Namely, they're slaughtered at the end of their milking life because....they're no longer giving (sufficient) milk. How does that reflect on the cows still in the milking cycle and what are the proportions of cows in at the end of their cycle vs the beginning?
.25/ (Age of cow slaughtered - age of cow start milking)
I chose .25 assuming the traifa happened in the last 3 months you can substitute it for .125 .5 or 1 or whatever number you feel like.
number i found for US (quick googling) is start milking at 3 slaughtered at about 7 so,
.25/(7-3)=1/16 or 1 out of 16