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/) = a + 6( 1 -e~c t ). 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~( c +*") 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). TWTT? O'DTTPTTO'WThere are essentially two methods of obtaining

Estimates of the degradability of protein in the rumen are essential for the application of new systems which have been suggested for the evaluation of the protein requirements of ruminants (Agricultural Research Council, 1980). The effective percentage degradability (P) of protein supplements in the rumen is dependent not only on the course of degradation of the protein particles in the rumen, but also on the time distribution of their stay in the rumen, and will decrease if there is an increase in the rate of passage of the particles. It was shown by Ørskov & McDonald (1979) that if the percentage protein disappearance (p) from samples incubated for time t is described by the equation and if k is the fractional rate of passage from the rumen, then the effective degradability can be calculated as . The calculation depends on the equation for p remaining valid from t =0 (time of ingestion) until a time when all the particles have passed beyond the rumen. It has been found that for some protein supplements, linseed meal for example, the equation for p does not hold true for low values of t. The present note proposes a modification to the formula for effective degradability so that it may remain valid under these circumstances.

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 NHI concentrations were assessed by measuring the disappearance of barley dry matter from polyester bags suspended in the rumen of these sheep.3. The minimal NH:, concentration for maximal rate of fermentation was estimated as 235 mg/l rumen fluid.The optimal ammonia concentration of rumen fluid may be defined as that which results either in the maximum rate of fermentation in the rumen or that which allows the maximum production of microbial protein per unit of substrate fermented. The two definitions may not always coincide; for instance Orskov, Fraser & McDonald (1972) showed with a barley feed that the microbial protein produced per unit of substrate fermented was not altered as a result of urea supplementation while the extent of rumen fermentation and digestibility was increased.The rate at which rumen fermentation proceeds has a great influence on both total and digestible feed intakes (Balch & Campling, 1962) and therefore feed intake may be reduced if NH, concentration is limiting the rate of fermentation.Diurnal variation in all constituents of rumen fluid, generally associated with the time of feeding, make it difficult to estimate optimal rumen NH, concentration. A steady state of rumen fermentation can be achieved by continuous feeding. Although urea can also be infused continuously, its absorption into grains (Orskov, Smart & Mehrez, 1974) offers another simple method for stabilizing rumen NH, concentration.The use of the 'polyester bag' technique (Mehrez & IZIrskov, 1977) enabled us to determine rapidly and accurately the rate of substrate fermentation in the rumen.This present experiment was carried out to investigate the relationship between rumen NH, concentration and rate of fermentation.

In experiments on digestive function in ruminants, markers can be used to estimate the rate of movement of digesta, the volume of a viscus and the rates of absorption of solutes from the gut. The most widely used water-soluble marker is polyethylene glycol (PEG), which was introduced by Sperber, HydCn & Ekman (1953). They found that the polymer with mean molecular weight 4000 was not degraded in the gut or absorbed, and described the use of PEG for the three purposes set out above.HydCn (1961) has given a full mathematical treatment of the principles involved in the use of soluble markers to measure the rate of flow and volume of rumen fluid, and has indicated the inherent limitations of the method. He found that PEG gave a satisfactory estimate of rumen fluid volume (PEG space was approximately 95 % of rumen water volume); in experiments of long duration (i.e. days or weeks), the flow rate could be calculated from the rate of fall in concentration of the marker; and in short-term experiments, the flow could be estimated from the amount of reference substance that had disappeared from the rumen during the experiment and the mean concentration of the marker in the rumen fluid.However, a serious limitation in the use of PEG is the lack of a specific, sensitive and accurate method of analysis. All published methods have been based on precipitation from aqueous solution with subsequent gravimetric or turbidimetric measurement. Analytical difficulties have been recorded by Smith (1958), and Corbett, Greenhalgh, Gwym & Walker (19 j8) obtained incomplete recovery of PEG from the faeces of grazing cattle and sometimes from stall-fed cattle.An alternative reference substance was sought and radioactivity was selected as a ineans for sensitive and completely specific analysis. The complex of 51Cr with ethylenediamine tetraacetic acid (j1Cr EDTA) was tested and compared with PEG.The results which are reported here showed j3Cr EDTA to be a highly satisfactory soluble marker in sheep. E X P E R I M E N T A L AnimalsNine Merino wethers, one Merino ewe, and four cross-bred sheep were used. Five of the Merino wethers were 'normal', the rest of the sheep were fitted with rumen cannulas. The Merinos were fed once daily and were housed indoors in metabolism cages of the type described by Reis & Schinckel (1961). Faecal collections from the available at https://www.cambridge.org/core/terms. https://doi

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In a previous communication attention was drawn to the occurrence in the rumen of the sheep of significant concentrations of ammonia (McDonald,

1. From a review of the literature it has been shown that there are two opposing views regarding the best method of interpreting growth data, which arise from conflicting opinions as to the role of fat deposition in the growth of the animal.2. Data of McMeekan and Palsson and Verges have been re-analysed and their own results are compared with results obtained when the effects of variation in fat content are eliminated.3. No evidence has been found of any effect of plane of nutrition on the total weights of bone and muscle relative to the weight of bone plus muscle together.4. The weight of bone plus muscle in the head and neck was increased relative to the total weight of bone plus muscle during periods of restricted nutrition. Apart from this there was no clear evidence of a relationship between the order of maturity of the joints and their relative retardation of development.5. Huxley's allometry equation was found appropriate for standardising the measurements, and the exponent was taken as a numerical expression of the relative maturity of each tissue or part.

SUMMARYThirty-six mature Finnish Landrace × Dorset Horn ewes, each suckling two lambs, were used in a comparative slaughter experiment to measure changes in body tissues during early lactation. Two levels of body fatness at lambing were established by giving ewes a complete diet containing 10 MJ metabolizable energy (ME) and 139 g crude protein (CP)/kg d.m. either close to requirements or ad libitum during the second half of pregnancy. In lactation half the ewes in each group were given a complete diet containing either 90 (diet A) or 60 (diet B) % milled hay ad libitum. These diets contained 7·9 and 9·2 MJ ME and 121 and 132 g CP/kg d.m. respectively.Ewes fed at the two levels in pregnancy contained 8·4 and 19·6 kg chemically determined fat 5 days after lambing but had similar amounts of body protein, ash and water. Over 6 weeks of lactation ewes given diet A lost 60 and 69% of these weights of fat respectively, while ewes given diet B gained 5% and lost 30% respectively. Up to 26 g of body protein was lost daily from ewes given diet A but none from ewes on diet B. During early lactation the weight of the empty digestive tract increased while the weights of most other body components, particularly the carcass, decreased. The ratio of body energy change to live-weight change varied from 24 to 90 MJ/kg. Thus live-weight change did not accurately reflect relative or absolute changes in body energy.Voluntary food intake was greater for ewes given the high-energy diet (B) than for those given diet A and was depressed in the fatter ewes. Differences in intake could be explained by the effects of body fatness and diet on the weight of gut contents. Milk yield was not significantly affected by body fat reserves but was higher on diet B than A. Fat content of milk was higher and protein content lower for ewes with the higher fat reserves at lambing.As the contribution of fat loss to energy available for milk synthesis increased there appeared to be a reduction in the energetic efficiency of milk synthesis. A number of possible reasons for this are discussed.