Overall, mean Comeau's Competition Index (CCI) was lower for grazed plots when compared to the control (ungrazed) plots (Fig 1). This relationship was observed in all trials, though in Grande Prairie in 2023 the difference was less prominent, while in 2024 for both sites, mean CCI of control plots was more than double that of the grazed treatment. Broken down by vegetation group, the relationship becomes more complex, with some vegetation groups showing little to no difference between grazed and ungrazed treatments (Fig 2). While sheep grazing consistently decreased forb and grass CCI, the impact on shrubs and deciduous trees was less notable. This aligns with what we know about sheep grazing habits, as sheep are unlikely to exert effort to reach higher branches or vegetation, and prefer to graze lower to the ground or around head height.

Careful flock management is essential to the success of sheep grazing for vegetation management. Even so, some seedlings will inevitably be damaged. Though browsing by sheep is sometimes presented as a concern, no evidence of browsing was observed on white spruce or lodgepole pine seedlings in this study. Damage to some seedlings did occur from trampling, though only a few seedlings were trampled fully to the ground. The percent of seedlings trampled to any degree, including slight lean but not fully down, is presented in Table 2. Overall, 4.53% of seedlings observed were trampled by sheep throughout the study.

The results of this study indicate that sheep grazing effectively reduces competition and vegetation biomass in regenerating forests in the short term. Grazed plots had consistently lower mean competition (CCI) compared to control (ungrazed) plots, particularly for forb and grass competition, though the effect on shrubs and deciduous trees was not as reliable. Blocks with higher initial competition (competition in ungrazed plots) tended to retain higher competition after grazing, suggesting that while grazing is effective, it does not entirely eliminate competition. On average, grazing reduced competition by an average of 43.9% with 95% certainty (p < 0.001).

Biomass data followed a similar trend; mean biomass was significantly lower for all grazed plots than control plots, and grazing reduced biomass by an average of 51.9% with 95% certainty (p < 0.001). Variability in mean biomass across blocks is likely due to differences in the amount of initial vegetation present and sheep grazing behavior, as sheep tend to graze at ground and head height, avoiding higher vegetation. The less notable difference in 2023 may result from a later grazing start due to delays from wildfires, and vegetation being less palatable to sheep in late summer. While 4.53% of seedlings were damaged by trampling, no browsing occurred on white spruce or lodgepole pine, and most affected seedlings were only partially damaged rather than completely trampled. Seedling damage of less than 5% is considered within an acceptable range (Newsome et al. 1995).

While operational challenges, costs, and initial site conditions may constrain the scalability of sheep grazing, it does present an important potential alternative that may be suitable for areas where herbicide use is undesirable or stakeholder collaboration is of higher value. This might include areas close to human settlements and Indigenous communities, or areas of cultural importance. Grazing is more expensive than aerially-applied herbicides, however, the average cost is less than that of manual removal of vegetation (Newsome et al. 1995). Understanding the effectiveness of grazing and how results vary across conditions provides an important baseline for how this method works and where it may be most effective.

Future research is needed to capture the effects of sheep grazing on tree growth in the long term. This research is part of a longer-term project that plans to monitor tree response over time. Continued work on this project will also evaluate the impact of grazing on ignition potential in regenerating cutblocks, as well as incorporate laser scanning and drone imagery to further assess grazing effects.