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Author

I. McDonald

Bio: I. McDonald is an academic researcher from Rowett Research Institute. The author has contributed to research in topics: Dry matter & Fish meal. The author has an hindex of 30, co-authored 57 publications receiving 7607 citations.


Papers
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Journal ArticleDOI
TL;DR: In this paper, a method 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 is proposed, where the potential degradability, p, is measured by incubating the supplement in artificial-fibre bags and is related to incubation time, t, by the equation p = a+b (1 − e -ct ).
Abstract: 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).

4,552 citations

Journal ArticleDOI
TL;DR: In this article, the authors proposed a modification to the formula for effective degradability so that it may remain valid under low values of t for some protein supplements, linseed meal for example.
Abstract: 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 Orskov & 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.

854 citations

Journal ArticleDOI
TL;DR: This approach was to a large extent successful in maintaining relatively steady states of rumen ammonia concentration and rates of barley fermentation in the rumen at various rumen NH3 concentrations.
Abstract: 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.

450 citations

Journal ArticleDOI
TL;DR: Ninety-nine Finnish Landrace x Dorset Horn ewes in lamb to Suffolk rams were used in a comparative slaughter experiment to assess the effects of numbers of foetuses on the growth of the products of conception, and the mathematical model implies that the differences originate in early pregnancy, when the factor is very close to unity, and that the mean weights gradually diverge.
Abstract: Seventy-nine Finnish Landrace x Dorset Horn ewes in lamb to Suffolk rams were used in a comparative slaughter experiment to assess the effects of numbers of foetuses on the growth of the products of conception. The mean number of foetuses was 2·7, with a range from 1 to 5. The ewes were individually penned and given a standard diet with metabolizable energy concentration 7·7 MJ/kg and N concentration 21 g/kg, rationed at 2 kg/day during the first month and 1·25 kg/day during the second and third. Allowances beyond that time were on two scales and depended on the number of foetuses being carried as diagnosed by radiography. The ewes were slaughtered between 50 and 145 days of gestation. The gravid uterus was dissected into foetal, placental, foetal fluid and empty uterus components. Equations were fitted to the weights of each component to describe the effects of stage of gestation, litter size and ewe weight. For the mean weight per foetus (Y, kg) the preferred equation isIn (Y)= 2·419–17·574e-0.01976t–00079ft+0–0046w,where tis the time in days from conception, / is the number of foetuses and wis the weight (kg) of the ewe at mating. This is a version of the Gompertz equation, with additional terms to express the effects of / and w.The weights of the placenta and of the empty uterus were similarly fitted by versions of the Gompertz equation but the mean weight of fluids per foetus (Z, kg) or, rather, its natural logarithm was best described by a third degree polynomial, which isIn(Z) = –11·518 + 0–326t;–0·00316t2+0·0000102t3.None of the weights was significantly affected by the level of feeding in late pregnancy.Estimates of mean weights from the equations and of daily rates of gain in weight from the first differentials of the equations are tabulated against stage of gestation and litter size, and the forms of the weight, growth rate and specific growth-rate curves are illustrated graphically.The equation for foetal weight estimates that at the end of pregnancy the mean weight per foetus is reduced by a factor of 0–89 for each additional foetus being carried. The mathematical model implies that the differences originate in early pregnancy, when the factor is very close to unity, and that the mean weights gradually diverge. In the absence of direct evidence this would appear to be the simplest hypothesis, rather than the assumption in most of the earlier literature that the effect is entirely confined to the last 4 or 5 weeks of gestation.Just before parturition the total daily weight gain of quadruplet foetuses was about 250 g and was associated with a similar gain in weight of foetal fluids, the ratio of fluid weight to foetal weight appearing to increase with litter size. The ewes were clearly under considerable physical stress. It is suggested that this aspect must be closely considered when greater prolificacy is sought.

170 citations

Journal ArticleDOI
TL;DR: It is suggested that barley diets for early-weaned lambs can with advantage be supplemented with non-protein N to increase the crude protein in the dry matter up to about 12%.
Abstract: 1. The effects of adding increasing supplements of urea to mainly barley diets for early-weaned lambs were investigated in two experiments. In the first experiment the passage of nutrients along the alimentary tract was studied by taking samples of abomasal, ileal and rectal contents and using a marker technique. In the second experiment, feed consumption and rate of gain were recorded over the growth period up to 40 kg live weight, and nitrogen balances were carried out. In the second experiment a barley-fish meal diet was also included.2. The fermentation of organic matter in the rumen increased with the amount of urea in the diet and levelled off when the diet contained about 12% crude protein.3. Urea supplementation had significant effects in increasing N retention and rate of live-weight gain and in decreasing feed conversion ratio, but supplementation beyond about 12% crude protein in dry matter had no further effect on these measurements. In each instance results with the barley–fish meal diet were better than the results with any of the barley-urea diets.4. From the concentrations of diaminopimelic acid (DAPA) in abomasal fluid it was estimated that microbial protein was produced in the rumen at a rate of 15.6% g/100 g organic matter fermented. This ratio did not appear to alter significantly with urea supplementation, but the comparison depends on the assumption that the concentration of DAPA in the bacterial protein did not itself change with urea supplementation.5. Using results from both experiments, it was calculated that the retained N on the urea-supplemented barley diets was approximately 47% of the amount of protein N absorbed in the small intestine.6. It is suggested that barley diets for early-weaned lambs can with advantage be supplemented with non-protein N to increase the crude protein in the dry matter up to about 12%. When barley diets are given with a protein supplement the addition of non-protein N is unlikely to be beneficial unless the protein supplement is given in such a way that it is not subject to degradation to yield ammonia in the rumen.

124 citations


Cited by
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Journal ArticleDOI
TL;DR: The results suggest that addition of non-protein N supplements to ruminant rations are warranted only if the prevailing concentration of ruminal ammonia is less than 50 mg NH3-N/l ruminal fluid.
Abstract: 1. The effect of ammonia concentration on microbial protein production was determined in continuous-culture fermentors charged with ruminal contents obtained from steers fed on either a protein-free purified diet, a maize-based all-concentrate diet, or a forage–concentrate (23:77) diet. Urea was infused into the fermentors to maintain various concentrations of ammonia in the incubating mixtures.2. Under nitrogen-limiting conditions, microbial protein yield measured as tungstic acid-precipitable N (TAPN) increased linearly with supplementary urea until ammonia started to accumulate in the incubating ingesta. Increasing the ammonia concentration beyond 50 mg NH3-N/l had no effect on microbial protein production.3. The molar proportions of volatile acids produced were not affected by the level of urea supplementation. Total acid production was decreased slightly under N-limiting conditions, but not to the same extent as microbial protein production.4. Estimated yield of microbial dry matter/mol ATP produced averaged 15·6 when non-limiting N as urea was provided with the purified diet.5. These results suggest that addition of non-protein N supplements to ruminant rations are warranted only if the prevailing concentration of ruminal ammonia is less than 50 mg NH3-N/l ruminal fluid.

1,731 citations

Journal ArticleDOI
TL;DR: Results obtained establish the pressure transducer as a suitable tool for determining the fermentation kinetics of ruminant feeds and ranking them with respect to their in vitro fermentability.

1,524 citations

Journal ArticleDOI
TL;DR: It was concluded that optimum bacterial growth in the rumen occurs when EMPS is 29 g of bacterial N/kg of fermented organic matter, and ENU is 69%, implying that bacteria would require about 1.31 x rumen-available N per unit ofacterial N.

1,144 citations

Journal ArticleDOI
TL;DR: In this article, the authors proposed a modification to the formula for effective degradability so that it may remain valid under low values of t for some protein supplements, linseed meal for example.
Abstract: 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 Orskov & 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.

854 citations

Journal ArticleDOI
TL;DR: In this article, an artificial fiber bag technique was evaluated for assessing the proportions of dietary D. M. and N which disappear in the rumen and the most important factor determining the variability in disappearance from bags incubated together was the sample size in relation to bag size.
Abstract: 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 NH3 concentration, on rate of fermentation.

804 citations