The impact of plant cell wall lignin on energy utilization in lactating Jersey cows.

Fiber from forages comprises a significant proportion of dairy cattle rations and by extension, it contributes largely to the energy concentration of the diet. While the proportion of fiber in the diet is important, the composition of fiber is probably more important as the different constituents of this fraction vary in their contributions to the nutritive value of the feed. Lignin has been described as an important factor limiting the digestion of NDF, reducing intake, and compromising milk production. Although lignin’s effects on these responses have been well characterized, the literature lacks data on the use of indirect calorimetry to evaluate the dietary lignin concentration. Therefore, there is a need to evaluate the energetics of lignin, and its relationship with the energy concentration of the entire NDF fraction. The first experiment used 16 NDF residues from individual feeds or mixed rations to analytically determine the GE concentration of feed NDF. This value was compared to that of fecal NDF, which was analytically determined from 34 fecal NDF residues. The GE concentration of feed NDF was found to be lower than that used in the Dairy NASEM (2021) model’s equations used to calculate dietary gross and digestible energy concentrations. If the observed NDF GE concentration is representative of the true GE concentration of NDF, this result suggests that the Dairy NASEM (2021) model is overpredicting the energetic contribution of NDF. Additionally, this study reports that feed NDF is of a greater energy concentration relative to fecal NDF. This result suggests that nutritional models likely do not capture the full scope of NDF digestibility in their predictions of energy utilization. Lignin’s impact on utilization of energy and nitrogen was examined using twelve multiparous lactating Jersey cows in a two period crossover design. Diets were formulated so to be equal in NDF concentration but differing in their NDF profiles. The LoLig diet contained 32.5% NDF (% DM) and 9.59% lignin (% NDF) while the HiLig diet contained 31.0% NDF (% DM) and 13.3% lignin (% NDF). Interestingly, increasing the concentration of lignin not only decreased the digestibility of NDF, but also CP and starch, likely due to decreased fermentability by ruminal microbes. The effects of reduced digestibility carried through to metabolizable energy concentration but not net energy concentration, likely due to an underpowered experiment or cumulative error associated with calculating net energy. Increasing the concentration of dietary lignin shifted nitrogen excretion from the urine to the feces, which is considered to be better for the environment. Feeding the HiLig diet resulted in lower yields of milk, fat, and protein, suggesting that the impacts of increasing dietary lignin concentration might impact more factors than NDF digestibility. Advisor: Paul J. Kononof