FINNISH DAIRY-COWS - ENERGY-BALANCE AND SEASONAL-VARIATION IN FERTILITY

The relationship between metabolic balance and seasonal variations in fertility was studied in middle Finland in 104 cows that had calved from February to June. The cows were kept indoors until the end of May when they turned out to pasture. Urea and acetone concentrations in milk were used to evaluate metabolic balance, and progesterone assays were used to determine the onset of ovarian activity. The length of ovarian quiescence was influenced by the season. All cows that calved in May had started their ovarian activity by 7 weeks postpartum, while 64% and 70%, respectively, of those that calved in March and April had increased levels of progesterone (p < 0.01). Cows that calved in March had the highest concentrations of acetone beyond 4 weeks postpartum. On pasture the energy balance reached equilibrium in five weeks postpartum, whereas cows kept indoors had an increased level of acetone even 9 weeks after parturition. No correlation was found between urea levels and reproductive performance. The most harmful effect of increased acetone concentration was found in late puerperium. The adverse effect of low energy level was also observed when cows were on pasture; increased level of acetone was significantly correlated with a long interval from calving to first insemination and conception (p < 0.05). Cows that calved in late spring had the shortest interval from calving to first insemination and conception. In dairy cows an imbalance in energy metabolism in late puerperium affects reproductive performance negatively. The effect of season on fertility was due to the better metabolic balance of cows on pasture. The positive effect of pasture may be due to exercise and to better nutrition.