Three rumen fermentation studies in combination with three feeding trials were carried out to investigate the effect of partial replacement of heavily fertilized perennial ryegrass by low protein feedstuffs on pH and concentrations of VFA and NH3 N in the rumen and on N excretion in milk, urine, and feces by dairy cows. Feedstuffs tested were high fiber concentrate mixtures based on sugar beet pulp and soybean hulls and high starch concentrate mixtures based on corn (Experiments 1 and 2), corn silage (Experiment 1), dried and ensiled pressed sugar beet pulp and high moisture ear corn silages with or without husks (Experiment 3). In the fermentation studies, N intake ranged between .43 and .57 kg/d. Partial replacement often increased DMI (maximal by 2.6 kg), resulting in minor effects on N intake. Urinary N excretion ranged between 30 and 58% of N intake and decreased by 30 to 40% when grass was partially replaced. Fecal N output was between 25 and 30% of N intake and tended to increase with the low protein feed. The reduction in urinary N excretion corresponded to a decrease of rumen NH3 N. Replacement by concentrate mixtures based on corn reduced milk fat content; for mixtures based on beet pulp, milk fat content was not changed.

The ileal digestibilities of maize starch and native pea starch do not differ. However maize starch is digested faster than pea starch and the ileal amino acid digestibility of a diet containing pea starch is lower. In the present study, the net portal fluxes of glucose, lactate, volatile fatty acids (VFA) and amino acids were measured for diets including 650 g maize starch or pea starchhcg. The diets were fed at a level 870 kJ digestible energykg'"' twice daily (06.00 and 18.00 hours) to four female pigs in a crossover design. Portal vein blood flow did not differ between maize and pea starches (1620 and 1484 mumin respectively; SED 100; P = 0.23). For maize starch portal glucose flux was significantly higher during the first 6 h after feeding, was not different 8 h after feeding and was significantly lower thereafter. Net portal glucose flux was higher for maize starch than for pea starch (1759 and 1265 mmoV12 h respectively; SED 182; P = 0.054). Net portal lactate flux was not significantly different between maize and pea starches (36.5 and 67.2 mmoUl2 h respectively; SED 24.1; P = 0.27) and net portal VFA flux was lower for maize starch than for pea starch (169 and 218 mmoUl2 h respectively; SED 18; P = 0.054). Net portal fluxes of valine, isoleucine, phenylalanine, tryptophan, arginine, serine, cystine, tyrosine, lysine, histidine and the sum of essential amino acids tended to be or were higher (Pc0.1 or P

Grass samples were harvested during the 1993 growing season after a precut on April 27, 1993 and were stored frozen or left to ensile in 30-L buckets. Effects on chemical composition and fermentation kinetics of the maturation of the grass and of ensiling were investigated. Chemical composition and fermentation kinetics were determined using the gas production technique, in vitro techniques, and the nylon bag technique. Two silage samples were also investigated in vivo. Maturation caused a decrease in crude protein content and organic matter degradability and an increase in neutral detergent fiber, acid detergent fiber, and lignin contents. Degradation rates were highest for the youngest samples and decreased as grass and silage matured. This result could be observed from the first derivative of the cumulative gas production curves (i.e., the rate of gas production) and from the gas production parameters. The rate of degradation of the nonsoluble fraction of the young samples, determined using the gas production technique, was relatively higher in rumen fluid from a cow fed silage from grass cut at a young stage. More mature samples were degraded relatively faster in rumen fluid from a cow fed mature grass silage, suggesting a specific adaptation of the rumen microorganisms to the grass properties. There was a good relationship among the second phase of gas production (i.e., fermentation of the nonsoluble fraction), maturity of the grass and grass silage samples, degradability determined with the Tilley and Terry technique, and degradability determined after 46 h of incubation in rumen fluid. Results obtained with both of the different in vitro techniques and the nylon bag technique were confirmed by the in vivo experiments involving the two silage samples.

Mechanistic rumen models of Baldwin (1995), Danfm (1990) and Dijbtra et al. (1992) were compared on identical inputs that were derived from trials with lactating dairy cows fed on grass herbage. Consistent differences were detected between models and between predicted and observed outputs. None of the models seemed to predict all nutrient flows best. The models particularly differed in the representation of microbial metabolism: degradation of insoluble substrate, fermentation of substrate into volatile fatty acids, and incorporation of substrate into microbial matter. Differences amongst models in the prediction of these processes compensated for each other and consequently all models predicted the duodenal flow of non-NH3 N, microbial N and organic matter reasonably well. Large differences remained in the prediction of individual nutrient flows, however, and it was stressed that in order to enhance prediction of the profile of nutrient flows, the mechanisms of microbial metabolism need to be tested on their ability to describe the intraruminal transactions. However, this requires more-detailed information on individual nutrient flows and on the microbial or non-microbial origin of duodenal contents. Parameter inputs for physical and chemical feed properties were identified that are improperly defined in extant models or susceptible to error. The description of these feed characteristics needs to be developed further and become identifiable for a wide range of dietary conditions. Rumen: Models: Lactation Current research into the nutrition of lactating dairy cows focuses on the profile of nutrients absorbed from the digestive tract, on the partitioning of these nutrients in intermediary metabolism, and on the utilization of these nutrients for milk production and body tissues. However, when theories of dietary influences on nutrient partitioning and animal production are to be investigated, the effect of dietary factors on feed digestion and the profile of nutrients absorbed from the gastrointestinal tract needs to be defined. In particular, an understanding of rumen function is essential for estimating these nutrient flows, because the profile of nutrients can change dramatically due to events in the rumen.Mathematical modelling is an important instrument for understanding and integrating knowledge (Baldwin, 1995). Detailed dynamic models that describe the mechanism of rumen function in cattle have been developed by Baldwin et al. (1987; BA), Danfzr (1990; DA) and Dijkstra et al. (1992; DY). Although the BA and DA models are whole-animal models, in contrast to the DY model, the part that describes rumen function can easily be extracted in both cases because it is formulated as a separate module. Although the models have differing objectives and consequently apply different concepts and theories, all three models basically simulate the digestion, absorption and outflow of nutrients from the rumen. The model inputs required include daily feed intake, the composition and other characteristics of the feed, and ...

No abstract