E. R. Ørskov

The estimation of protein degradability in the rumen from incubation measurements weighted according to rate of passage

A method is proposed for estimating the percentage of dietary protein that is degraded by microbial action in the rumen when protein supplement is added to a specified ration. The potential degradability, p , is measured by incubating the supplement in artificial-fibre bags in the rumen and is related to incubation time, t , by the equation p = a+b (1 – e -ct ). The rate constant k , measuring the passage of the supplement from the rumen to the abomasum, is obtained in a separate experiment in which the supplement is combined with a chromium marker which renders it completely indigestible. The effective percentage degradation, p , of the supplement, allowing for rate of passage, is shown to be p = a +[ bc/(c+k) ] (1- e -(e+k)t ) by time, t , after feeding. As t increases, this tends to the asymptotic value a+bc /( c+k ), which therefore provides an estimate of the degradability of the protein supplement under the specified feeding conditions. The method is illustrated by results obtained with soya-bean meal fed as a supplement to a dried-grass diet for sheep. The incubation measurements showed that 89% of the soya-bean protein disappeared within 24 h and indicated that it was all ultimately degradable with this diet. When the dried grass was given at a restricted level of feeding the allowance for time of retention in the rumen reduced the estimate of final degradability to 71% (69% within 24 h). With ad libitum feeding there was a faster rate of passage and the final degradability was estimated to be 66% (65% within 24 h).

A study of artificial fibre bag technique for determining the dig estibility of feeds in the rumen

The artificial fibre bag technique was evaluated for assessing the proportions of dietary D. M. and N which disappear in the rumen. The most important factor determining the variability in disappearance from bags incubated together was the sample size in relation to bag size. For incubation of 5 g air dry feed, a bag size of 17 ✗ 9 cm was found to be adequate. There were also variations in substrate disappearance between animals and between days of incubation. It was estimated that three sheep and the measurement of substrate disappearance twice were necessary in order to obtain acceptable repeatability. The technique was found to be satisfactory as a simple and rapid guide for measuring nutrients disappearance in the rumen such as protein degradation and carbohydrate fermentation. It has been used in subsequent work to measure the effect of features of the rumen environment, such as pH and NH 3 concentration, on rate of fermentation.

Associative effects of mixed feeds. I. effects of type and level of supplementation and the influence of the rumen fluid pH on cellulolysis in vivo and dry matter digestion of various roughages

Abstract 1. (1) The effect of various levels and types of concentrate supplementation of roughage-based diets on dry matter degradation, rumen cellulolysis and rumen pH, as well as changes in the rumen microflora and in the proportion of volatile fatty acids (VFA) produced were investigated. 2. (2) In the first experiment it was found that the amount and proportion of barley necessary to depress rumen pH, dry matter (DM) degradation and cellulolysis depended on the availability of readily fermentable barley starch to the rumen microorganisms, and was, therefore, influenced by the degree of processing. A pH threshold of 6.0–6.1 was obtained below which cellulolysis was totally inhibited. 3. (3) When a range of fibrous roughages were supplemented with various concentrates at 65% on a dry matter basis, it was observed that the extent of depression of DM degradation was influenced by both the rumen pH and the rate of solubilisation of the concentrate supplement. This effect was further confounded by the degradability of the roughage, since the percentage reduction in the rate of cellulolysis was greatest with those roughages with a low DM degradability. 4. (4) The depression in the rate of DM degradation could be almost totally alleviated when the rumen pH levels of the sheep were maintained at 6.70 ± 0.15. The reduction in DM degradation which could not be alleviated by increasing the rumen pH was termed the ‘carbohydrate effect’. This effect increased with the degree of processing of grain supplements and was greatest with molasses. 5. (5) It was concluded that in order to maintain rumen cellulolysis when roughages are supplemented with a readily fermentable carbohydrate, the rumen pH has to be maintained above 6.0–6.1, which appears to be the cellulolysis threshold, although the value appeared to vary with the quality of the roughage.

Comparison of in vitro gas production and nylon bag degradability of roughages in predicting feed intake in cattle

Abstract The Hohenheimer gas production test has been adapted to describe the kinetics of fermentation based on the exponential model p = a + b (1 − e − ct ). Ten cereal straws previously fed ad libitum to growing steers and consisting of two varieties of winter barley, two varieties of spring barley and one variety of winter wheat either untreated or treated with anhydrous ammonia, were examined. In vitro gas production was compared with in vivo results and with nylon bag degradabilities; the sources of the gas were determined. Total gas production a + b as described by the exponential equation were correlated with intake (0.88), digestible dry matter intake (0.93) and growth rate (0.95) in a multiple regression model. Use of the rate of gas production, c , did not improve the precision of correlation. The gas volume could be explained by the amount of volatile acids produced and their proportions. About 50% of the gas volume consisted of CO 2 and CH 4 arising from fermentation, the remainder being CO 2 released from the buffer solution.

Rates of rumen fermentation in relation to ammonia concentration

1. Four sheep were fed from automatic continuous feeders on whole barley fortified with graded levels of a urea solution. This approach was to a large extent successful in maintaining relatively steady states of rumen ammonia concentration. 2. Rates of barley fermentation in the rumen at various rumen NH3 concentrations were assessed by measuring the disappearance of barley dry matter from polyester bags suspended in the rumen of these sheep. 3. The minimal NH3 concentration for maximal rate of fermentation was estimated as 235 mg/l rumen fluid.

Manipulation of rumen fluid pH and its influence on cellulolysis in sacco, dry matter degradation and the rumen microflora of sheep offered either hay or concentrate

1. (1) It was possible to alter and maintain the rumen pH of three sheep given hay and three given concentrate by the continuous infusion of various amounts of an acid solution (H2SO4:HCl:H2PO4; 1:1:1, w/w) and a bicarbonate solution (NaHCO3 (66%):KHCO3 (34%), w/w), respectively. 2. (2) Reduction of the rumen pH (to 6.0–6.1) in sheep given roughage led to the inhibition of cellulolysis and partial destruction and washout of the rumen microflora. Dry matter degradation in the rumen and dry matter intake were also depressed. 3. (3) Increasing the rumen pH of sheep offered the concentrate diet did not greatly alter the rumen microflora nor did it increase cellulolysis, dry matter degradation or dry matter intake. 4. (4) It appears that both the type of substrate present in the rumen and the rumen pH have important effects on ruminal cellulolytic activity.

Excretion of purine derivatives by ruminants. Effect of microbial nucleic acid infusion on purine derivative excretion by steers.

Two steers totally nourished by intragastric infusion of volatile fatty acids and casein were given an abomasal infusion of a microbial protein preparation (Pruteen) at eight rates with a purine input ranging from 0 to 170 mmol/day over 11 successive 5-day periods. The urinary excretion of purine derivatives relative to the purine input was measured. Negligible amounts of xanthine plus hypoxanthine were present in the urine. The relative contributions of allantoin and uric acid to the total excretion were not affected by the rate of purine infusion. Total purine derivative excretion (uric acid and allantoin) was linearly correlated with purine input. Recovery in the urine of the infused purines was 0·77. It is suggested that utilization of exogenous purines may only occur in the intestinal mucosa and that the remaining purines may be completely converted, before entering the liver, to uric acid and allantoin, which are subsequently eliminated by the renal and extrarenal routes. The differences between cattle and sheep in excretion of purine derivatives, and the implications of these differences for the use of purine excretion values in order to estimate microbial protein supply to the ruminant, are discussed.

Excretion of purine derivatives by ruminants : effect of exogenous nucleic acid supply on purine derivative excretion by sheep

The present study examined the relationship between the supply of exogenous nucleic acid (NA) purines and their recovery as the derivatives hypoxanthine, xanthine, uric acid and allantoin in urine. Six lambs, totally nourished by intragastric infusions of volatile fatty acids (VFA) and casein (i.e. no rumen fermentation), were given by abomasal infusion a microbial NA concentrate at six levels (from zero to 24.5 mmol purines/d). The true digestibility between the abomasum and terminal ileum of the NA purines was measured in a separate experiment using three lambs. The relative proportion of urinary allantoin increased, and that of other derivatives decreased, as the amount of NA infused was increased. The relationship between total excretion of purine derivatives (Y; mmol/d) and exogenous purines absorbed (X; mmol/d) was Y = 0.84X + 0.150W0.75 e-0.25X, where W is body-weight (kg). This implies that the endogenous contribution to the total excretion of derivative decreased as the supply of exogenous purines increased, with an associated progressive replacement of de novo synthesis by exogenous purines. The model also implies that 0.16 of the purines were eliminated through routes other than derivative excretion in urine. Once excretion exceeded 0.6 mmol/kg W0.75 per d, endogenous excretion was effectively zero and thus Y = 0.84 X. In normally fed sheep, derivative excretion should therefore relate to the microbial purines and, hence, microbial protein absorbed according to these models. The changing proportions of allantoin and other derivatives in urine were probably due to changes in the relative importance of endogenous and exogenous purines as precursors.

The effect of protein infusion on urinary excretion of purine derivatives in ruminants nourished by intragastric nutrition

Two experiments were carried out to determine endogenous excretion of purine derivatives in steers and lambs, and to investigate the relationship between microbial nucleic acid input and urinary excretion of purine nitrogen. The endogenous excretion of allantoin after conversion of hypoxanthine, xanthine and uric acid to allantoin, was calculated to be 72 and 26 mg/kg W 0·75 per day in steers and lambs, respectively, when the dietary protein contained no nucleic acid nitrogen. The excretion of purine derivatives increased linearly with increasing microbial nucleic acid input in lambs. The excretion of purine derivatives in excess of endogenous contribution was closely related to the theoretically expected values. The average recovery was calculated as 0·96 for one sheep and 1·0 for the other.

The nutritive value of rumen micro-organisms in ruminants: 1. Large-scale isolation and chemical composition of rumen micro-organisms

A method is described whereby a large quantity of rumen microbial dry matter of high purity was isolated from whole rumen contents obtained from abattoirs, by means of a continuous process of one filtration through four sieves followed by three differential centrifugations. The contents of ash, carbohydrate, lipid, nitrogen, RNA, DNA and individual amino acids of the three centrifugal fractions are given and compared with values summarized from more than sixty published reports on the chemical composition of rumen micro-organisms isolated from both whole rumen contents and pure cultures. The amino acid composition of isolated rumen micro-organisms, in particular that of the bacteria, was found to be remarkably constant.