An average cow producing 18,000 pounds per lactation would peak at 66 pounds milk in her first lactation, at 82 in her second and 88 pounds in her third lactation. The primary factors are feed intake and its nutrient content. For each pound more of feed intake, you can expect 2 pounds more milk. Anything that can affect high feed intake negatively will quickly result in lower peak production. The ration's protein and energy adequacy and density together usually make up the number-one culprit.
Cows build body reserves during the dry period.
During early lactation and towards peak production, cows normally are milking more off their bodies than they replace in their daily feed intake. Here is where the art and science of stretching peak production comes in. Cows will lose body condition, but the slower this happens and the longer you can keep her from dropping down to the expected 2.5 score, the more you will stretch peak production. If at calving cows are already down at a 2.5 body condition score, they will be further down at 1.5 during peak production. But it won't last long.