Does Zakat change with the change in livestock expenses, or does it remain fixed?

Zakat (definition) is not obligatory on livestock unless it is grazing (feeding on permitted pasture) and kept for milk and offspring. If it is for حمل (carrying loads) and riding, or if it is fed (not grazing), then there is no Zakat on it, unless it is intended for trade, in which case Zakat al-Tijara (trade Zakat) becomes obligatory on it by assessing its value and paying one-fourth of one-tenth (2.5%) of its value.