Effect of partitioning the nonfiber carbohydrate fraction and neutral detergent fiber method on digestibility of carbohydrates by dairy cows

Abstract

Many nutrition models rely on summative equations to estimate feed and diet energy concentrations. These models partition feed into nutrient fractions and multiply the fractions by their estimated true digestibility, and the digestible mass provided by each fraction is then summed and converted to an energy value. Nonfiber carbohydrate (NFC) is used in many models. Although it behaves as a nutritionally uniform fraction, it is a heterogeneous mixture of components. To reduce the heterogeneity, we partitioned NFC into starch and residual organic matter (ROM), which is calculated as 100 - CP - LCFA - ash - starch - NDF, where crude protein (CP), long-chain fatty acids (LCFA), ash, starch, and neutral detergent fiber (NDF) are a percentage of DM. However, the true digestibility of ROM is unknown, and because NDF is contaminated with both ash and CP, those components are subtracted twice. The effect of ash and CP contamination of NDF on in vivo digestibility of NDF and ROM was evaluated using data from 2 total-collection digestibility experiments using lactating dairy cows. Digestibility of NDF was greater when it was corrected for ash and CP than without correction. Conversely, ROM apparent digestibility decreased when NDF was corrected for contamination. Although correcting for contamination statistically increased NDF digestibility, the effect was small; the average increase was 3.4%. The decrease in ROM digestibility was 7.4%. True digestibility of ROM is needed to incorporate ROM into summative equations. Data from multiple digestibility experiments (38 diets) using dairy cows were collated, and ROM concentrations were regressed on concentration of digestible ROM (ROM was calculated without adjusting for ash and CP contamination). The estimated true digestibility coefficient of ROM was 0.96 (SE = 0.021), and metabolic fecal ROM was 3.43 g/100 g of dry matter intake (SE = 0.30). Using a smaller data set (7 diets), estimated true digestibility of ROM when calculated using NDF corrected for ash and CP contamination was 0.87 (SE = 0.025), and metabolic fecal ROM was 3.76 g/100 g (SE = 0.60). Regardless of NDF method, ROM exhibited nutritional uniformity. The ROM fraction also had lower errors associated with the estimated true digestibility and its metabolic fecal fraction than did NFC. Therefore, ROM may result in more accurate estimates of available energy if integrated into models.

Estimating digestible energy values of feeds and diets and integrating those values into net energy systems

“…Over time, some weaknesses in the original equation were noted ( White et al, 2017 ), and methods to account for additional sources of variation were suggested ( Tebbe et al, 2017 ). Improvements to the discount equation used in NRC (2001) , which used diet TDN as a proxy for starch, have also been suggested ( de Souza et al, 2018 ).…”

“…Therefore, Tebbe et al (2017) partitioned NFC into starch and residual organic matter (ROM; ROM = NFC − starch, where concentrations are as percentages of dry matter) so that additional sources of variation could be included in the equation. Based on the Lucas test ( Tebbe et al, 2017 ), NFC, starch, and ROM behaved as nutritionally uniform fractions ( Figure 4 ); however, the overall fits were better and standard errors of the intercept coefficients (i.e., estimated metabolic fecal content) were decreased for ROM and starch compared with NFC. Using average true digestibility coefficients for ROM and starch should result in greater accuracy in estimating DE than using the average true digestibility of NFC.…”

“… Lucas tests for NFC (solid line, red triangles), starch (long dashed line, blue squares), and residual organic matter [(ROM) short dashed line, black circles] in lactating cows (data from Tebbe et al (2017) ): digestible NFC = −2.08 (±0.800) + 0.903 (±0.020) × NFC intake [Bayesian information criterion (BIC) = 806]; digestible starch = −0.17 (±0.359) + 0.931 (±0.013) × starch intake (BIC = 578); digestible ROM = −3.43 (±0.295) + 0.961 (±0.295) × ROM intake (BIC = 688). The slopes (±SEM) are estimates of the true digestibility and intercepts (±SEM) are estimates of metabolic fecal content.…”

“…In the modified equation, endogenous fat was set at 0 as discussed above and endogenous fecal ROM energy was set at 0.137 Mcal/kg. This was calculated by multiplying endogenous ROM ( Figure 4 ; Tebbe et al, 2017 ) by an assumed enthalpy of 4.0 Mcal/kg (i.e., 0.0343 kg/kg DM × 4.0 Mcal/kg = 0.137 Mcal/kg). Metabolic fecal CP remained the same as in Weiss et al (1992) with an assumed enthalpy of 5.65, yielding an energy value of 0.166 Mcal/kg (i.e., 0.0294 kg metabolic fecal CP/kg DM × 5.65 Mcal/kg = 0.166 Mcal/kg).…”

Effects of magnesium source and monensin on nutrient digestibility and mineral balance in lactating dairy cows

“…Concentrates and dried, ground samples of silages, feces, and refusals were assayed for DM (100°C for 24 h), ash (muffle oven at 600°C 2000). The NDI-ash and NDICP values were then used to calculate NDF as ash-free, CP-free, or both (Tebbe et al, 2017). Nonacidified urine, water, milk, and dried ground feed, refusal, and fecal samples were analyzed for minerals (OARDC STAR Laboratory, Wooster, OH) by inductively coupled plasma emission spectroscopy (Isaac and Johnson, 1985) after microwave digestion in nitric acid (Jones et al, 1991).…”