Impact of Grazing Management on Ruminant Nutrient Intake and Emissions in Temperate Pastures

Thesis Abstract

Grazing management in temperate pastures influences ruminant nutrient intake and greenhouse gas emissions, yet empirical evidence linking adaptive grazing strategies to both animal nutrition and environmental performance remains fragmented. This study addresses the gap by evaluating how rotational, continuous, and adaptive grazing regimes affect nutrient intake, forage quality, rumen fermentation indicators, and methane and nitrous oxide emissions under real-farm conditions. The aim is to quantify the trade-offs and synergies between optimized nutrition and emission reductions to inform sustainable pasture-based systems. Specific objectives are (1) to compare forage dry matter intake (DMI), crude protein (CP), neutral detergent fiber (NDF), and energy availability across grazing treatments; (2) to assess ruminal metabolite profiles (volatile fatty acids, ammonia) and body condition scores as indicators of animal performance; (3) to measure enteric methane and soil/animal-driven nitrous oxide emissions using close-interval measurements and static chamber methods; (4) to identify relationships between grazing management, sward botanical composition, and emission intensities (g CH4 per kg DMI, g CH4 per kg gain, and g N2O per kg N intake); and (5) to develop a parsimonious predictive model linking grazing structure to both nutrient intake and emissions. The research adopts a longitudinal, field-based experimental design conducted across three temperate commercial grazing farms over two complete pasture growth cycles. A randomized complete block design assigns treatments—rotational grazing (RG), continuous grazing (CG), and adaptive strip-grazing (ASG)—to each farm, with n=40 lactating dairy cows per farm evenly distributed among treatments, yielding a total sample size of 120 animals. Data collection integrates forage analysis, animal nutrition metrics, and emissions measurements forage samples collected biweekly for crude protein, neutral detergent fiber, digestibility, and minerals; daily individual milk yield and body condition scoring; biweekly rumen fluid sampling for volatile fatty acids and ammonia; methane emissions measured monthly using the sulphur hexafluoride (SF6) tracer technique complemented by spot headspace gas analysis, and soil/animal nitrous oxide emissions quantified through static chamber measurements and micrometeorological data; and animal performance traits including weight gain and reproductive status. Validity and reliability are ensured via calibration of forage NDF/CP with near-infrared spectroscopy, standardized SF6 protocols, replicate chamber measurements, and inter-laboratory validation for rumen metabolite assays. Data will be analyzed using mixed-effects models to account for hierarchical farm-by-treatment-by-time structure, with fixed effects for grazing regime, forage quality, and interaction terms, and random effects for animal ID and farm. Regression analyses will quantify the relationships between nutrient intake variables and emission outcomes, while ANOVA will compare means across grazing systems. Mediation analysis will test whether forage quality mediates the effect of grazing management on emissions. A theoretical framework integrating optimal foraging theory with the Eco-Physiological Emissions Model will guide interpretation, and the study will test hypotheses derived from the Environmental Kuznets Curve concept as applied to pasture-based systems. Expected findings anticipate that RG and ASG will improve forage quality and DMI relative to CG, while reducing methane intensity (g CH4 per kg DM intake) and nitrous oxide fluxes through improved pasture utilization and sward diversity. However, total annual emissions may differ due to variations in stocking density and grazing duration, necessitating a nuanced interpretation of emission intensity versus total emissions. The study contributes to knowledge by providing robust, field-based estimates of how grazing management modulates nutrient intake and emissions in temperate pastures, offering validated predictive models for decision-support tools and informing policy-relevant recommendations for emission reporting and pasture-based nutrition strategies. The main conclusion is that strategically designed grazing management can reconcile high-quality nutrient intake with lower emission intensities without compromising animal performance, with adaptive strip-grazing presenting a particularly favorable balance. Recommendations include adopting dynamic grazing plans informed by forage quality targets, integrating emission monitoring into routine farm management, and pursuing further research on sward species diversification to enhance both animal nutrition and environmental outcomes.