Timing and Frequency of Goat Grazing Affect Invasive Species Cover and Fuel Loads in Southern California1

Goat grazing has increasingly become an attractive solution to invasive plant removal and wildfire fuel reduction in Southern California. Despite the potential advantages of grazing when compared with other removal methods, little is known regarding its effectiveness in this region, particularly in forb-dominated landscapes. We aimed to determine how the timing and frequency of summer goat grazing affect three focal invasive plant species: Brassica nigra, Bromus diandrus, and Silybum marianum, all of which are common in Southern California as well as our study site near Whittier, CA. We found that grazing in both the early and late summer during the first year reduced the cover of our three focal species by at least 86% on average. This reduction did not carry over into the second year, likely due to heavy winter precipitation after the first year of grazing, but a second grazing period resulted in lower total invasive cover compared with the treatment that was only grazed in the first year, which appears to be driven by a significant decrease in cover of rip-gut brome (Bromus diandrus). We did not find strong evidence of grazing decreasing fine fuel loads, although black mustard (Brassica nigra) biomass was significantly decreased (87% on average) for the early and late summer grazing periods in the first year. These results highlight the importance of long-term grazing studies that consider seasonal differences between years as well as how timing, intensity, and frequency of grazing affect invasive removal.