Ruminal protein degradation and protein value of feeds

In situ and in vitro determination of the protein value of feeds for ruminants

The objective of this study was to estimate the amount of post-rumen crude protein (prCP), a precursor to metabolisable protein for ruminants, of feeds and feed mixtures using an in situ and in vitro method, and to contrast the results of both approaches. For this, 34 samples were examined: 9 feeds, 13 feed mixtures made thereof and 12 feed mixtures provided by commercial dairy farms. As a minor aspect, additivity of protein values from feeds was evaluated by comparing measured and calculated values of feed mixtures. Effective prCP was calculated for assumed rumen passage rates (k) of 0.05 h^-1 (prCP₅) and 0.08 h^-1 (prCP₈) by in situ measurements of rumen undegraded CP and fermented organic matter (FOM) assuming an efficiency of 181 g microbial CP/kg FOM. Additionally, effective prCP in vitro was estimated using the modified Hohenheim gas test (modHGT) through incubation in rumen-fluid buffer solution for 8 and 24 h followed by ammonia distillation. In vitro estimations were highly correlated with in situ values for both passage rates (k = 0.05 h^-1: p < 0.001, R² = 0.68; k = 0.08 h^-1: p < 0.001, R² = 0.76). The in vitro method yielded higher values for effective prCP than the in situ approach with 29 g/kg OM (k = 0.05 h^-1) and 37 g/kg OM (k = 0.08 h^-1) on average for all samples. Small positive associative effects - reflecting non-additivity - were found, averaging at 2.2 g prCP₈/kg OM (p < 0.05) for the in situ and 10.7 g prCP₈/kg OM (p < 0.001) for the in vitro approach. Due to the need of an assumption of a certain value for microbial efficiency in situ, effective prCP might be more accurately estimated in vitro, accounting for nutrient-specific efficiencies as well as interactions between carbohydrate and protein degradation by rumen microbes. Furthermore, the modHGT highlighted associative effects more pronounced and seems suitable as a routine method due to the comparably low effort and high sample throughput. The potential of the modHGT to determine the protein value of feeds could be demonstrated by our study.

Calculation of utilizable crude protein at the duodenum of cattle by two different approaches

The duodenal flow of utilizable crude protein (crude protein minus endogenous protein) in cows was estimated using dietary parameters, first by multiple regression and secondly by the addition of microbial protein and undegraded feed protein. These estimates were compared with 327 results from experiments conducted with fistulated cows in Braunschweig-Völkenrode and Rostock-Dummerstorf. The regressions and the measurements for microbial protein synthesis as well as feed protein degradation and organic matter fermentation in the rumen were based on the same experimental data set. The prediction of utilizable crude protein (uCP) at the duodenum by regression with digested organic matter (kg DOM) and undegraded feed protein (g UDP) as predicting variables, was more accurate than the value given by microbial protein synthesis and rumen protein degradability. The regression model [g uCP = [188.5-(116.5 (UDP/CP))] DOM + 1.03 UDP] had the highest coefficient of determination (r2 = 0.91) and the lowest coefficient of variation (cv = 8.6); indicating the model's superiority over the other method of estimation.

Some environmental effects on relationships between in sacco degradability of protein and dry matter and chemical composition of Italian ryegrass

The environmental effects on chemical composition and the rate and extent of crude protein (CP) and dry matter (DM) degradation of Italian ryegrass (Lolium multiflorum Lam.) in the rumen were studied. More intensive nitrogen (N) fertilizing (40, 80 and 140 kg N/ha) increased the content of the water, CP, true proteins and the effective degradability of CP (P < 0.05). Especially in the spring harvest such fertilizing increased the content of acid detergent fibre (ADF), neutral detergent fibre (NDF), cellulose and crude fibre (CF) as well as the rate of DM degradation (P < 0.05) and caused lower DM solubility while the content of hemicellulose was not affected. The most rapid changes due to grass ageing occurred in springtime. The content of DM, ADF, NDF, cellulose, hemicellulose and CF increased with maturity while the degradation rate and effective DM degradability decreased. The season and the trial location influenced the chemical composition of grass and the degradability of both DM and CP (P < 0.05). The closest multiple regression (the lowest RSD) was between the effective protein degradability and CP, NDF, ADF and and acid detergent lignin (ADL) (R2 = 0.97). Relatively good estimation of effective protein degradability was obtained using an exponential regression equation on the basis of CP content (R2 = 0.72), NDF content (R2 = 0.67) or effective DM degradability (R2 = 0.71) while the effective DM degradability may be estimated on the basis of DM solubility (R2 = 0.94), CF content (R2 = 0.93), ADF content (R2 = 0.94), ADL content (R2 = 0.85) or NDF content (R2 = 0.96). When estimating the effective CP and DM degradability from chemical composition, the error that occurs is most evidently influenced by N fertilizing and by time of harvest.