Does supplementing dairy cows with β-carotene during the dry period affect postpartum ovarian activity, progesterone, and cervical and uterine involution?

β-carotene is the main natural precursor of vitamin A and plays an important role in reproductive efficiency and immune function in dairy cows. The objective of this study was to investigate whether a supplement of β-carotene given during the dry period is able to 1) increase blood concentrations of β-carotene postpartum, 2) improve ovarian function and progesterone production, and 3) enhance uterine involution and uterine health. This study was conducted using 40 Holstein cows. On the day of drying-off, cows were allocated to one of two dietary treatments: control diet (C, n = 20) or control diet plus 1g/d β-carotene (BC, n = 20). The β-carotene supplement was given individually to the cows until calving. Blood samples were obtained regularly before and after calving from the cows to measure the concentrations of β-carotene. The diameters of the cervix and uterine horns were measured regularly using ultrasonography. Endometrial cytology samples were acquired from the cervix and uterus to determine uterine health. Milk samples were obtained three times per week for progesterone assay. Additional blood samples were taken on the day of calving, 7 and 21 days postpartum to determine the plasma concentrations of amino acids. Blood concentrations of β-carotene were not different before the start of the experiment (C, 3.03 ± 0.22 mg/L vs BC, 3.12 ± 0.22 mg/L, P > 0.05). Blood concentrations of β-carotene in the BC group peaked (7.45 ± 0.24 mg/L) 1 month after drying-off while the concentrations in the C group remained constant. β-carotene concentrations then decreased in both groups. The difference in blood concentrations of β-carotene between groups became significant 2 weeks after the start of the supplement until 2 weeks postpartum. There was no significant difference in the interval from calving to ovulation between groups (C, 27.8 ± 3.46 d vs BC, 35.8 ± 3.55 d, P > 0.05). The dietary supplement of β-carotene during the dry period had no effect on ovarian activity, progesterone production, cervix and uterine horn diameters. Plasma concentrations of hydroxyproline in the BC group were higher than in the C group on day 21 postpartum (BC, 20.8 ± 1.33 μmol/L vs C, 15.0 ± 1.33 μmol/L; P < 0.01). On day 28 postpartum the percentage of neutrophils in the BC group was lower than in the C group (cervical smear; C, 21.0 ± 3.22% vs BC, 9.7 ± 3.14%, P < 0.05 and uterine smear; C, 32.0 ± 3.86% vs BC, 20.9 ± 3.76%, P < 0.05). In the present experiment a dietary supplement of β-carotene had no effect on ovarian activity. However, due to effects of β-carotene on hydroxyproline profiles and their potential relationship with uterine function we speculate that uterine involution may have been more complete and that uterine inflammation may have been reduced in cows which received the β-carotene compared to controls.