200 Using canopy structure to improve interpretation of animal responses in pasture-based research.

Abstract In forage-based animal research, responses are often evaluated under grazing or fresh-cut conditions without detailed characterization of pasture structure. However, variation in canopy architecture, shaped by defoliation management, can influence both forage yield and composition. When defoliation strategy and the resulting pasture structure are not clearly defined or controlled, it may become difficult to interpret treatment effects associated with a forage diet. To establish a clear conceptual approach, we conducted a greenhouse experiment from August 2022 to August 2023. The study employed a completely randomized 2 × 5 factorial design with three replicates, evaluating the effects of two defoliation targets based on canopy light interception (~95% and ~100%) on five functionally diverse forage grasses: tall fescue, orchardgrass, Kentucky bluegrass, big bluestem, and indiangrass. Light interception was measured regularly using a LICOR quantum sensor, and defoliation was applied when target thresholds were reached. Data from February to August 2023 were used in this analysis to ensure that plants had structurally adapted to the defoliation regimen. For each regrowth cycle, canopy height was recorded, and forage was harvested. Tiller population density was assessed monthly. Tiller weight and green forage mass were determined at the final harvest cycle. Agronomic traits included leaf, stem, and dead biomass yield. Despite differences in morphology and growth strategy, all species showed consistent structural adaptations to defoliation. In general, the 95% light interception target generated shorter and denser canopies, with greater tiller population density (1813.8 vs. 1673.7 tillers m⁻²; P = 0.021) and lower tiller weight (0.234 vs. 0.320 g DM; P < 0.001) compared to the ~100% target. Canopy height varied by species, with a significant interaction between species and defoliation target (P = 0.009). The 95% treatment also resulted in higher leaf yield (1,725.7 vs. 1,440.5 g DM m⁻²; P = 0.002) and lower accumulation of stem (111.4 vs. 142.1 g DM m⁻²; P = 0.050) and dead material (13.1 vs. 42.8 g DM m⁻²; P < 0.001). Conversely, green mass was higher under the ~100% light interception treatment (409.5 vs. 305.7 g DM m⁻²; P = 0.009). This indicates that defoliation management, not species type, was the primary driver of pasture structure and forage yield. Because canopy structure directly influences the amount and type of harvested biomass, variation in defoliation strategy can lead to meaningful changes in the forage offered to animals. Thus, it becomes difficult to attribute animal responses to the intended dietary treatment when such structural conditions are not defined or controlled. These findings emphasize the importance of characterizing pasture structure in forage-based studies and offer a conceptual framework for incorporating structure-based management into animal science research.