Energy partitioning during lactation: where the calories go
Published 14 June 2026 · Updated 2 September 2026
A lactating cow runs an energy budget in which the largest outflow, milk, is also the one the animal gives priority. Maintenance is fixed in the short term, growth is a minor claim after first calving, and the remaining allocation is set by hormonal signals that favour the mammary gland over adipose tissue and over the reproductive system. The consequence is that in early lactation the cow produces milk from her own body whatever the ration contains.
The accounts
In the net energy for lactation systems (NRC 2001 in Mcal, CVB in VEM, INRA in UFL, where 1 UFL equals 1.7 Mcal NEL), maintenance is set at about 0.080 Mcal NEL per kg of metabolic body weight. For a 650 kg Holstein, with metabolic weight near 127 kg, that comes to roughly 10 Mcal per day. Milk energy follows from composition: with 3.7 per cent fat, 3.1 per cent protein and 4.85 per cent lactose, a kilogram of milk carries about 0.70 Mcal. A cow at 45 kg a day therefore exports about 32 Mcal in milk, three times her maintenance. Pregnancy adds very little before the last two months, and activity and thermoregulation cost more or less depending on housing and weather.
The conversion from metabolisable energy to net energy is the next layer. The efficiency of ME use for lactation, kl, is about 0.64 at maintenance level feeding and drifts down as the energy concentration of the ration falls and intake rises. Mobilised body tissue converts to milk with a higher efficiency, about 0.82, which is the reason a cow cannot be starved out of production by moderate underfeeding in the first weeks: her own fat is a cheaper source than the ration for the same joule.
The early lactation deficit
Milk yield rises quickly after calving and peaks at 5 to 8 weeks. Dry matter intake lags: it reaches its maximum at 10 to 14 weeks, after the peak. In the gap the cow is in negative energy balance, with the deficit reaching its nadir in the first or second week and closing, on average, around week 8 to 10 in high-yielding herds. Typical cumulative loss is 40 to 70 kg of body weight in a multiparous cow, equal to 200 to 350 Mcal of NEL if the loss is largely fat (the NRC factor is around 5 Mcal per kg of body weight lost), and a body condition score decline of 0.5 to 1.0 units on the five-point scale.
The metabolic pattern behind the number is well characterised. Insulin falls and growth hormone rises, and the uncoupling of the growth hormone axis in the liver lowers IGF-1. Adipose tissue releases non-esterified fatty acids (NEFA) in proportion to the deficit. The liver takes up NEFA in proportion to supply, oxidises part of it, converts another part to ketones, and re-esterifies the rest to triglyceride, which accumulates when export as VLDL cannot keep pace. Blood beta-hydroxybutyrate above 1.2 mmol per L marks subclinical ketosis, a threshold that between 20 and 40 per cent of cows cross in the first weeks in many herd surveys.
What the deficit costs
The price is paid in health and fertility rather than in milk. Cows in deeper negative energy balance resume ovarian cycling later, since LH pulse frequency is suppressed and the dominant follicle is less responsive. Immune function, in particular neutrophil activity around calving, is impaired by high NEFA and low glucose. The incidence of displaced abomasum, retained placenta and metritis tracks the severity of the deficit, though causation is partial, because a cow with a metabolic disorder eats less and deepens her own deficit.
Breeding for milk yield has widened the gap. Genetic selection has moved the priority given to the mammary gland upward and left the intake response behind; the correlation between genetic merit for yield and the depth of negative balance is positive and well documented. The cow is not deficient in any single nutrient; she is outrunning her gut.
Managing the gap
Three levers exist and none is complete. The first is the dry period: avoiding over-conditioned cows at calving (BCS above 3.75) reduces NEFA mobilisation, since fatter cows mobilise more and eat less. The second is energy density after calving: ration starch can be raised, but starch above about 28 per cent of dry matter invites rumen acidosis, and the cow then loses intake instead of gaining energy. Rumen-protected fat supplements raise energy density without that risk, though the response in intake is often negative and the net gain is small. The third is the supply of glucose precursors, propionate being the principal one, since the cow has a gluconeogenic demand of 2.5 to 3 kg of glucose per day at peak and ruminal fermentation delivers very little glucose directly.
Protein enters the same budget. Amino acids are used for gluconeogenesis when glucose is short, and the supply that reaches the intestine is calculated by a metabolisable protein system that has its own uncertainties. An energy deficit therefore shows up as a protein problem in the milk analysis: low milk protein concentration in early lactation is often a signal of too little fermentable energy, not too little protein.
The cow will lose weight in the first month; the aim is to keep the loss inside a range where the liver and ovary cope with it, and to have intake rising as fast as the rumen allows. Anything promising to remove negative energy balance entirely has not accounted for the arithmetic of 45 kg of milk.