PART II (Contd.)

CHAPTER 12
MAXIMIZING NPN USE IN FEEDING SYSTEMS BASED ON AGRO-INDUSTRIAL BY-PRODUCTS

by

J. Kowalczyk

Institute of Animal Physiology and Nutrition,
Polish Academy of Sciences, Jablonna nr. Warsaw, Poland

Resumen

Los compuestos de nitrógeno no proteínico (NNP) se utilizan normalmente en la alimentación de rumiantes; esta utilización es rentable, especialmente en países que disponen de recursos limitados de piensos proteínicos.

Entre los distintos compuestos de NNP que se utilizan para alimentar a los rumiantes figuran en primer lugar la urea y sus derivados y las sales de amonio. A pesar de las muchas investigaciones que se han hecho sobre los factores que influyen en la utilización del NNP por los rumiantes, no se ha elaborado todavia un modelo apropiado para tener en cuenta todos los parámetros pertinentes. La medida en que se aprovecha el NNP depende principalmente del índice de síntesis proteínica microbiana, el cual está determinado principalmente por la disponibilidad de sustratos. También desempeñan un papel importante Las propiedades y la dosis de los compuestos nitrogenados que se emplean, la cantidad, calidad y solubilidad de la proteína y la naturaleza de los carbohidratos que componen la dieta, la productividad de los animales y sus necesidades de proteínas, la adaptación al NNP y otros constituyentes de la ración, el contenido de minerales de la dieta, el régimen de alimentación, el índice de recirculación del nitrógeno en el organismo y otros factores.

Para que los organismos del rumen utilicen al máximo el NNP, deben realizarse simultáneamente dos procesos en el rumen: la degradación del NNP en amoníaco y la fermentación de los carbohidratos para suministrar energía para la síntesis proteínica microbiana.

El amoníaco se desprende con relativa rapidez del NNP, especialmente de la urea, en el rumen. Se producen ahora algunos preparados de urea con un índice reducido de liberación de amoníaco.

El modo de preparar el NNP y de completar con él las dietas, compuestas por forrajes de cultivo propio o subproductos industriales deficientes en proteínas, depende de las condiciones locales, de la disponibilidad de estos piensos y de la naturaleza del NNP.

Los compuestos de NNP y los piensos completados con NNP que se utilizan más comunemente en la alimentación de rumiantes son:

1. Urea, sales de amonio, biuret -

1. mezclados con piensos concentrados;
2. preparados de urea o de urea y minerales;
3. complemento líquido compuesto de melazas, urea, minerales y vitaminas;
4. complemento de NNP para ensilajes o para forrajes verdes pobres en proteínas durante el ensilamiento;
5. piensos comprimidos formados por un compuesto con una gran proporción de paja triturada, tratada con alcalí o sin tratar, y completada con NNP.

2. Forrajes amoniados que se producen tratando productos tales como paja, pulpa de remolacha azucarera, bagazo, pulpa de agrios, resíduos de destilería, ensilajes o forrajes verdes en la época del ensilamiento.

El complemento de NNP servirá para la finalidad que se pretende, solamente si se añade a los forrajes pobres en proteínas, pero ricos en energía fácilmente disponibles, tales como melazas, pulpa de remolacha azucarera, resíduos de destilería y otros subproductos industriales, plantas enteras de maíz y otros forrajes verdes, patatas, remolacha azucarera, etc. El completar estos forrajes con compuestos baratos de NNP, en lugar de utilizar los piensos ricos en proteínas que son más caros, como la harina de soja o harina de pescado, queda justificado desde el punto de vista económico ya que son semejantes los efectos obtenidos de ambos casos en cuanto a la producción. La utilización de estos forrajes sin el complemento de compuestos de nitrógeno es ineficaz y puede influir negativamente en la salud de los animales.

Si el NNP se emplea en raciones que son pobres en proteína y contienen mucha fibra, es necesario, completar dichas raciones con alimento ricos en energía o hacer más digestible la fibra mediante un tratamiento apropiado. De lo contrario, la utilización de NNP es ineficaz y puede incluso producir intoxicación de amoníaco.

Si se emplean compuestos de NNP para alimentar a los rumiantes, podrá dedicarse una gran proporción de piensos ricos en proteína a la alimentación de otros animales de cría.

Research and discussion on the use of non-protein nitrogen (NPN) in rumlnant feeding have continued for over 80 years since Zuntz (1891) formulated the hypothesis that rumen microorganisms are capable of decomposing cellulose and converting NPN into true protein.

In practical animal husbandry the use of urea in feeding increases steadily. In the U.S.A., where it amounted to only 68 000 tons in 1958 (Loosli and McDonald, 1968), it rose from 465 000 to 685 000 tons between 1965 and 1970 (Fonnesbeck et al., 1975) and exceeded 1 million tons in 1973 (Huber, 1975). Urea and other NPN compounds are also used in large amounts in other countries in America and Europe, including the U.S.S.R. The above figures are a convincing argument for the general and extensive use of NPN in feeding ruminants, the more so as the shortage of protein feeds in the world market is conductive to their rational and economic use and also to the search for new sources of feeds.

Protein and nonprotein nitrogen compounds are, along with carbohydrates, fat, vitamins and minerals, the basic constituents of animal diets. In monogastric animals the breakdown of dietary protein in the stomach and small intestine consists mainly in enzymic hydrolysis to amino acids, which are absorbed from the intestine into the bloodstream for utilization by the animal. In ruminants, dietary protein is already broken down in the rumen, to a greater or lesser extent depending on its nature, by the action of microbial enzymes. However, this composition does not stop at the hydrolysis level: the liberated amino acids are further degraded by deaminating enzymes to ammonla and the corresponding acids. The extent of protein degradation in the rumen, ranging from 30 to 80%, is primarily a direct function of its solubillty.

Apart from protein, the diet contains also low molecular weight nitrogenous compounds which are usually readily soluble and, consequently, especially susceptible to degradation in the rumen with the liberation of ammonia; the latter enters the ammonia pool of the rumen, which also includes ammonia derived from the breakdown of protein and of urea, the latter transferred from the blood across the rumen wall or entering the rumen with saliva. Ammonia liberated in the rumen is utilized by the microorganisms for growth and thus for increasing the amount of microbial protein. In this way a part of dietary protein is converted into microbial protein of high nutritive value. Not all ammonia of the ruminal pool is utilized by the microorganisms. A part, depending in the first instance on rumen ammonia concentration and acidity, is absorbed from the rumen into the blood and is converted in the liver to urea, of which a part is recirculated to the rumen with saliva or across the rumen wall, while the remainder is excreted in urine as the end product of nitrogen metabolism in the ruminant.

There is a close relationship between the quantity of energy obtained from the feed by anaerobic fermentation in the rumen and available to the microorganisms and the yield of microbial matter (Hungate, 1966; Hobson, 1971). The yield of ATP from the fermentation of feeds is proportional to the quantity of organic matter fermented. The type of rumen fermentation, and the kind of end-products, i.e. the molar proportion of volatile fatty acids, have but little effect on the yield of microbial matter (Ørskov et al., 1968; 1974). If, however, a certain microorganism causing a different type of fermentation predominated in the rumen, and if that microorganism were particularly efficient in utilizing ATP, then a more pronounced relationship between type of fermentation and bacterial cell yield would exist.

The rate of fermentation is undoubtedly an important factor influencing the economy of microbial growth. With a low rate of fermentation the rate of microbial growth is also reduced; with rapid fermentation the utilization of the ATP formed is impaired.

Some aspects of bacterial protein synthesis in the rumen

Carbohydrates are the main substrate for rumen bacteria and also the main constituent of the ruminant diet. They are the only substrate from which large amounts of energy can be derived under anaerobic conditions. In contrast to the bacteria for which carbohydrates are the energy source, the growth of those utilizing protein to cover their energy needs under anaerobic conditions is very limited (Stadtman, 1970). Fat is not an energy source under the anaerobic conditions of the rumen, but the energy value of its hydrolytic product, glycerol, is similar to that of carbohydrate. Fat also exerts an inhibitory effect on some rumen microorganisms (Czerkawski, 1966).

The composition of the rumen microbial population varies according to the diet, but even under stabilized feeding conditions the rumen harbours a great variety of species, and composition of microbial protein is relatively constant. The ratio of nucleic acids to protein in the microbial matter also changes little and is therefore used as an index of the synthesis of bacterial protein in the rumen and of its passage to the abomasum (Smith, 1969).

McNaught et al. (1954) and Bergen et al. (1968) found in experiments on rats that the digestibility of bacterial protein ranged from 70 to 75%; Mason and Palmer (1971) obtained similar values. Digestibility values for protozoal protein were higher: 89% (McNaught et al., 1954). The lower digestibility of bacterial protein is probably due to the resistance of bacterial cell walls to the action of enzymes (Mason and White, 1971).

A substantial part, about 40%, of microbial protein produced in the rumen is digested in the abomasum (Kowalczyk et al., 1970).

During recent years the digestion in the rumen of energy-yielding constituents of the ration and the production of microbial protein have been extensively studied. Demeyer et al., (1972) and Thomas (1973) summarized this work and found considerable differences between the results obtained by different workers, due not only to different experimental conditions but also to technical difficulties in estimating the rate of production of microbial matter in the rumen. No satisfactory method has yet been developed for separating dietary and microbial protein. Nuclelc acids or 2,6 Diaminoplmelic acid (DAPA) are frequently used as indicators of the proportion of microbial protein in the digests, but the amounts of these compounds in individual bacterial species differ and some bacteria contain no DAPA at all. More precise estimates can be obtained with the use of 15N, 32P or 35S labelled compounds.

The rate and extent of microbial protein synthesis in the rumen depend on the rate and extent of ammonia liberation, the rate of fermentation of carbohydrates, rumen pH, the rate of absorption of the fermentation products and the number of bacteria. About 20 to 30 g of microbial nitrogen are produced per kg of organic matter digested (Hogan and Weston, 1970; Kowalczyk, 1971; Ørskov, 1972; Miller, 1973; Thomas, 1973).

The nitrogen requirement of rumen bacteria on a given diet can be estimated from the amount and digestibility of organic matter ingested by the animal, bearing in mind that there should be at least 30 g nitrogen, to be hydrolysed to ammonia in the rumen, per kg of digestible organic matter in the diet (Ørskov, 1976). If the basal diet offered to the animals is deficient in nitrogen but contains large amounts of readily fermentable carbohydrates, it is advisable to supplement the diet with NPN so as to ensure the correct proportions of nitrogen and energy for microbial protein synthesis. The magnitude of the latter is rarely limited by nitrogen deficiency, as nitrogen can readlly be added in form of NPN or, in the first instance by the intake of sufficiently large amounts of rumen-digestible organic matter, mainly carbohydrates.

Supplementing diets of relatively low digestible organic matter content with NPN is useless, as the bacteria are unable to utilize all the ammonia released in the rumen, the excess being absorbed into the bloodstream, converted to urea and excreted in the urine.

A rapid ingestion of large amounts of readily digestible organic matter and nitrogenous compounds does not by itself provide the proper conditions for efficient bacterial protein synthesis, as in this instance the rate of ammonia release may exceed the rate of protein synthesis and again ammonia will accumulate and be absorbed from the rumen. It will be recirculated to a certain extent to the rumen, but this entails a considerable loss of nitrogen (Nolan and Leng, 1973; Kowalczyk et al., 1975a, b). It is better, therefore, to offer such diets more frequently but in small amounts. A good example is the use of diets based on molasses and urea, which resulted in fairly good utilization of nitrogen and energy with no accumulation of ammonia or lactic acid in the rumen (Preston and Willis, 1970; Kowalczyk, 1971).

Addition of NPN to diets containing large amounts of readily digestible carbohydrates but small amounts of protein, or protein protected against deamination in the rumen, is particularly advisable, as in such diets there may be a shortage of nitrogen available to the bacteria for growth.

A correct synchronization of the rate of carbohydrate fermentation with the rate of ammonia release is of particular importance for the efficient utilization of NPN. In this respect feeds containing sugars (molasses, sugarcane, sugar beet, etc.) or starch (potatoes, cereals or maize) are better suited for supplementation with NPN than those rich in fibre, as the rate of fermentation of sugars or starches matches better the rate of release of ammonia from NPN; the rate of cellulose degradation is slow, and adequate amounts of energy are not supplied in the presence of excess ammonia.

If NPN is used as a protein replacement, the diets should be supplemented with minerals, especially sulphur, which is a constituent of protein. Sulphur deficlency in the diet can seriously reduce bacterial protein synthesis in the rumen. The N:S ratio should be from 10 to 13.5 : 1 in rations for sheep and from 13.5 to 15 : 1 in those for cattle. The form in which sulphur is added is of no importance because sulphates, thiosulphates or elementary sulphur alike are converted, in the strongly anaerobic environment of the rumen, to H₂S, which is utilized by rumen bacteria for the synthesis of sulphur amino acids.

NPN compounds and NPN preparations

NPN compounds used as protein replacements for ruminants should be inexpensive and well utilized by microorganisms and should not adversely affect the health of animals. Table 12.1 lists some of the compounds of possible importance as protein replacement. Apart from these, others which may be of some importance in the feeding of ruminants can be found in the literature.

Acetamide, glycine and glutamine are hydrolised at a slower rate than urea, and their nitrogen is efficently utilized by ruminants, but their use in practical feeding is limited by their high price.

Biuret is relatively resistant to hydrolysis in the rumen, and a long adaptation period is needed for the microbial population to utilize it efficiently. The conditions for its use and the extent of biuret utilization by the animals are still controversial (Fonnesbeck et al., 1975; Loosli and McDonald, 1968).

Urea and urea preparations

From among these nitrogenous compounds, urea finds widest use in ruminant feeding. The conditions of its effective utilization are also best known (Chomyszyn, 1976).

These are:

1. adaptation of the animals, or rather of the rumen microbial population, to urea feeding. The amounts given should be small at the start and increase gradually throughout the adap tation period, which should be no less than 10 days;

2. careful and uniform mixing with other feeds, as crystalline urea tends to lump and needs grinding. Various substances are added to urea to prevent lumping. Crystalline or granulated urea added to feed mixtures tends to separate during transport and should be redispersed before feeding. Better ways of preventing separation are pelleting of the feeds containing urea or the use of special urea or urea-mineral preparations like “Starea” or “Grysik”;

3. the presence in the diets of adequa te amounts of sugars, starch and minerals, especially S, P, Ca and Co. As previously mentioned, when urea is fed with roughages containing no readily digestible carbohydrates, it is poorly utilized. Best results are obtained if all the necessary components, in proportions appropriate for the given type of production, are combined in a balanced feed given to appetite. Under these conditions the animals eat small amounts of feed frequently, and this favours the effective utilization of NPN by rumen microbes.

As compared to other NPN compounds urea is inexpensive, easy to produce and generally available, but it has also certain disadvantages:

1. too rapid hydrolysis, leading to the accumulation of ammonia in the rumen and its absorption into the bloodstream, reducing its utilization for protein synthesis;

2. urea is highly hygroscopic and tends to aggregate (lump); this impairs its adequate mixing with other feeds;

3. the unpleasant, bitter taste makes feed mixtures with a high proportion of urea unpalatable and adversely affects voluntary intake, especially by cows.

In the course of studies on the use of urea for feeding cattle and sheep, various methods of addition of urea to feeds and diets have been developed.

Urea-mineral preparations

Special preparations with a high content of urea have been developed with a view not only to obviating the disadvantages of urea and eliminating the labour-consuming ways of adding it to feeds, but also to improving its utilization by ruminants. Among preparations extensively tested in vitro and verified in practical feeding are “Starea” and “Dehy-100” (Helmer et al., 1970a, b; Stiles et al., 1970; Conrad and Hibbs, 1968), manufactured on a large scale in the U. S. A. according to technologies protected by patents. They contain, apart from 20 to 32% urea, ground cereal grains or dried potatoes (“Starea”, “Golden-Pro”) and lucerne meal (“Dehy-100”); 5% bentonite or 0.5% propionic acid acting as moisture absorber or conservant are often added. The advantage of these preparations is to moderate the bitter taste of urea, which therefore may be given in fairly large amounts (up to 1 g/kg body weight) with a minimum risk of intoxication. During processing at high temperature and pressure, the starch mixed with urea gelatinizes, forming a homogeneous mass (“Starea”, “Golden-Pro”) which decomposes slowly in the rumen, gradually releasing urea and thus supplying the bacteria simultaneously with ammonia and energy for protein synthesis. Under these circumstances there is little accumulation of ammonia in the rumen and, consequently, only a limited absorption of ammonia into the bloodstream. These preparations proved beneficial in the feeding of high-yielding dairy cows and sheep and replaced soybean meal to a large extent without adversely affecting production.

In other countries, different urea preparations have been developed and tested in practical feeding.

In Hungary, satisfactory results have been obtained in fattening lambs and feeding lactating cows with a preparation in meal form, “Urea-Abdukt”, based on paraffin oils (50%) and urea (50%) (Juhas and Lukas, 1973).

In Czechoslovakia, a pelleted preparation “DL-100” has been manufactured with the following composition: urea 31.8 %; lucerne meal 64%; sodium propionate 0.4 %; Na₂HPO₄ 1,5%; CaCO₃ 1.3 %; and praemix 1%. The preparation was given together with cereal mixture and maize silage to cows with a daily milk yield of 19 liters. There were no differences in milk yield between the cows given “DL-100” and control cows on protein (Vlasak and Mala, 1970).

In Bulgaria, a urea-mineral preparation called “Carbisale” consists of urea 42%; NaCl 34,5%; Ca₃(PO₄)₂ 17%; CaSO₄ 6%; and trace element mixture 0,5%. The performance of fattening lambs given 15 g of this preparation daily was similar to that of lambs on sunflower meal and better than that of lambs given crystalline urea (Aleksiev et al., 1974).

In Scotland, a method has been developed of supplementing barley with urea. The grain is soaked in urea solution and dried. The resulting product is similar in quality to other urea preparations (Ørskov et al., 1974).

In Poland several urea preparations are being produced and tested in experiments with sheep, cows and fattening cattle. The pelleted preparation “KBM” contains 13% urea, 30% rapeseed meal, 35% ground cereals, 5% molasses, 5% grass meal and 12% minerals. It should be used especially with home-produced feeds like silages and root crops. In experiments with cows fed on home-grown feeds and dried maize (whole plants) supplemented with up to 1 kg “KBM” daily, a yearly milk yield of 4 000 liters was obtained with no oilseed meals.

Another urea preparation, called “Grysik”, consists of 32% urea, 42% ground barley and 26% minerals. The thoroughly mixed constituents are steamed at 1500; the starch gelatinizes while urea and minerals dissolve in it. The cooled and ground product can be added to concentrate feeds for cattle and sheep. It can also be added, instead of high-protein feeds, to raw potatoes, beet roots or maize silage. The amount added varies depending on the protein deficiency in the diet. In experiments with cows given 1 g of urea in the form of the “Grysik” preparation per kg body weight, a good daily milk yield was obtained, about 4 000 liters/year, with no signs of intoxication or digestive disorders (Kotarbinska et al., 1975).

Briquets and pellets with NPN

In recent years the manufacture of balanced feeds for winter and summer feeding in the form of pellets and briquets has been started in different countries, including Denmark, France, G.D.R., U.S.S.R. and U. K. Apart from cereal grains they contain large amounts (up to 50%) of straw, sometimes previously treated with sodium hydroxide, and urea (up to 2%), ammonia or ammonium carbonate, as well as other feeds. During processing at high temperature and pressure, urea condenses partly to bluret, releasing ammonia, which acts on the fibre of the feeds, rendering it more readily digestible. Attempts to feed cows and fattening cattle on such briquets have given encouraging results (Bergner and Görsch, 1974).

Molasses with urea

Molasses, the main by-product of sugar manufacture, is a good source of readily fermentable carbohydrates. Combined with urea and a small proportion of concentrates and roughages it can provide the basal diet for fattening cattle. The use of molasses and urea for feeding cattle is dealt with in Chapter 13 of this book.

Ammoniated feeds

Ammoniating is one of the ways of enriching low-protein feeds with nitrogen. Ammonia is released at a slower rate from ammoniated feeds than from urea, and this favours bacterial protein synthesis. The technology of impregnating different feeds with gaseous ammonia or ammonia solution has been developed and patented (Miller, 1942; Stiles, 1952; Chomyszyn, 1966). The quantity of ammonia that can be fixed in the feeds depends on their chemical composition and on the physical conditions of the treatment, especially temperature and pressure.

Ammonia in ammoniated feeds appears in the form of ammonium salts, amide compounds in pectins, and imidazoles; it can also be bound by sugars and lignin. If processing is carried out at high temperature and pressure, products with higher ammonia contents are obtained, but the nitrogen utilization of such feeds by animals is often poorer than that of feeds treated with ammonia under less drastic conditions although the latter contain less nitrogen.

Sugar beet pulp, citrus pulp and apple pomace

Sugar beet pulp, citrus pulp and apple pomace contain relatively large amounts of pectins which readily bind ammonia, forming a stable compound; the nitrogen content of the ammoniated products increases twice or more (Ferguson and Neave, 1943; Miller, 1944; Davis et al., 1952; Ryś and Sokól, 1963; Chomyszyn, 1966). The results of numerous experiments carried out in Poland, with ruminants given different amounts of ammoniated sugar beet pulp, suggest good utilization of nitrogen with this feed. Ammoniated sugar beet pulp is more readily eaten by the animals than untreated pulp, is safe in practical feeding and increases the digestibility of dietary fibre (Abgarowicz et al., 1962; Chomyszyn et al., 1960, 1962, 1972; Loos li and McDonald, 1968; Trela et al., 1970; Chomyszyn and Kowalczyk, 1966). These have been confirmed by German workers in experiments on sheep and dairy cows (Bergner and Krieghoff, 1966; Görsch and Bergner, 1969, 1970; Richter and Oslage, 1961).

Straw

Ammonia treatment of the straw of various cereals doubled its nitrogen content (Bondariev and Gutrych, 1962; Zafren, 1959; Chomyszyn et al., 1964). Experiments on sheep and cattle showed that ammoniating improved the digestibility of fibre (Chomyszyn et al., 1964; Najdenov et al., 1964). Straw contains little readily fermentable carbohydrate, and the addition of feeds such as molasses, ground cereals etc., is therefore advisable. Ammoniated maize meal, prepared from whole plants harvested at the milk-wax stage of ripeness, provided a valuable feed (Chomyszyn et al., 1973); supplemented with minerals it was used as a monodiet for lambs and growing-fattening cattle. Efficiency of feed utilization and liveweight gains were better than with untreated maize meal supplemented with rapeseed meal.

Ammonia treatment of sugarcane bagasse (Davis, 1957), rice hulls (Eng, 1964) and wood wastes (Erlinger et al., 1975) has not taken on importance because of the poor ammonia binding capacity of these products, which consequently yielded little improvement of animal performance. Better results were obtained with fattening cattle fed on sugarcane bagasse enriched with molasses and urea (McDowell et al., 1975). More work is needed to improve the utilization of the cellulose energy of these high-fibre feeds.

Ammoniated molasses and distillers slops

The results obtained with feeding ammoniated molasses are less good than those with molasses supplemented with urea; apart from this, there are reports of harmful effects of ammoniated molasses on the health of animals. Its use is therefore not to be recommended (Richardson, 1954; Tillman et al., 1957; Bartlett and Broster, 1958).

Distillers' slops (by-products of alcohol production from molasses or potatoes), ammoniated and condensed, gave good results in the feeding of dairy cows and fattening cattle when added to straw or hay (Chomyszyn, 1968); Tillman and Kidwell, 1951).

An ammonium lactate preparation is manufactured from molasses fermented to lactic acid, neutralized with ammonia and condensed. The resulting product, a thick brown syrup, can be added to straw, hay or sugar beet pulp. Sheep and cows fed on diets in which up to 40% of oilseed meal nitrogen was replaced by ammonium lactate showed good liveweight gains and milk yield; the milk fat content was slightly higher than with oilseed meals (Bieliński et al., 1965; Chomyszyn et al., 1964, 1966).

Conclusion

These examples of the use of NPN compounds for feeding ruminants do not exhaust all the possibilities of NPN use in livestock feeding. The choice of the best method of use depends on local conditions, the supplies of agro-industrial by-products that can be used as feed, and the equipement available. The agro-industrial by-products are largely unbalanced in their chemical composition, but their supplementation with nitrogen will be useful only if they are deficient in nitrogen and at the same time contain an adequate amount of readily available energy for the conversion of dietary nitrogen into bacterial protein in the rumen. Rations composed of such feeds may be supplemented with additional nitrogen in the form of expensive vegetable or animal protein concentrates or in the form of inexpensive NPN. Replacement of protein with NPN in feeding ruminants would free large amounts of protein for feeding pigs and poultry. Well balanced rations for ruminants, with a correct energy/nitrogen ratio and also containing such other essential nutrients as fibre and minerals, ensure effective conversion of NPN into tissue and milk proteins, as has been proved in numerous experiments with different NPN combinations in all parts of the world.

The classical experiment of Virtanen (1966) with cows fed over several years on a synthetic diet containing NPN as the sole source of nitrogen showed that production of up to 5 000 liters per year of milk of normal composition was possible; a further increase of milk yield was limited by the insufficient voluntary intake of feed and the capability of bacteria to synthesize protein. The maximum intake of urea in this experiment reached 600 g per cow daily with no adverse effect on health and fertility. With partial (up to 40%) replacement of protein with NPN, an average milk yield exceeding 6 000 liters per year was obtained (Conrad and Hibbs, 1968).

Rations for high-yielding cows should be precisely balanced with regard to all nutrients and should be given and libitum, as in this case the intake is spread over a long period during the day, favouring effective conversion of NPN into microbial protein.

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