We know that nutrition plays a tremendous role in milk production of your dairy herd. Without the proper calories, fibers, and nutrients, animals are not at their optimal level to produce efficiently.
However, there are other factors that influence milk production also.
Genetics: Different breeds of cattle produce milk at different rates. Not only does production vary between breeds, but the level of quality with their milk is diverse as well.
Reproduction: The age a heifer is chosen to first calf plays a part in the overall lifetime production. The younger the animal, the more opportunity they have throughout their life cycle to produce milk. This also varies depending on how long you choose to keep cows dry for.
Environment: From macro to micro, the environment your herd lives in influences milk yield. The general climate of the area you live in can stress cows out if the weather is too hot or too cold. Inside the barn, animals need enough room to lay around and chew cud. Keeping the stalls freshly bedded and alleys clean help to reduce the number of bacteria in the barn to avoid infection or sick animals.
Even within herds containing a similar genetic base and eating the same ration, there were differences in milk production between animals. A study done in Spain showed that the presence or absence of feed refusals and whether feed was pushed up in the feed bunk influenced yield.