The following answer was provided by Jo Leroy, An Langbeen, Veerle Van Hoeck and Peter Bols with the Laboratory for Veterinary Physiology, Department of Veterinary Sciences at the University of Antwerp in Belgium.
Q: Why do modern dairy cows prioritize milk production at the expense of sustained reproductive efficiency?
A: From a biological point of view, it makes sense for mammals in early lactation to favor milk production over fertility: this we can refer to as nutrient prioritization. As nutrition becomes scarce, the lactating dam will preferentially invest the limited resources in the survival of living offspring rather than gambling on the oocyte that has yet to be ovulated, fertilized and nourished through an entire gestation. This maternal catabolic mechanism, also genetically programmed, should maximize the chance of survival of the newborn offspring. Over the past 40 years, the focus of the dairy industry has been on maximizing milk yield, thereby creating a "nutrient highway" from the daily ration and body reserves directly to the udder to sustain milk production.
Nutrient requirements of the gravid uterus late in gestation impose a catabolic status on the dairy cow. Following parturition, an additional demand for glucose, fatty acids and protein is established as milk production starts. During this transition period, cows are unable to compensate for such increased energy by increasing feed intake, resulting in negative energy balance. Drastically reduced insulin concentrations bring about energy mobilization and partitioning of energy to the udder. Hypoinsulinemia promotes the production of glucose in the liver (up to 4 kg. glucose each day) and acts as a massive trigger to mobilize the fat reserves in the body. The mobilized non-esterified fatty acids serve as an alternative energy source for other tissues to preserve glucose, which is preferentially used by the mammary gland to form lactose…. A series of biological mechanisms bring about this prioritization for milk production at the cost of body reserves in early-postpartum dairy cows.
- Dairy cows selected for high milk yields display an absolute priority for their energy to be partitioned to the milk production process in the udder.
- This severe energy priority is at the cost of the cow’s own body reserves and functioning, being recognized as a significant loss in body condition and a higher sensitivity to diseases.
- The degree of body reserve mobilization, and thus body condition loss, is correlated with poor reproductive performance.
- There is growing scientific evidence that this energy saving and mobilization status in the dairy cow early postpartum affect fertility negatively at the level of the 1) brain; 2) ovaries; 3) uterus; 4) oocyte and embryo quality. These interactions are complex.
- Only attention to management practices around parturition that optimize health, feed intake and body condition loss, and direct treatment of disorders will limit fertility disorders postpartum. Accurate and repeated assessment of body condition to estimate changes in body reserves is also critical. Minimizing changes in body condition and thus the "exhaustion of energy reserves" early postpartum requires an optimal dietary strategy. (The authors discussed nutrition more specifically in this paper presented at the Western Canadian Dairy Seminar.)