This is what I was thinking too but wasn't quite sure.
I was thinking open...like in heat maybe though...bleeding.
yaks as least do not "bleed" when in season like dogs/bitches do, They also "cycle" every 28 days usually in late summer, fall and late fall. At least according to the info I have read a Yak cow has a 3 hour window to conceive if not it's another 28 days IF you are lucky.