Pattern of change in the concentration of milk acetone and its association with ovarian activity for pasture-based cows in early lactation

The variation in milk acetone concentration between cows and the resumption of ovarian activity was determined in 68 Holstein–Friesian cows grazing pasture. Cows calved in two groups approximately three weeks apart and were assessed over the first 60 d of lactation.The largest coefficient of variation (152%) for milk acetone concentration between cows was found in the first week of lactation. This variability in milk acetone concentration was largely explained by a consistent cyclic pattern, in relation to the day of the study, with regular peaks at 21–26 d intervals. These peaks were synchronised between cows, and cows in the second group synchronised with cows in the first group.Milk acetone peaks greater than 0.4 mmol/L were associated with a significant drop in milk yield, possibly due to energy deprivation around this time. In this respect, liveweight change over the first 60 d of lactation was significantly negatively correlated (r = − 0.29, p < 0.05) with the mean milk acetone concentration taken over the same period. However, there was no significant relationship (p > 0.05) between mean or maximal milk acetone concentration and the resumption of ovarian activity for the cows that resumed ovarian activity in the first 9-weeks post-partum.The results of this study suggest that the use of milk acetone as a practical indicator of energy balance is limited due to its variability over time. Milk acetone concentration may be useful in experimental situations or when regular milk samples can be obtained in the field, such as in-line analysis. There is a need to undertake more research to elucidate the implications of the cyclic pattern of acetone concentration in milk on reproductive performance.