Influence of Feeding Level on Digestion and Protein Passage to the Duodenum in Cows Fed High Concentrate Diets

Predictions of ruminal outflow of essential amino acids in dairy cattle

The objective of this work was to update and evaluate predictions of essential AA (EAA) outflows from the rumen. The model was constructed based on previously derived equations for rumen-undegradable (RUP), microbial (MiCP), and endogenous (EndCP) protein outflows from the rumen, and revised estimates of ingredient composition and EAA composition of the protein fractions. Corrections were adopted to account for incomplete recovery of EAA during 24-h acid hydrolysis. The predicted ruminal protein and EAA outflows were evaluated against a data set of observed values from the literature. Initial evaluations indicated a minor mean bias for non-ammonia, non-microbial nitrogen flow ([RUP + EndCP]/6.25) of 16 g of N per day. Root mean squared errors (RMSE) of EAA predictions ranged from 26.8 to 40.6% of observed mean values. Concordance correlation coefficients (CCC) of EAA predictions ranged from 0.34 to 0.55. Except for Leu, all ruminal EAA outflows were overpredicted by 3.0 to 32 g/d. In addition, small but significant slope biases were present for Arg [2.2% mean squared error (MSE)] and Lys (3.2% MSE). The overpredictions may suggest that the mean recovery of AA from acid hydrolysis across laboratories was less than estimates encompassed in the recovery factors. To test this hypothesis, several regression approaches were undertaken to identify potential causes of the bias. These included regressions of (1) residual errors for predicted EAA flows on each of the 3 protein-driven EA flows, (2) observed EAA flows on each protein-driven EAA flow, including an intercept, (3) observed EAA flows on the protein-driven EAA flows, excluding an intercept term, and (4) observed EAA flows on RUP and MiCP. However, these equations were deemed unsatisfactory for bias adjustment, as they generated biologically unfeasible predictions for some entities. Future work should focus on identifying the cause of the observed prediction bias.

Dietary energy losses of cattle influenced by body type, size, sex and intake

Digestibility (DE) and metabolizability (ME) of a single pelleted diet was examined with growing Angus and Holstein cattle. One hundred and ninety-one individual digestion trials were conducted with bulls, steers and heifers. Food was offered ad libitum or restricted with a daily meal frequency of four. Data were analysed by two covariate models. The first included relative dry matter intake (RDMI) defined as the ratio of DMI to body weight (M0·73). In the second model, covariates were defined by genetic-size scaling; the first covariate scaled DMI by mature weight (A) raised to the power 0·73 (DMI/A0·73) and the second covariate was degree of maturity (μ) defined as body weight divided by A. Model 2 also allowed separation of size and DMI effects. No breed differences were detected. There was a consistent trend for increasing ME and ME/DE from heifers to steers to bulls which was significant between heifers and the average of bulls and steers (575·3 v. 589·3 and 832·8 v. 844·3 J/kJ for ME and ME/DE, respectively). Independent of animal effects, DE, ME and urinary energy (UE) decreased and ME/DE and faecal energy (FE) losses increased as RDMI or DMI/A0·73 increased. Per unit rise in RDMI equivalent to maintenance, the corresponding changes for DE, ME, ME/DE, FE and UE were –33·9, –13·6, 21·3, 34·1 and 9·6 J/kJ, respectively. DE increased 1·5 and ME increased 0·8 J/kJ GE for each 01 unit increase in degree of maturity.

Microbial protein synthesis and flows of nitrogen fractions to the duodenum of dairy cows

Attempts have been made to increase nutrient availability for milk production by increasing feed intake, optimizing ruminal fermentation, and supplementing nutrients to the diet that will escape ruminal degradation. Energy and N are the nutritional factors that most often limit microbial growth and milk production. Ruminal fermentation and flow of microbial and dietary protein to the small intestine are affected by feed intake and by the amount and source of energy and protein in the diet. Feeding protein and carbohydrate that are not degraded in the rumen increases the quantity of dietary protein that passes to the small intestine but may decrease the quantity of microbial protein that is synthesized in the rumen. This often results in only small differences in the total NAN that passes to the small intestine. Because microbial protein supplies a large quantity of total AA that passes to the small intestine, differences in passage of individual AA often are only slight. Additional research with cows consuming large amounts of feed are needed to identify combinations of feed ingredients that synchronize availabilities of energy and N for optimizing ruminal digestion, microbial protein synthesis, nutrient flow to the small intestine, and milk production and composition.

[The ruminal nitrogen metabolism in calves and sheep. 2. Studies in wethers]

Energetically differing rations which received ca. 45% of their crude protein from a supplement of soybean oil meal or urea were tested in a model experiment with adult wethers fitted with duodenal re-entrant cannulae or rumen cannulae resp. The N-equivalent exchange of soybean oil meal by urea in wethers resulted in a distinct increase of the NH3-N content of the rumen fluid, an increase of irreversible N losses and thus a worsening of the utilization of the available N for bacterial N-efficiency. A significant influence on duodenal NAN passage, on the apparent fermentation of the organic matter, on postruminal NAN digestibility and on the content of amino acid nitrogen as well as the amino acid composition of the duodenal NAN could not be detected. The increase of the energy level of the rations did not have a significant influence on the parameters of the ruminal N metabolism in the wethers. The studies with calves and adult wethers showed that a transposition of the parameters of ruminal N metabolism obtained to calves is not possible under the conditions given.