Not all cows with a high Body Condition Score have problems with their transition to lactation

We are all familiar with the term ‘negative energy balance’. It is often perceived negatively, and even sometimes discussed as if it was a ‘disease’. These connotations are most likely driven by numerous associations found between negative energy balance and risk of diseases during the periparturient period. It is also frequently assumed that negative energy balance is the consequence of a shortfall of energy supply relative to milk energy yield. Yet most, if not all, nutritional tentative aiming to abolish the negative energy balance have failed.

Indeed, most mammals, including cows, mobilize fat after parturition, and this strategy is believed to have conferred evolutionary advantage. As ruminants first evolved in the wild with the potential threats of predators, the ability to rely on resources that have already been acquired, such as body fat, to produce milk for the calf, is highly precious. First because cows could afford to move away from area of high availability of food, and therefore from area where predators are also most likely to be. And then because cows are particularly vulnerable after calving. It is now well recognized that variation in body fat relative to parturition are for a large part genetically driven. Evolutionary, losing fat in early lactation ensures the survival of the new-born calf, and the gain of fat in mid-to late lactation is the preparation of the next calving, so that body fat can then be mobilized at next calving to complete the cycle. Therefore ‘negative energy balance’ after calving is just the natural expression of normal physiological adaptation to calving. However, it is with no doubt that genetic selection for greater milk yield has exacerbated the amount of fat being lost in early lactation, and that such amount represents a challenge for metabolic health, and in particular for hepatic health.

It is not uncommon that up to 40 kg of body fat are mobilized within the first few weeks after calving for modern high producing cows. This provides a substantial amount of energy to produce milk but also imposes great challenge on the liver. As the amount of fat mobilized increases, plasma NEFA rise and, as hepatic NEFA uptake is proportional to plasma NEFA, the liver has to process more fat. Simultaneously the demand for glucose increases, and this endogenous glucose production through gluconeogenesis may compete with some metabolic substrates (e.g. oxaloacetate) required to fully oxidized NEFA. As a result, some NEFA are incompletely oxidized and exported as ketones bodies (i.e. acetoacetate and BHBA) or stored as triglycerides in the liver. When excessive, these processes can lead to ketosis and fatty liver. A well described nutritional strategy known to consistently ameliorate hepatic health is limiting energy supply during the dry-period by feeding a diet that contain a high amount of forage NDF. Such strategy primes the cow to better deal with the increased concentrations of NEFA, and often results in greater dry-matter intake postpartum with less incidence of metabolic diseases.

Whereas avoiding excessive accumulation of body fat at calving is known to be critical for success of transition, it is also apparent that some cows have an inherently better ability to process fat. Recent work from researchers of the University of Bonn has highlighted this aspect with the use of metabolomics and machine learning. The profile of metabolites observed in plasma around the transition period was used to predict if dairy cows had a normal BCS (< 3.5) or a high BCS (> 3.75). Most cows with high BCS were correctly predicted to be high BCS (13 out of 19) but 6 fat cows were identified as normal BCS. These 6 apparently metabolically healthy but fat cows had similar BCS loss and plasma NEFA dynamic post calving than the other 13 fat cows, but had much lower plasma BHBA, demonstrating substantial difference in NEFA oxidation capacity. These metabolically healthier cows had also greater dry-matter intake post calving, emphasizing once again the importance of feed intake after calving. Understanding the factors that shape the metabolism of dairy cows into more resilience is of great importance to improve the health and sustainability of Dairy production systems. The journey towards an adult lactating cows start very early, and it is now clear that even the amount of milk fed in the first weeks after birth influences the likelihood of a smooth transition period.