Is zakat obligatory on animal feed made from the remnants of grain stalks (straw), especially since its sale is for a not insignificant price?

Zakat is not obligatory on straw because it is not among the categories subject to Zakat. However, Zakat becomes obligatory on its price if it reaches the minimum threshold (nisab) either on its own or when combined with money or trade goods, and a full year (hawl) has passed over it. In this case, it is subject to Zakat on money (a quarter of a tenth).