Predicting omasal flow of nonammonia N and milk protein yield from in vitro-determined utilizable crude protein at the duodenum

Abstract

This study evaluated the relationship between utilizable crude protein (uCP) at the duodenum estimated in vitro and omasal flow of crude protein (CP; omasal flow of nonammonia N × 6.25) measured in lactating dairy cows. In vivo data were obtained from previous studies estimating omasal digesta flow using a triple-marker method and N as microbial marker. A total of 34 different diets based on grass and red clover silages were incubated with buffered rumen fluid previously preincubated with carbohydrates for 3 h. The buffer solution was modified to contain 38 g of NaHCO and 1 g of (NH)HCO in 1,000 mL of distilled water. Continuous sampling of the liquid phase for determination of ammonia-N was performed at 0.5, 4, 8, 12, 24, and 30 h after the start of incubation. The ammonia N concentrations after incubation were used to calculate uCP. The natural logarithm of uCP [g/kg of dry matter (DM)] at time points 0.5, 4, 8, 12, 24, and 30 h of incubation was plotted against time to estimate the concentration of uCP (g/kg of DM) at time points 16, 20, and 24 h using an exponential function. Fixed model regression analysis and mixed model regression analysis with random study effect were used to evaluate the relationships between predicted uCP (supply and concentration) and observed omasal CP flow and milk protein yield. Residual analysis was also conducted to evaluate whether any dietary factors influenced the relationships. The in vitro uCP method ranked the diets accurately in terms of total omasal CP flow (kg/d) or omasal CP flow per kilogram of DM intake. We also noted a close relationship between estimated uCP supply and adjusted omasal CP flow, as demonstrated by a coefficient of determination of 0.87, although the slope of 0.77 indicated that estimated uCP supply (kg/d) was greater than the value determined in vivo. The linear bias with mixed model analysis indicated that uCP supply overestimated the difference in omasal CP flow between the diets within a study, an error most likely related to study differences in feed intake, animals, and methodology. Predicting milk protein yield from uCP supply showed a positive relationship using a mixed model (coefficient of determination = 0.79), and we observed no difference in model fit between the time points of incubation (16, 20, or 24 h). The results of this study indicate that the in vitro method can be a useful tool in evaluating protein value of ruminant diets.

Evaluating the Protein Value of Fresh Tropical Forage Grasses and Forage Legumes Using In Vitro and Chemical Fractionation Methods

“…It was expected that the in vitro method poorly predicts the PRCP supply from tropical forages as a result of its low accuracy and precision caused by the overestimation of NH 3 -N release of the blank and the underestimation of NH 3 -N release from the feed sample. The NH 3 -N release from the inoculum is overestimated because microbial lysis is greater in the blank than those syringes containing feed substrate due to a lack of fermentable substrates [8]. It is also possible that NH 3 -N release of the feed sample in an in vitro system is underestimated because NH 3 -N release and NH 3 -N uptake by microorganisms occur simultaneously [8] with a higher rate of uptake than release in the early stage of incubation [49].…”

“…The PRCP supply at Kp of 2, 5, and 8%/h was obtained by plotting the log of the time of incubation (i.e., ln (8) and ln( 48)) against PRCP supply after 8 and 48 h in vitro incubation, respectively. From the resulting non-linear regression equation, the intercept and slope were obtained.…”

“…Forages grown in tropical regions differ in their chemical composition [9] and are characterized by a slower rate and lower extent of carbohydrate and CP degradation in the rumen than forages grown in temperate regions [10], which may hamper the estimation of PRCP supply with the in vitro and chemical methods. In the present study, it was therefore hypothesized that accuracy and precision of the in vitro method in estimating the PRCP supply from tropical forages might be poor due to early microbial lysis in the blank and higher rate of ammonium-nitrogen (NH 3 -N) uptake than release in the early stage of incubation of the feed samples [8], the latter being more pronounced in tropical than in temperate forages, because of their slow rate and low extent of carbohydrate and CP degradation in the rumen [10].…”

“…The PRCP supply to the small intestine has been studied for temperate ruminant feedstuffs using in vivo and in situ methods; however, these methods are costly, timeconsuming, require ruminally and duodenally fistulated animals, and thus compromise animal welfare [2,3], rendering these methods unsuitable for routine evaluation of ruminant feedstuffs in tropical husbandry systems. Alternative methods such as the in vitro method developed by Steingaβ et al [4] and the chemical method proposed by Zhao and Cao [5] have been tested in a wide range of temperate ruminant feeds; however, the adequacy (i.e., accuracy and precision; [3,[6][7][8]) of these methods to predict the PRCP supply of common feedstuffs used in tropical husbandry systems is still questioned.…”

Metabolizable Protein: 1. Predicting Equations to Estimate Microbial Crude Protein Synthesis in Small Ruminants

“…Therefore, main feeding systems preconize models to estimate MCP based on energy compounds as digestible OM (dOM), metabolizable energy (ME), or TDN (7)(8)(9)(10)(11)(12)(13), once these models could be used to calculate rumen degradable protein (RDP) requirements from metabolizable protein (MP) requirements. Moreover, the general importance of the estimation of microbial protein for ruminants must be considered, as it accounts for between 50 and 90% of the protein entering the duodenum and provides most of the amino acids needed for growth and milk protein synthesis (14,15).…”