Yes, the gains from grazing livestock during the year are added to the threshold of their kind; because the obligation of zakat is considered on the gains with the year that has passed on the original amount. So, if someone has two hundred dirhams and a year has passed on them, and in the middle of the year he has gained one hundred dirhams, he adds the one hundred to the two hundred. See: Fath Bab al-Ianaya 1: 505.