Reference for Bekhorot 33:43

עד שיהא ראשו ורובו דומה לאמו:

And from the former text alone I might have inferred that this is only the case with an obligatory sacrifice, but in the case of a freewill-offering there is no prohibition as regards nidmeh.<span class="x" onmousemove="('comment',' For the words, 'a bullock or sheep', refer to a freewill-offering. The obligatory sacrifice mentioned here includes not only a goat for the New Moon but also Festival goats as the word 'One' is used of those offerings as well.');">²⁴</span>

Jastrow