The Relationships Between the Ruminal Digestibility Kinetics of Fiber, Total-Tract Nutrient Digestibility, and Methane Emissions from Lactating Dairy Cattle

Abstract

The efficiency of fiber utilization in dairy cattle plays a central role in optimizing production performance while minimizing environmental impacts, particularly enteric methane (CH4) emissions. Ruminal degradation and passage kinetics of fiber fractions, potentially degradable (pdNDF) and undegraded (uNDF) neutral detergent fiber (NDF), are influenced by both forage quality and dietary composition. This dissertation examined how forage type, maturity, and dietary inclusion level affect nutrient digestibility, fiber passage dynamics, milk production, and CH4 emissions in high-producing dairy cows. Chapter 2 evaluated the effects of alfalfa hay and orchardgrass hay on ruminal NDF degradation and passage kinetics. Diets were formulated to provide similar total NDF concentrations but differed in uNDF, with alfalfa contributing more uNDF relative to its total NDF content. Cows consuming alfalfa exhibited faster ruminal passage rate (Kp) and smaller ruminal pools of dry matter (DM), NDF, and pdNDF. Despite faster passage, cows fed alfalfa consumed more DM and uNDF and exhibited greater total-tract digestibilities of NDF and pdNDF. However, milk production and components were not affected by treatment. These findings suggest that the kinetics of ruminal degradation and passage influence NDF degradation in ways beyond uNDF concentration or forage quality. Chapters 3 and 4 evaluated the influence of triticale silage maturity [boot (BT) vs. soft-dough (SFT) stage] on production performance, nutrient digestibility, ruminal volatile fatty acid (VFA) profile, enteric CH4 emissions, and fecal biomethane potential. Chapter 3 implemented a 2×2 factorial arrangement of treatments with high-forage (HF; 52% forage) or low-forage (LF; 37% forage) diets containing BT or SFT silages. Forage maturity did not affect DM intake (DMI) in LF diets, but cows fed HFBT consumed more DM than those fed HFSFT. While energy-corrected milk (ECM) production and feed efficiency did not differ by silage maturity, cows fed LF diets produced more ECM than those fed HF diets, though at the expense of digestibility. Cows consuming BT silage exhibited improved total-tract digestibilities of DM and NDF compared to SFT. Cows consuming HF diets also exhibited greater digestibilities of DM, NDF, and starch compared to LF. Cows consuming BT diets produced 23 g/d less enteric CH4 than SFT, with no difference between HF and LF diets in absolute enteric CH4 output. Cows consuming BT silage had lower enteric CH4 yields on a DMI basis and DM-digested basis than SFT. Thus, harvesting triticale at the BT stage can enhance digestibility and milk yield while reducing enteric CH4 intensity when fed to lactating dairy cattle, whereas LF diets boost milk yield and lower enteric CH4 yield but compromise NDF digestibility. Chapter 4 further explored these effects using diets composed of either BT or SFT triticale silage, with additional evaluation of ruminal VFA profiles, fiber passage kinetics, and fecal biomethane potential. Although dietary Kp did not differ, marker-based estimates showed a faster Kp for the individual BT silage compared to the SFT silage. Cows fed BT silage had greater total VFA concentrations and a lower acetate-to-propionate ratio, reflecting more active fermentation. These cows also emitted less enteric CH4 and exhibited reduced enteric CH4 intensity. However, greater pdNDF intake in the BT diet resulted in greater fecal pdNDF output. While fecal biomethane potential did not statistically differ, cows consuming BT diets were estimated to emit 18 L/d more CH4 from their fecal material, suggesting a potential trade-off between enteric and manure-derived CH4 emissions. Collectively, these studies highlight the complex trade-offs between forage quality, animal performance, and environmental outcomes. These findings challenge the assumption that faster ruminal Kp uniformly reduces enteric CH4 emissions by limiting fermentation and emphasize the importance of integrated assessments of digestion kinetics, animal performance, and environmental outcomes in dairy nutrition.