Evaluation of gross energy concentration of neutral detergent fiber contained in feed and fecal samples

Starch and NDF are usually assumed to contain the same concentration of gross energy (GE), but NDF is more variable in chemical composition and varies more in the extent of digestion. The variable chemical composition of NDF may have direct implications on dairy nutrition models that predict dietary GE and use this estimate for also predicting digestible energy. For example, when NDF is enriched in lignin and protein, the concentration of GE would increase, whereas NDF enriched in ash would have the opposite effect. Current nutritional models, such as the NASEM (2021) and CNCPS (6.55), assume a GE coefficient of 4.20 Mcal/kg for NDF. This study aimed to determine the heat of combustion of NDF and to consider if it is a contributing factor to the variance in digestible energy. To do so, NDF residues were isolated from 9 feed and 8 fecal samples and then combusted. Approximately 0.20 g of NDF residues from 16 feeds (corn silage, n = 2; grass hay, n = 2; alfalfa hay, n = 2; wheat straw, n = 1; cottonseed hulls, n = 1; soyhulls, n = 1; distillers dried grains with solubles, n = 1; and total mixed ration, n = 6) and 34 fecal samples were collected. A bomb calorimeter (Parr 6400 Calorimeter, Parr Instrument Company) was used to determine concentration of GE in each NDF residue sample. The GE concentration of feed NDF was observed to be 4.03 ± 0.245 Mcal/kg, which was similar to that of fecal NDF (3.94 ± 0.245 Mcal/kg). The lack of difference between feed and fecal NDF GE implies that digested NDF is of a similar GE concentration as total feed NDF and that current nutritional models are validated in their current approach in predicting digestible energy from NDF. However, our observed estimate of GE in NDF is lower than what is assumed and across feed types varied from 3.85 to 4.19 Mcal/kg.