A new perspective on management of reproduction in dairy cows: the need for detailed metabolic information, an improved selection index and extended lactation.

For lactating dairy cows, we need management tools, that are "clean, green and ethical", cost-effective and easy to use. Specific tools are needed for artificial insemination (AI) after oestrus detection within a few months of calving, and for managing the complex nutritional requirements of cows between successive calvings. Assessment of energy deficit by measurement of body condition score (BCS) has been useful in the past but we now need more sophisticated ways to measure the relationship between adipose tissue and fertility. For this reason, we have focused our attention on the cells of the adipose tissue, the adipocytes, and the role of the hormone that they produce, leptin. This hormone affects pulsatile LH release and, in dairy cows, it seems to be linked to the first postpartum ovulation. Adipocytes are always sensing energy status and they control leptin secretion dynamically, so blood leptin concentrations can change acutely, even when there is no detectable change in BCS. Leptin secretion seems to be determined by the secretory activity of each adipocyte as well as the total mass of adipocytes in the body of the animal (as measured by BCS). The strong relationship between BCS, leptin concentration and reproductive function in dairy cows suggests that we should reconsider the interval of the recovery from prepartum and postpartum damages, the need for high milk yields at the last lactation causing the dry-off stress and the subsequent troubles. We should also re-assess the current drive to reduce calving interval because milk yields during the early stages of lactation are economically very important but high yields seem to cause several metabolic and reproductive disorders in modern dairy cows. In general, the thinking has been that calving interval must be short because short intervals are more profitable. However, if we remember that main product from dairy cows is milk and that a short calving interval is very difficult without reproductive problems, then a longer calving interval might be more sensible and also more profitable. We have example of an extended calving interval in Japan, Supercows which are very rare cows yielding remarkable high milk. Finally, we probably need to improve dairy cows genetically if we are to achieve the goal of "clean, green and ethical" dairy farming. This paper reviews data relevant to these strategies and we conclude that more basic and applied research will be required if we are to find ways to reach that goal.