From bite to system : How grazing management shapes dairy production

The overarching objective of this thesis was to elucidate how post-grazing sward height modulates the contribution of grazed forage to dairy cow diets, the associated grazing behaviour strategies, and the downstream consequences for intake, milk production, methane emissions, and system performance at both animal and land-use scales. In addition, the thesis evaluated whether the infrastructure used for supplementation in mixed feeding systems influences productive and economic outcomes. Chapters 2 and 4 examined how dairy cows behaviour respond to contrasting post-grazing sward heights when grazing tall fescue pastures. Chapter 2 focused on unsupplemented, late-lactation cows with low milk yield, whereas Chapter 4 addressed supplemented cows producing at medium to high levels across early and mid-lactation. Despite these contrasts in physiological state and supplementation level, the responses to residual sward height were remarkably consistent. Daily grazing time was primarily driven by supplementation level and pasture access time, and only marginally affected by residual height. In contrast, the structure of grazing activity within the day differed: cows grazing taller residuals adopted a pattern of shorter but more frequent meals, whereas cows grazing shorter residuals compensated with fewer but longer meals. At the spatial scale, cows managed on taller residuals exhibited a distinct “rapid sampling” strategy, characterized by higher feeding station turnover and fewer bites per station. This strategy enabled higher grazed forage intake, reflecting both improved sward structure and enhanced selectivity for higher-quality forage. Across production levels, these behavioural adjustments translated into higher milk yield and improved body condition when taller post-grazing sward heights were maintained. Chapter 3 expanded the inference from short-term responses to a near full-lactation perspective, integrating animal performance with pasture dynamics over 215 days. Although cows managed under taller residuals produced more milk per cow and recovered body condition earlier, these individual-level gains did not fully offset the lower stocking rate associated with laxer grazing. As a result, milk production per hectare tended to be lower. Nevertheless, grazed forage intake per hectare remained similar between treatments, while supplementary feed input per hectare was reduced under taller residual management, leading to comparable income over feed costs at the land-use scale. Chapter 5 demonstrated that higher post-grazing sward height improved whole-animal energy utilization and reduced methane emission intensity without increasing absolute methane production. Greater intake of higher-quality forage enhanced feed efficiency and diluted maintenance energy requirements, allowing a larger proportion of metabolizable energy to be retained as milk and body tissue. Although higher dietary crude protein increased nitrogen losses, cumulative methane emissions per hectare were lower under taller residual management. Finally, Chapter 6 addressed the role of supplementation infrastructure. While fully confined systems achieved the highest milk yields, differences between compost-barn and outdoor soil-bedded systems within mixed feeding were modest. Environmental exposure did not consistently affect cow performance, although compost barns offered episodic advantages under heat stress. Overall, this thesis shows that integrating grazing management, supplementation, and infrastructure design can enhance nutrient capture and use efficiency, reduce methane intensity, and sustain competitive milk yields.