In the initial weeks of lactation, the typical Irish dairy cow physically cannot eat enough to meet her requirements for milk production and maintenance. Yes, her capacity for intake grows as her time in milk progresses, but it lags around a fortnight behind where it needs to be for the initial months. In other words, her peak intake comes two weeks after her peak daily milk production.
What does this mean? Chances are that many late autumn- and winter-calving cows are in a slight negative energy balance (NEB) at present – putting out more than they can take in. Provided it is indeed slight and relatively short-lived, there should be minimal residual effects.