Here in the UK you may be aware that most herds are housed during winter and this means that they will be fed a TMR consisting of a base of grass/maize silage with soya etc. to supply the extra protein.
In summer they milk very well off grass but as with most breeds in the UK will be given a top-up in the parlour of 16-22% protein 'cake' depending on their stage of lactation/yield.
I should just explain that most farms here supplement their cows diet as the yields expected of them are just not possible off grass alone. [this goes for the large and small breeds]
It's not unusual to have a herd average yield of 7,200 and with some of the modern genetic based cows giving yields of 13-14,000 ltrs in their 1st lactation you will appreciate why supplements are needed ?
It probably explains the very tight margins we have to work to !
I hope this helps, how do you find them ?