Die Verdaulichkeit des Proteins beim Wiederkäuer1

Invited review: Nitrogen in ruminant nutrition: A review of measurement techniques

Nitrogen is a component of essential nutrients critical for the productivity of ruminants. If excreted in excess, N is also an important environmental pollutant contributing to acid deposition, eutrophication, human respiratory problems, and climate change. The complex microbial metabolic activity in the rumen and the effect on subsequent processes in the intestines and body tissues make the study of N metabolism in ruminants challenging compared with nonruminants. Therefore, using accurate and precise measurement techniques is imperative for obtaining reliable experimental results on N utilization by ruminants and evaluating the environmental impacts of N emission mitigation techniques. Changeover design experiments are as suitable as continuous ones for studying protein metabolism in ruminant animals, except when changes in body weight or carryover effects due to treatment are expected. Adaptation following a dietary change should be allowed for at least 2 (preferably 3) wk, and extended adaptation periods may be required if body pools can temporarily supply the nutrients studied. Dietary protein degradability in the rumen and intestines are feed characteristics determining the primary AA available to the host animal. They can be estimated using in situ, in vitro, or in vivo techniques with each having inherent advantages and disadvantages. Accurate, precise, and inexpensive laboratory assays for feed protein availability are still needed. Techniques used for direct determination of rumen microbial protein synthesis are laborious and expensive, and data variability can be unacceptably large; indirect approaches have not shown the level of accuracy required for widespread adoption. Techniques for studying postruminal digestion and absorption of nitrogenous compounds, urea recycling, and mammary AA metabolism are also laborious, expensive (especially the methods that use isotopes), and results can be variable, especially the methods based on measurements of digesta or blood flow. Volatile loss of N from feces and particularly urine can be substantial during collection, processing, and analysis of excreta, compromising the accuracy of measurements of total-tract N digestion and body N balance. In studying ruminant N metabolism, nutritionists should consider the longer term fate of manure N as well. Various techniques used to determine the effects of animal nutrition on total N, ammonia- or nitrous oxide-emitting potentials, as well as plant fertilizer value, of manure are available. Overall, methods to study ruminant N metabolism have been developed over 150 yr of animal nutrition research, but many of them are laborious and impractical for application on a large number of animals. The increasing environmental concerns associated with livestock production systems necessitate more accurate and reliable methods to determine manure N emissions in the context of feed composition and ruminant N metabolism.

Efficiency of utilization during pregnancy and lactation in the ewe of the protein reaching the abomasum and truly digested in the small intestine

In six experiments carried out on individually penned Finn Dorset ewes estimates were made of the quantities of non-ammonia nitrogen (NAN) reaching the abomasum and truly digested in the small intestine. For experiments 1 and 2 which each involved 33 ewes given a complete diet of milled hay and concentrates supplemented with varying amounts of a good-quality fish meal during late pregnancy, the overall regression estimate for the daily amount of truly digested NAN required for zero N retention (ewe plus conceptus) was 438 mg/kg M0·75 (441 and 434 for experiments 1 and 2 respectively). The coefficients for the efficiency of utilization of increments of truly digested NAN and maternal tissue N for net N accretion in the conceptus were 0·48 (s.e. 0·039) and 0·84 (s.e. 0·127) respectively.

In experiment 3, 24 lactating ewes had their diet supplemented with either soya-bean meal or fish meal. There was no effect of protein source on the amounts of NAN reaching the abomasum in ewes fitted with an abomasal cannula or on milk yield and these observations confirmed the unusually high degradability (measured by the polyester bag technique) of the fish-meal protein in the rumen. For experiments 4, 5 and 6 a total of 36 ewes were used to test the efficiency with which the NAN truly digested in the small intestine was used for the synthesis of milk protein. In the absence of data on the possible contribution of body tissue N to milk N the coefficient for the apparent efficiency of utilization of truly digested NAN when the ewes were given a basal diet containing approximately 10 g crude protein per MJ metabolizable energy was 0·63. For those ewes receiving the basal diet supplemented with either soya-bean meal, fish meal or blood meal the coefficients for the efficiency of utilization for the production of milk N of the increments in truly digested NAN supplied by the three protein sources were 0·61, 0·54 and 0·29 respectively. It is suggested that the low coefficient for blood meal may be due to its low content of methionine.

Supplementation of Acacia mearnsii tannins decreases methanogenesis and urinary nitrogen in forage-fed sheep

The objective of this experiment was to assess the effects of a partial replacement of ryegrass (Lolium perenne) by red clover (Trifolium pratense) or alfalfa (Medicago sativa) supplemented with 0 or 41 g Acacia mearnsii extract (containing 0.615 g/g condensed tannins)/kg dietary dry matter on nitrogen turnover and methane release by sheep, using the respiration chamber technique. Across all variables, there was no significant interaction between basal diet and tannin supplementation. The partial replacement of the grass by the legumes remained without effect on the amounts of nitrogen excreted through faeces or urine. Nitrogen and energy utilisation was lower (P < 0.05) with ryegrass–alfalfa than with ryegrass alone, and methane release (kJ/MJ gross energy intake) was higher (P < 0.05) with ryegrass–red clover than with ryegrass alone. Tannin supplementation decreased (P < 0.05) ruminal ammonia concentration and urinary nitrogen excretion without affecting body nitrogen and energy retention, and reduced (P < 0.001) methane release by 13% on average. The results suggest that supplemented Acacia mearnsii tannins can be useful in mitigating methane and potential gaseous nitrogen emissions, whereas a replacement of grass by legumes obviously shows no advantage in this respect.

[The ruminal nitrogen metabolism in calves and sheep. 1. Studies in calves]

In a model experiment with calves supplied with duodenal re-entrant and rumen fistula resp. rations were tested that were energetically different (535 and 585 EFU cattle/kg DM) due to changing quotas of wheat starch and straw resp., dry matter intake being equal, and in which 45% of the crude protein came from soybean oil meal or urea resp. Due to the N-equivalent exchange of soybean oil meal by urea in rations for ruminating calves (130th day of life) the ammonia-N content in the rumen fluid, the quota of bacteria-N in the duodenal non-ammonia N (NAN) as well as the fermentation of crude plant protein in the ration were clearly increased. The iso-acid content in the rumen fluid and the utilization of the N available for bacteria N synthesis (N intake minus not degraded feed N in the duodenum) were reduced. An influence on duodenal NAN passage, on the apparent fermentation of the organic matter, on postruminal NAN digestibility and on the content of amino acid nitrogen as well as the amino acid composition of duodenal NAN could not be proved. A ca. 20% higher energy intake by ruminating calves did not have a clear influence on other parameters, with the exception of the reduction of the molar quota of acetic acid in favour of the quota of propionic acid in the rumen fluid.

Protein and energy relationships in dairy cattle. 1. Dry cows

Nitrogen metabolism in ruminants is a dynamic process depending on level of intake and composition of dietary dry matter and on the physiological state of the animals. These parameters were analysed in a regression model for the requirements of nitrogen absorbed in the small intestine in relation to nitrogen balance of dry cows fed at maintenance level. The amount of total dietary nitrogen and the apparent nitrogen digestibility vary notably according to energy intake and energy concentration in dietary dry matter. On the contrary, the amount of nitrogen absorbed in the small intestine is only slightly affected by these parameters. The discrepancy between these statements lies mainly in differences in intestinal nitrogen absorption and in altered fermentation processes of the large intestine in response to changes in energy supply and ME concentration in dietary dry matter.

[Protein and amino acid metabolism in the digestive tract of growing young bulls. 4. Apparent digestibility of NH3-free raw protein reaching the duodenum and critical evaluation of apparently digestible raw protein]

The experimental determination of the apparent digestibility (AD) of the NH3-free crude protein at the duodenum (NH3 fr. CPD) showed a value of 65.7% at a standard error of +/- 3.6% for the section of the digestive tract between proximal duodenum and end of rectum. This standard error can only be insignificantly diminished due to the existing relation between AD of NH3 fr. CPD in dependence on the CP concentration in the ration (y = 56.4 + 0.598 x +/- 3.17; r = 0.501xx) so that this relation has no practical significance. The criticism of AD CP is correct from a point of view of digestion physiology. There is only agreement between AD PC and AD NH3 fr. CPD in a CP concentration range between 12.5 and 14.0% CP in the DM, which, however, applies to a large range of the production sphere in the rearing of young bulls. It excludes, however, the low live weight range with high live weight gains.

[Effect of type of starch and quantity in rations on apparent and true digestibility of nitrogen and nitrogen balance in sheep]

In a digestibility trial 5 semisynthetic rations were fed in 3 periods to 10 male sheep to examine the effects on N balance, components of faecal nitrogen and N digestibility. The rations contained constant amounts of nitrogen but different contents of cellulose and two different types of starch (untreated and steamflaked). Content and type of starch did not show any noticeable effect neither on excretion of undigested dietary nitrogen nor on true digestibility. There could not be noticed any effects on the apparent N digestibility by changing contents of cellulose or untreated starch. If the rations contained steamflaked corn starch, the animals excreted more faecal nitrogen and therefore showed a lower apparent N digestibility. Especially the water soluble N fraction of the faecal nitrogen was clearly higher. Compensation took place through a lower N excretion with the urine. The reason for increased faecal N excretion may be higher microbial protein synthesis in the rumen. This was accented by the allantoin excretion with the urine.

Measurement of the postruminal digestibility of crude protein by the bag technique in cows

A new method has been developed which permits the crude protein digestibility of feedstuffs in the intestine of cattle to be measured with little effort in terms of samples and experimental work. It consists of welding 0.4 ... 0.8 g of the feedstuff (particle size: 125 ... 1000 micron) into polyamide fabric bags (25 X 40 mm) which are inserted via cannulae into the digestive tracts of fistulated cows from the abomasum/duodenum to the ileum or from the abomasum/duodenum to the faeces. The mean retention time of the bags in the animal was 8.5 +/- 2.7 h from the abomasum to the end of the ileum and (13.3 +/- 1.9 h from the abomasum to the faeces. Up to 15 bags per day and cow may be used. The random error of the method is 1.3% (absolute) when the measurements are performed on two animals using two bags each. Intestinal digestibilities of over 90% were measured for concentrate proteins (except linseed meal) and of 72 ... 95% for forage proteins. Post-ruminal digestion was virtually finished at the end of the small intestine.

[Protein and amino acid metabolism in the digestive tract growing young bulls. 1. Flow of bacterial raw protein in the duodenum determined by 2,6-diaminopimelic acid as a marker]

The influence of the composition of the rations on the content of diamino pimelic acid (DAP) and N in the bacteria dry matter and on the flow of bacteria crude protein into the duodenum, determined with DAP as marker, was determined on the basis of experimental investigations with young bulls provided with duodenal cannulae and with 28 different rations whose details are described. At a production level corresponding to dry matter intake and a variation range of the crude fibre content of between 11.3 and 29.1% in the DM, a content of N-free extractives between 50.9 and 77.4% in the DM, a content of soluble carbohydrates ibetween 5.3 and 6.4% in the DM, a crude protein content of between 6.4 and 17.1% in the DM and a pure protein content of between 4.9 and 15.5% in the DM, the DAP content of the bacteria DM amounts the 0.350 g 100 g DM +/- 0.090 the N-content of the bacteria DM amounts to 7.37 +/- 1.08 g/100 g DM, there are no relations between DAP- and N-content in the bacteria DM and the content of the individual carbohydrate fractions of the ration, there are positive relations between DAP- and N-content of the bacteria DM, the flow of bacterial crude protein into the duodenum amounts to 133 +/- 14 g/kg truly fermentable organic matter or 130 +/- 14 g/kg apparently digestible organic matter, there is a negative relation between bacteria crude protein at the duodenum (BCPD)/kg truly fermentable organic matter and the crude fibre content of the ration, there is a positive relation between BCPD/kg truly fermentable organic matter and N-free extractives and soluble carbohydrate content as well as the digestibility of the organic matter of the ration, there is no specific influence of the flow rate (kg digesta/kg intake of org. matter) or the dilution rate (g bacteriafree org. matter D/kg LW075/b) on BCPD/kg truly fermentable org. matter, there is a dependence of BCPD/kg truly fermentable org. matter on crude protein concentration in the ration in the concentration range of 6.4-9.0% crude protein in the ration (provided endogenous CP equivalents are used).

[Protein digestion in ruminant calves]

The influence of various protein sources on N-digestion in forestomachs was investigated with 10 calves of the average age of 12 weeks which were supplied with duodenal re-entrant cannulae. 50 and 100% resp. of the soybean coarse meal-N in the standard ration (A) were replaced by urea-N (B) and lucerne-N (C). The crude protein concentration in the test rations varied between 17 and 19%. In the rumen the average NH3-concentration for rations A, B and C amounted to 7.7, 18.9 and 4.5 mg/100 ml resp., the pH-value was 6.4, 6.8 and 6.3 resp. 2.2, 2.4 and 2.7 g bacteria-N were synthesised per 100 g fermented organic matter. There were significant differences as regards the flow of non-ammonia N measured at the duodenum. Related to N-intake, values of 90, 79 and 104% were registered. 3.7, 3.7 and 4.3 g resp. non-NH3-N per 100 g digested organic matter were found at the duodenum. Under consideration of the high protein level of more than 17% (in the dry matter of the ration) required for this phase of growth, a partial replacement of soybean coarse meal-N by urea-N is of little effectivity. In contrast to this, the complete replacement of soybean protein by lucerne protein could guarantee the protein supply of 12-week-old calves.

[Effect of crude protein on fertility and urea content of the body fluids of high-yielding cows]

A total of 36 high-yielding cows were -- post partum -- divided into two analogous groups. During the following 90 days they received rations containing 14.5 resp. 19.1% crude protein. The daily milk yield ranged between 27.8 and 28.8 kg resp. 29.4 and 31 kg FCM. The urea content of the milk reached 16.8 and 33.8, that in the blood plasma 19.7 and 38.8 mg/100 ml. The correlation between urea content in milk and blood serum was r = 0.94, between milk and urea content in urine r = 0.69. The period between gravidities of 82 days (14.4%) in contrast to 127 days (19.1%) was distinctly improved, and so was the insemination index (2.0 resp. 2.8). We consider 14.5% crude protein meet the requirement for yields of 30 kg milk, which is approximately 15% below the norm.