Serrated tussock (Nassella trichotoma) is a perennial grass that causes both environmental and economic losses in several parts of the world. This study examines chemical composition changes in N. trichotoma under defoliation. Results showed a rise in fibre that may indicate an anti-herbivore defence, reducing the plant’s nutritional quality, leading to cattle avoidance. These findings highlight the importance of considering defoliation-induced defences in integrated weed management strategies for controlling N. trichotoma.