by 6.6 d. We concluded that dairy cattle kept at high latitudes during winter should be exposed to dim illumination at night and a minimum photoperiod of 12 h to stimulate reproductive performance and milk yield.

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Dolezel, R., K. Karlberg, E. Ropstad, Kristina Landsverk: Ultrasound Observations of Ovarian Structures in Cows during Intravaginal Progesterone Device Application and in the Following Sexual Cycle (Preliminary Report). Acta vet. Brno, 62,1993: 179-185. Pluriparous dairy cows (Norwegian cattle) were used in experiment. Intravaginal progesterone devices (CIDR) were inserted in 4 experimental cows from day 14 -15 to day 24 -25 postpartum. Four cows in the same postpartal period were included in a control group. The cows were examined by transrectal ultrasonography at 3 and 4 day alternating intervals from day 9 -10 post partum until the 2nd ovulation. Three times weekly the samples of whole milk were taken for progesterone investigation by RIA. Follicle population was divided into 3 groups: small (4-6 mm), medium (7 -10 mm) and large (> 10 mm). None of experimental cows had CL before and during CIDR period. Dominant large follicles slowly grew during CIDR period and ovulated day 3 and 4 after CIDR removal. The first dominant medium follicle occurred in one experimental cow days 1 after CIDR insertion, it persisted and became atretic. Second dominant large follicle was observed at the Ist day after CIDR removal and ovulated 8 days later. Medium follicles did not change and number of small follicles increased during CIDR period. One and two follicular waves were discovered in control cows which ovulated before and after day 20 postpartum. Term of the 1st postpartal ovulation (day postpartum 30 ± 3.37 vs 19.5 ± 7.85), length of the 1st sexual cycle (24.0 ± 2.45 vs 18.0 ± 5.35 days), and number of follicular waves during the ISt sexual cycle (2.5 ± 0.58 vs 2.0 ± 0.82) in experimental and control cows were different. Values of milk progesterone during CIDR period (0.7-1.4 ng/ml) were significantly higher (p < 0.01) in comparison with values before CIDR insertion and during the same postpartum period in non-ovulatory control cows. A 10-day-CIDR treatment is a possible method for induction of the 1st ovulation followed by a normal sexual cycle in postpartal cows. Dominant follicle, follicular wave, postpartal ovulation, milk progesterone, CIDR deviceInitiation of early postpartal luteal phase using progestagen treatment (or pretreatment) for several days is considered to be a useful method for induction and/or synchronization of the first postpartal ovulation followed by normal luteal phase in cows. Progestagen implants (Gauthier et al. 1985), vaginal devices (Smith et al. 1987) and parenteral or oral drugs (Fulton et aI., 1978;Rutter et al., 1985) have been used for this purpose. Long term progestagen treatment is followed by a lower conception rate (Zimbelman 1963;Thimonier et al. 1975). Reduced fertility may

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