Knowledge of the fractional outflow rates of rumen digesta constituents is required for the quantitative prediction of rumen function. However, there are few data available for these parameters because of the problems associated with specific labelling of digesta constituents and with the need to ensure that samples are truly representative of rumen digesta. Methods are described here by which two markers can be used together to overcome the problem of representative sampling of the rumen, so eliminating the need for specific labelling, with 'ideal' markers, of digesta constituents. Thus, from the measurement of the amount and composition of true digesta in the rumen and the amount and composition of true digesta flowing from the abomasum, values for the fractional outflow rates (and its reciprocal, mean retention time) of rumen digesta constituents can be calculated. Appropriate application of these methods will allow the derivation of functions to predict fractional outflow rates. The results presented show that the fractional outflow rate of an adsorbable marker, 103Ru-P, was about half that of water but was about twice that of particulate matter; it was also greater than the net value for microbes. On the assumption that microbial material leaving the rumen is drawn from a free-floating microbial population and a population associated with particulate matter, it was calculated that 0.29 � SE 0.07 (range 0.09-0.49) of the rumen microbial population could have been free-floating.
Aust. 1. BioI. Sci. 1987, 40, 365-77 Fetal and placental growth, and fetal and maternal urea synthesis in late gestation, were studied in 2-yearold Corriedale ewes on a maintenance ration (M) except when subjected to moderate dietary restriction from day 50 to day 100 (RM), day 100 to day 135 (MR) or day 50 to day 135 (RR). In comparison with fetuses of ewes maintained throughout the experiment (MM), RR fetuses were smaller and RM fetuses were larger whereas MR fetuses were unaffected; all restrictions were associated with increased placental size. Fetal urea synthesis at day 133 in the well-nourished ewes (MM) was 21.5 mg N h-i kg-i increasing to, respectively, 25.7, 27.3 and 38.8 mg N h-i kg-i in groups MR, RM and RR; these values were 1.6, 3.9,2.2 and 3.8 times the maternal rates of synthesis. On the basis of the observed urea synthesis rates, amino acid oxidation could have accounted for up to, respectively, 32, 38, 40 and 571110 of fetal oxygen consumption in groups MM, MR, RM and RR. Amino acids, in addition to their role in tissue accretion, may be key energy substrates for the fetus.
SummaryTwo methods for calculating the flow of digesta and its constituents from the concentrations of two markers, 51Cr-labelled chromium EDTA (51Cr-EDTA) and 103Ru-labelled ruthenium phenanthroline (103Ru-P), and digesta composition were compared in fistulated sheep fed once daily and in other fistulated sheep fed continuously. One method uses the marker concentrations to calculate the flow of whole digesta directly (double-marker) whereas the other uses the markers to calculate separately the flow of fluid and non-filtrable particles, the sum of which is digesta flow (two-marker).When feed was given continuously, flow values calculated using the double-marker method and the two-marker method, which requires a correction to be made for nonideal behaviour of 51Cr-EDTA, did not differ. When feed was given once daily, the double-marker method consistently overestimated flow relative to the corrected twomarker method but the differences were extremely small (less than 0-8% for digesta flow and 0-5% for nitrogen flow). 51Cr-EDTA as a single marker slightly overestimated digesta flow (up to 2·3%) but grossly overestimated nitrogen flow (up to 22·5%); 103Ru-P as a single marker grossly underestimated both digesta flow (up to 30·5 %) and nitrogen flow (up to 23·1%).It was concluded that the double-marker method is to be preferred when the diet is given continuously or frequently at fixed intervals. When the diet is given once daily, the two- and double-marker methods can be expected to give similar values. Single markers should not be used for estimating the flow of digesta and its constituents when samples are taken from a simple cannula.
I . Fistulated sheep given a concentrate diet were used to study the behaviour of solute ([WrIEDTA) and particle ([103Ru]phenanthroline) markers in the stomach under conditions of continuous feeding.2. An injection of a mixed dose of [WrIEDTA and [103Ru]phenanthroline was given into the rumen and the time course of marker concentrations in the rumen and the abomasum was recorded. The curves were analysed on the assumption that the stomach of the sheep could be represented as two mixing compartments (reticula-rumen and abomasum) and a time delay (omasum). This model provided a very good description of the data.3.[103Ru]phenanthroline associated with small particles was retained in the rumen much longer than [WrlEDTA. Although exchange of [103Ru]phenanthroline occurred between large and small particle fractions, the results suggested that small particles may have been retained somewhat longer in the rumen than solutes. However, it was clear from the results that the mean retention times for particulate matter in the rumen could not be simply obtained using adsorbable markers.4. Cyclical fluctuations in the concentration of [WrIEDTA in the rumen indicated that there were daily variations in net water flux in the rumen.5. The presence of protozoa was associated with much shorter retention times of both solutes and particles in the rumen. Protozoa were also associated with reduced rumen volumes.
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