- Email: [email protected]
Feeds rich in cellulose in ruminant and non-ruminant nutrition
Agriculture and Environment, 6 (1981) 195--204
195
Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands
FEEDS RICH IN CELLULOSE IN RUMINANT AND NON-RUMINANT NUTRITION
A.J.H. VAN ES
Institute for Livestock Feeding and Nutrition Research, Lelystad, and Department of Animal Physiology, Agricultural University, Wageningen (The Netherlands) (Accepted 23 March 1981)
ABSTRACT Van Es, A.J.H., 1981. Feeds rich in cellulose in ruminant and non-ruminant nutrition. Agric. Environm., 6: 195--204. Because m a n is a monogastric, m u c h of the world's plant production cannot be used by him directly as food. This applies not only to pasture and forest produce but also to a considerable part of the produce of arable land (bran, beet- and citrus pulp, oilseed residues, etc.).With the present methods of agriculture, maximal production of food for mankind can best be obtained by a combination of plant and animal husbandry, as will be shown. This is mainly due to the fact that ruminants (through their symbiosis with microbes) can utilize feeds rich in cellulose. Even ruminants often have difficulty in utilizingsuch feeds, as their volume and slow digestion lower voluntary intake and as the nutritive value of the ingested feed per kg is often low. S o m e physiological data are presented which determine the rate of this digestion. A n y treatment which would speed up and improve digestion would result in a higher productivity of these animals. Several methods are k n o w n for this purpose. To test their effectiveness,trialswith animals have to be performed but analytical laboratory measurements are also useful. Such techniques were discussed at a recent E E C workshop at Lelystad, which is reported. Research with pigs fed feeds with higher contents of cellulose is presented and suggestions are made as to h o w these animals might benefit from treatments of these feeds.
INTRODUCTION
The view that food production b y means of animal husbandry is accompanied by considerable energy and nitrogen losses is wide-spread, especially among persons knowing little a b o u t this t y p e of production of food for mankind. It is argued that direct consumption of animal feed by man is often possible and that man might benefit more from this feed, which thus becomes food, than from the small quantity of edible food which the animal is said to make of its feed. However, there are many examples where these arguments do n o t apply. Few persons are willing to eat forages, so important for ruminants. Moreover, little of these feeds, especially if grown in a warm climate, will be of
0304-1131/81/0000--0000/$02.50 © 1981 Elsevier Scientific Publishing Company
196
of value to man as his digestive system lacks cellulase to digest the cellulose of these feeds. Plant cell walls, especially in warmer climates, contain much cellulose. The dairy cow, like all ruminants, does n o t have cellulase itself, b u t the microorganisms in its forestomachs do. They not only convert a part of the plant cell walls into substances which are valuable for the cow b u t also make the plant cell contents more accessible for the digestive enzymes of the host. Moreover, they provide essential amino acids and vitamins of the B complex. Dairy cows and other ruminants also consume, feeds other than forages. These, the so~alled concentrates, may contain ingredients like grains which are directly competitive with man's food supply. Such feeds are therefore expensive, so modern cattle concentrates contain cheaper feedstuffs with higher cellulose contents. The food industry produces large quantities of byproducts like bran (bread), molasses and beet pulp (sugar), glutens (starch), brewers' grains (beer, spirits), oilseed expellers and extracted meals (oils and fats), whey (cheese), and animal fats (meat). These are n o t liked much b y man b u t are well suited for animals, those higher in cellulose especially for ruminants. Due to the ruminant's symbiosis with microorganisms, ingested sugar, starch and protein is utilized 10--20% less efficiently than in the case of non-ruminants. Single-stomach farm animals like pigs and poultry are far more competitive with man with regard to food consumption than ruminants. But even in this category some compensation takes place, usually a consequence of economy: potential foods for man which are polluted with soil, dirt, or contaminants and thus rejected for human use, are cheap and are often still good feed for these animals which are less particular as to what they eat, and better able to defend themselves against contaminants, the more so as they do not reach an advanced age. Good softs usually produce per hectare more food for man (energy: up to 5 times as much; protein: up to twice as much) via plant husbandry than via animal husbandry. However, because of their high water level, peaty softs are often only suited for animal husbandry. Moreover, most low-quality softs in warmer countries can be used only for grazing animals because they produce such small quantities of natural vegetation, often high in cellulose, or they are t o o hilly. In The Netherlands, b u t also over the world as a whole, only one third of the land used as pasture can be used for plant husbandry. Such a conversion is at present economically not attractive. Thus for maximizing a country's or the world's production of food for man, both plant and animal husbandry are needed. Farm animals should graze the softs n o t suited for arable crops and furthermore eat all the byproducts of plant husbandry and the food industry as well as rejected edible products. However, farm animals need much feed to maintain themselves, so that for them to contribute considerably to man's food production they need to be fairly productive. In the following sections ways in which farm animals m a y assist most in the production of food for man are examined.
197 MAN'S REQUIREMENTS FOR ENERGY, PROTEIN, MINERALS AND VITAMINS
Man requires his food mainly to maintain himself and to be able to perform mental and physical activity. Mental activity requires little food energy. Most of it is passive and included in man's maintenance metabolism. Active use of the brain hardly increases this metabolism at all. The major fuel for the maintenance functions of the b o d y is the free energy stored in the terminal phosphate bond of adenosinetriphosphate (ATP). The same is true for physical activity, a combination of several muscle contractions. Thus we can say that the major energy need is a sufficient supply of ATP. Even during pregnancy and lactation, maintenance is 70% or more of total metabolism so that even in this case a sufficient ATP supply is of greatest importance. This applies also to the growth of the human baby: it grows relatively slowly; more than 70% of its metabolism is maintenance metabolism. Man needs protein for enzyme and hormone synthesis, for renewal of worn tissues or cells and for pregnancy, lactation and growth. The amount of protein needed for enzymes and hormones is small. Part of the amino acids of the protein of these enzymes and hormones and of worn tissues is utilized again. Man's total protein requirements are fairly low. In synthesizing protein from amino acids, ATP is needed to make peptide bounds. Again there is a requirement for ATP. The same applies to some degree also to fat deposition. The needs for minerals and vitamins from a quantitative point of view are much lower than those for protein. Therefore for our purpose we can evaluate feeds and food by their ability to supply man with ATP and protein at the tissue level. This means that we have to predict h o w much of these substances can be obtained from different feeds. The ATP potential o f a feed is closely related to its content of metabolizable energy (ME) (Nehring and Schiemann, 1966; Armstrong, 1969). According to Atwater, for human foods the ME content in kcal/g or in kJ/g is calculated by multiplying the protein and carbohydrate content (g/kg) by 4 or 16.74 and the fat content by 9 or 37.66. This calculation assumes high digestibilities of the foods, of 92, 98 and 95% respectively. Feeds for farm animals, especially those for ruminants, have much lower digestibilities for man, a monogastric with little microbial activity in the hindgut. Therefore when predicting the ME content of the feeds or foods to predict their ATP potential for man, we should use the actual digestibilities for man rather than Atwater's rule. Few actual digestibility measurements have been done with man, however. Fortunately, pigs of 30--100 kg and mature cocks digest feeds and foods a b o u t as well as man. So we can estimate man's ME content (ME h) for feedstuffs simply b y using the digestibility data listed in the various feeding tables (DLG, 1970; CVB, 1977) for pigs. The MEh content (in kcal or kJ) can be derived from composition and digestibility with good precision b y multiplying digestible protein, fat, crude fibre + Nfree extractives by 4.4, 9.4 and 4.1 or 1 8 . 4 1 , 3 9 . 3 3 and 17.15 respectively
198 and adding the results (Schiemann et al., 1971). For poultry these feed tables list the ME content for this animal species, so here no further calculations are needed. Also for protein it holds true that man can only utilize the digestible part of it. Here again for feedstuffs we cannot use Atwater's high digestibility of 92% b u t should use digestibilities from the above-mentioned feed tables for pigs to predict the c o n t e n t of digestible protein for man (DPh). For simplicity we shall neglect the differences in nutritive value between (digestible) proteins resulting from their amino acid composition. We can therefore assess the nutritive value for man of foods and feeds (MEh and DPh) with good precision from their c o n t e n t of ME and (apparent digestible protein for pigs and poultry. FOOD ACCEPTABILITY Bad taste or smell, high cellulose content, and contamination with dirt or chemicals are some of the reasons w h y man is rather particular as to which feeds and foods to eat. We have to account for these differences in acceptability. Two situations will be distinguished: (1) great food shortage; and (2) ample f o o d supply. It is assumed that man will eat all feedstuffs for farm animals except forages in the first situation, while in the other man is assumed to eat none of the forages, 50% of the concentrate mixtures for ruminants, 60% of those for pigs, 75% of those for poultry and all of the artificial milk mixtures for veal calves. For countries like The Netherlands where the concentrate mixtures contain a high percentage of by-products which are not very competitive with foods for man, man very probably will eat still less of these feeds than is assumed above. For countries where the concentrate mixture contains much grain, some of the figures are t o o low. In fact correct acceptability figures can only be given when the composition of the concentrate mixture is known. The figures for the second situation given above hold true for The Netherlands. Multiplication of the MEh and DPh contents of the feeds of our farm animals by the acceptability indicates the nutritive value of these feeds for man (MEh,a and DPh,a, respectively). With regard to animal products, their nutritive value for man should include only the edible part. Thus for meat only the MEh and DPh of the carcass should be used while calculating MEh,a and DPh, a. For all edible animal products it holds true that ME h and DPh are close to 90% of total energy and 95% of total protein, respectively. COMPARISON BETWEEN INPUT AND OUTPUT OF MEh,a AND DPh,a FOR VARIOUS ANIMAL SPECIES DURING THEIR LIVES When we k n o w the average intake for a c o u n t r y of the various feedstuffs by a given t y p e of farm animal and the animal's average production during its
199
0
°~
D-~°~
"~
0
~o
°~
LO
D'-
0 °~
P~
:o
0
Q;
,.~ 0 0
.o 0
~o
0
0
0
0
~
0
~
0
~
~w o
o
~o
°~
z~ t-i
~
200 whole life, we can calculate the output--input ratio for MEh, a and DPh,a calculated according to the rules given above. It should be stressed that in the input the feed needed for the parent animals and for rearing young animals has also been included. Table I (Van Es, 1975) gives a survey of all the results of output--input ratios obtained for The Netherlands. Columns are added which give the ratios for a situation of great food shortage, for example, during a war. It is clear that in monogastrics input of MEh,a and DPh,a considerably surpasses the output. Obviously there is a loss with regard to food supply if man prefers meat and eggs over the edible part of the feeds eaten by these animals. The higher the animal's rate of production, the smaller are the losses. Hence a further reduction of these losses could be achieved by using animals with a higher production level t h r o u g h o u t life. Another possibility is using more feeds which are less favoured by man. Table I also contains a column giving the ratio of edible energy o u t p u t versus total energy input for some farm animals as computed by Holmes (1970). Holmes' ratio figures are very low in comparison with the other ratios. They disregard the fact that only part of the input energy, especially in the case of ruminant feeds, can be of any nutritive use to man. For the older ruminants the ratios are above one, a result of the fact that these animals, with the aid of their rumen microbes, can utilize cellulose so well and can even convert non-protein nitrogen into protein. The high-yielding dairy cow has the highest ratio. However, owing to an intensification of dairy husbandry in The Netherlands during recent years the ratio has decreased somewhat (Table I). Because o f the low net profit per animal (product prices went up much more slowly than those of labour and feeds) the Dutch dairy farmer increased his herdsize. The required extra feed was derived mainly from concentrates, as pasture in The Netherlands is scarce. Thus, the ratio decreased but still remained well above one, because higher milk yields per animal were obtained and because the concentrate mixtures used contained more by-products. Research aimed at improving knowledge of the nutritive value of forages and by-products helped greatly in promoting their use. FURTHER IMPROVEMENT IN THE CONVERSION OF FEED INTO FOOD It follows from the last section that there are two main ways to improve the conversion of feed into food: (1) using in the feed more materials which are less acceptable and utilizable by man; and (2) using high production levels t h r o u g h o u t the animal's life. The first needs no comment. The second is a consequence of the fact t h a t high amounts of energy are needed to maintain the animal, i.e. to keep its body in a good state. For example, the dally a m o u n t of feed needed for maintenance is equal to the feed required (above the a m o u n t needed for maintenance) to produce 12 kg milk in a dairy cow, about 0.4 and 1 kg live weight gain in a pig of 90 kg and in a steer of 500 kg, respectively, and about 1.5 egg in a laying hen. Therefore, in the dairy cow,
201 a short rearing period, high production levels during lactation, short dry periods and a high number of lactations improve the conversion efficiency most. Similar reasoning holds true for the other types of animals.
Monogastrics High production levels can be reached only b y animals which eat a lot and which are genetically productive. However, when monogastrics are offered feeds which, due to their higher cellulose content, are not acceptable or utilizable by man, feed intake, and in particular, intake of metabolizable nutrients, may be low. As mentioned above, the cellulose and hemicelluloses cannot be digested by the animals' own enzymes. The indigestible part of the feed fills the digestive tract and this may influence feed intake negatively. Finer grinding of the feed might prevent this reduction of intake; it might also make the feed more suitable for enzymatic digestion. Perhaps in the future a pretreatment with (hemi)cellulases might help. Hydrolysis of the (hemi) celluloses by other means might be beneficial also. However, such methods will be costly. Another possibility is to make use of the (hemi)celluloses of the feed via microbial fermentation in the hindgut since the resulting fermentation products (such as volatile fatty acids, VFA) are easily absorbed into the blood. However, in poultry there is little fermentation in the hindgut. Henkel (1980) concluded from his own work that the pig does not benefit from hindgut fermentation. Just (1979) found a small benefit in pigs b u t it decreased at higher plant cell wall levels in the feed. At this institute, in energy balance experiments with rapidly growing boars fed rations with high levels of byproducts, hardly any increase in methane production was found in comparison with a diet based on cereals and soybean meal. This suggests a low level of fermentation at this high feeding level and also, therefore, little benefit to the animal. This may be due to a high rate of passage of the by-product feeds, leaving t o o little time for microbial fermentation in the hindgut (in man fibre has been found to stimulate the rate of passage of digesta). In these experiments the digestibility of the diets was measured as well as the utilization of the digested energy for maintenance and production. Fermentation of plant cell walls in the hindgut might be of some benefit to the energy supply to mature breeding pigs which are kept at lower feeding levels, giving more time for fermentation. It may be concluded that pigs and poultry obtain little benefit from fermentation of untreated (hemi)cellulose in the hindgut. Treatment of the (hemi)cellulose might help if its reaction products could be absorbed in the small intestine. The use of feeds with higher (hemi)cellulose content than preferred b y man should be stimulated to achieve maximum food production because besides (hemi)cellulose such feeds often contain considerable amounts of starch, protein and fat. The other alternative is to offer such feeds to ruminants which can utilize part of the (hemi)cellulose b u t are
202 a b o u t 10--20% less efficient than monogastrics in utilizing the feed's starch, sugar and protein. This lower efficiency of utilization by ruminants is also the reason why, for maximizing food production, low-cellulose materials rejected as food for man because of reduced palatability, bad smell, contamination etc., might better be fed to productive pigs or poultry than to ruminants. With increasing (hemi)cellulose content of feeds, the advantage of the better utilization of starch, sugar and protein b y the pig is soon outweighed by the utilization of (hemi)cellulose by the ruminant.
Feed analysis The evaluation of the nutritive value of feeds for monogastrics would benefit from an analytical m e t h o d which separates those parts of the diet which can be utilized only via microbial fermentation. Treatment of the feed with neutral detergent gives a fairly good estimate of that part b u t feeds rich in starch or pectins need pretreatments to facilitate filtration (EAAP, 1980). Another b u t more laborious way is to analyse for protein, fat, starch, sugar and ash and calculate cell wall by difference.
Ruminants Although ruminants, by their symbiosis with microbes, can benefit from feeds rich in (hemi)cellulose, the benefit may differ considerably from feed to feed. Roughly, plant cell wall can be said to contain a lignified and an unlignified component. The first c o m p o n e n t is partly, the other c o m p o n e n t is easily degradable by microbes. With advancing maturity the first c o m p o n e n t becomes more lignified while the hemicelluloses, lignin and some other chemical groups form a network around cellulose which becomes more and more dense. It blocks the cellulases from hydrolyzing the cellulose; moreover, the hemicelluloses of the network become less degradable (Morrison, 1979). For optimal microbial degradation the pH should be above 6.0 and there should be available ammonia, some peptides and probably also some soluble carbohydrate to overcome the initial period of energy shortage during which the microbes attach themselves to the cell wall particles. The degree of degradation under these optimal conditions differs with plant species, with plant tissue, with maturity and m a y b e even with the climate during the plant's growth due to changes in plant growth with age which differ between species and tissues. In the case of a slow cell wall degradation the breakdown of long forage particles will take much time. It is a long time until these particles are small enough to leave the forestomachs through the narrow reticulo-omasal orifice. While in the forestomachs, they further hamper feed uptake. As a result, total nutrient absorption will be low so that the production level will also be low. Grinding the long particles increases voluntary feed intake considerably (Van der Honing, 1975) but, due to the lower retention time in the forestomachs,
203 microbial breakdown of cell wall is less, resulting in a lower digestibility. However, the gain due to the higher voluntary intake far outweighs the loss due to the lower digestibility. Treatment of the feeds with alkali, discussed by others at this workshop, is more useful. The long feed particles will disintegrate sooner because the possibility of microbial attack of the plant cell walls has been increased and this allows higher feed intake. Due to an increased fermentation a greater quantity of absorbable nutrients will become available to the animal.
Feed analysis It is very useful to have laboratory techniques to predict the nutritive value of feeds for ruminants and to test the efficacy of procedures to improve their quality. It is clear that not only a m e t h o d is needed to partition the feed into a -- readily available -- cell c o n t e n t part and a -- less available -cell wall part, but also one which partitions the cell wall part into a degradable and an undegradable part. Treatment with neutral detergent does the first separation. In the case of feeds with higher starch contents, filtration may be difficult. In forages we found that the m e t h o d of drying the sample (by freeze-drying or by drying at 60--70 ° C) influenced the residue after neutral detergent considerably. Neither the determination of crude fibre nor of acid detergent fibre gives the required content of degradable cell wall as the separation techniques of these methods do n o t partition in terms of degradability. The best way would be to incubate neutral detergent fibre with rumen fluid or with a cellulolytic enzyme preparation. The first procedure necessitates the presence of donor animals. Cellulolytic enzyme preparations at present on the market contain other enzymes than rumen fluid; moreover, their strength is variable. The various difficulties mentioned probably explain why the determination of the in vitro digestibility using 48-h incubation with rumen fluid followed by one of similar length with pepsin of the ground feed sample is still the most popular for precise prediction of apparent digestibility. It has become clear that for the highest comparability with in vivo digestibilities, samples with known in vivo digestibilities, treated in the same way as the samples to be tested, should be used as standards together with the test samples in the same in vitro run (EAAP, 1980; IRDC, 1980) . The necessity of using such standards of similar type is because this in vitro procedure is still a poor imitation of what actually happens in the animal. In vivo the feed particles may stay shorter or longer than 48 h in the forestomachs, and the rate of microbial degradation in vitro of the ground sample m a y differ from that in the forestomachs. The use of standards makes it possible to correct the in vitro results toward the in vivo situation. Other prediction methods based on relationships between in vivo digestibility at about maintenance feeding level and e.g. crude fibre, acid detergent fibre, stage of maturity, etc. have a considerably lower precision. This is even more so for feeds treated mechanically or chemically.
204 REFERENCES Armstrong, D.G., 1969. Cell bioenergetics and energy metabolism. In: W. Lenkeit, K. Breirem and E. Crasemann (Editors), Handbuch der Tierern~'hrung I. Allgemeine Grundlagen. Paul Parey, Hamburg/Berlin, pp. 385--414. CVB, 1977. Veevoedertabel. Centraal Veevoederbureau in Nederland, Wageningen, The Netherlands, pp. B 1--B 17. DLG, 1970. Futterwerttabelle fiir Schweine. DLG-Verlag, Frankfurt am Main, 32 pp. EAAP, 1980. A.J.H. van Es and J. van der Meer (Editors), Workshop on Methodology of Analysis of Feedingstuffs for Ruminants. Preprint N 5.4 of 31. Annual EAAP Meeting, 106 pp. Henkel, H., 1980.34. Tagung Gesellschaft ffir Ern~/hrungsphysiologie der Haustiere, GSttingen, Z. Tierphysiol. Tierern~ihr. Futtermittelkd., 44: 34--36. Holmes, W., 1970. Animals for food. Proc. Nutr. Soc., 29: 237--244. IDRC/IUNS, 1980. W.J. Pigden, C.C. Balch and M. Graham (Editors), Workshop on Standardization of Analytical Methodology of Feeds. IDRC, Ottawa, Ont., 134e: 7--14, 49--78. Just, A., 1979. The influence of diet composition and site of absorption on the efficiency of utilization of ME in growing pigs. Proc. 8th Syrup. Energy Metab. Farm Animals, EAAP. In: L.E. Mount (Editor), Energy Metabolism. Butterworths, London/Boston, pp. 27--30. Morrison, I.M., 1979. Carbohydrate chemistry and rumen digestion. Proc. Nutr. Soc., 38: 269--274. Nehring, K. and Sehiemann, R., 1966. Die energetische Bewertung der Nahrungs- und Futterstoffe. In: A. Hock (Editor), Handbuch der vergleichenden Ernahrungslehre. VEB Fischer Verlag, Jena, pp. 581--633. Scheimann, R. et al., 1971. Energetische Futterbewertung und Energienormen. VEB Deutscher Landwirtschaftsverlag,Berlin, 344 pp. Van der Honing, Y., 1975. Intake and utilization of energy of rations with pelleted forages by dairy cows. Diss. Agric. Res. Rep. 836, Pudoc, Wageningen, The Netherlands, 156 pp. Van Es, A.J.H., 1975. Losses and gains of energy during production of food for human consumption in animal husbandry. Agricultura (Leuven, Belgium), 23: 359--374.
195
Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands
FEEDS RICH IN CELLULOSE IN RUMINANT AND NON-RUMINANT NUTRITION
A.J.H. VAN ES
Institute for Livestock Feeding and Nutrition Research, Lelystad, and Department of Animal Physiology, Agricultural University, Wageningen (The Netherlands) (Accepted 23 March 1981)
ABSTRACT Van Es, A.J.H., 1981. Feeds rich in cellulose in ruminant and non-ruminant nutrition. Agric. Environm., 6: 195--204. Because m a n is a monogastric, m u c h of the world's plant production cannot be used by him directly as food. This applies not only to pasture and forest produce but also to a considerable part of the produce of arable land (bran, beet- and citrus pulp, oilseed residues, etc.).With the present methods of agriculture, maximal production of food for mankind can best be obtained by a combination of plant and animal husbandry, as will be shown. This is mainly due to the fact that ruminants (through their symbiosis with microbes) can utilize feeds rich in cellulose. Even ruminants often have difficulty in utilizingsuch feeds, as their volume and slow digestion lower voluntary intake and as the nutritive value of the ingested feed per kg is often low. S o m e physiological data are presented which determine the rate of this digestion. A n y treatment which would speed up and improve digestion would result in a higher productivity of these animals. Several methods are k n o w n for this purpose. To test their effectiveness,trialswith animals have to be performed but analytical laboratory measurements are also useful. Such techniques were discussed at a recent E E C workshop at Lelystad, which is reported. Research with pigs fed feeds with higher contents of cellulose is presented and suggestions are made as to h o w these animals might benefit from treatments of these feeds.
INTRODUCTION
The view that food production b y means of animal husbandry is accompanied by considerable energy and nitrogen losses is wide-spread, especially among persons knowing little a b o u t this t y p e of production of food for mankind. It is argued that direct consumption of animal feed by man is often possible and that man might benefit more from this feed, which thus becomes food, than from the small quantity of edible food which the animal is said to make of its feed. However, there are many examples where these arguments do n o t apply. Few persons are willing to eat forages, so important for ruminants. Moreover, little of these feeds, especially if grown in a warm climate, will be of
0304-1131/81/0000--0000/$02.50 © 1981 Elsevier Scientific Publishing Company
196
of value to man as his digestive system lacks cellulase to digest the cellulose of these feeds. Plant cell walls, especially in warmer climates, contain much cellulose. The dairy cow, like all ruminants, does n o t have cellulase itself, b u t the microorganisms in its forestomachs do. They not only convert a part of the plant cell walls into substances which are valuable for the cow b u t also make the plant cell contents more accessible for the digestive enzymes of the host. Moreover, they provide essential amino acids and vitamins of the B complex. Dairy cows and other ruminants also consume, feeds other than forages. These, the so~alled concentrates, may contain ingredients like grains which are directly competitive with man's food supply. Such feeds are therefore expensive, so modern cattle concentrates contain cheaper feedstuffs with higher cellulose contents. The food industry produces large quantities of byproducts like bran (bread), molasses and beet pulp (sugar), glutens (starch), brewers' grains (beer, spirits), oilseed expellers and extracted meals (oils and fats), whey (cheese), and animal fats (meat). These are n o t liked much b y man b u t are well suited for animals, those higher in cellulose especially for ruminants. Due to the ruminant's symbiosis with microorganisms, ingested sugar, starch and protein is utilized 10--20% less efficiently than in the case of non-ruminants. Single-stomach farm animals like pigs and poultry are far more competitive with man with regard to food consumption than ruminants. But even in this category some compensation takes place, usually a consequence of economy: potential foods for man which are polluted with soil, dirt, or contaminants and thus rejected for human use, are cheap and are often still good feed for these animals which are less particular as to what they eat, and better able to defend themselves against contaminants, the more so as they do not reach an advanced age. Good softs usually produce per hectare more food for man (energy: up to 5 times as much; protein: up to twice as much) via plant husbandry than via animal husbandry. However, because of their high water level, peaty softs are often only suited for animal husbandry. Moreover, most low-quality softs in warmer countries can be used only for grazing animals because they produce such small quantities of natural vegetation, often high in cellulose, or they are t o o hilly. In The Netherlands, b u t also over the world as a whole, only one third of the land used as pasture can be used for plant husbandry. Such a conversion is at present economically not attractive. Thus for maximizing a country's or the world's production of food for man, both plant and animal husbandry are needed. Farm animals should graze the softs n o t suited for arable crops and furthermore eat all the byproducts of plant husbandry and the food industry as well as rejected edible products. However, farm animals need much feed to maintain themselves, so that for them to contribute considerably to man's food production they need to be fairly productive. In the following sections ways in which farm animals m a y assist most in the production of food for man are examined.
197 MAN'S REQUIREMENTS FOR ENERGY, PROTEIN, MINERALS AND VITAMINS
Man requires his food mainly to maintain himself and to be able to perform mental and physical activity. Mental activity requires little food energy. Most of it is passive and included in man's maintenance metabolism. Active use of the brain hardly increases this metabolism at all. The major fuel for the maintenance functions of the b o d y is the free energy stored in the terminal phosphate bond of adenosinetriphosphate (ATP). The same is true for physical activity, a combination of several muscle contractions. Thus we can say that the major energy need is a sufficient supply of ATP. Even during pregnancy and lactation, maintenance is 70% or more of total metabolism so that even in this case a sufficient ATP supply is of greatest importance. This applies also to the growth of the human baby: it grows relatively slowly; more than 70% of its metabolism is maintenance metabolism. Man needs protein for enzyme and hormone synthesis, for renewal of worn tissues or cells and for pregnancy, lactation and growth. The amount of protein needed for enzymes and hormones is small. Part of the amino acids of the protein of these enzymes and hormones and of worn tissues is utilized again. Man's total protein requirements are fairly low. In synthesizing protein from amino acids, ATP is needed to make peptide bounds. Again there is a requirement for ATP. The same applies to some degree also to fat deposition. The needs for minerals and vitamins from a quantitative point of view are much lower than those for protein. Therefore for our purpose we can evaluate feeds and food by their ability to supply man with ATP and protein at the tissue level. This means that we have to predict h o w much of these substances can be obtained from different feeds. The ATP potential o f a feed is closely related to its content of metabolizable energy (ME) (Nehring and Schiemann, 1966; Armstrong, 1969). According to Atwater, for human foods the ME content in kcal/g or in kJ/g is calculated by multiplying the protein and carbohydrate content (g/kg) by 4 or 16.74 and the fat content by 9 or 37.66. This calculation assumes high digestibilities of the foods, of 92, 98 and 95% respectively. Feeds for farm animals, especially those for ruminants, have much lower digestibilities for man, a monogastric with little microbial activity in the hindgut. Therefore when predicting the ME content of the feeds or foods to predict their ATP potential for man, we should use the actual digestibilities for man rather than Atwater's rule. Few actual digestibility measurements have been done with man, however. Fortunately, pigs of 30--100 kg and mature cocks digest feeds and foods a b o u t as well as man. So we can estimate man's ME content (ME h) for feedstuffs simply b y using the digestibility data listed in the various feeding tables (DLG, 1970; CVB, 1977) for pigs. The MEh content (in kcal or kJ) can be derived from composition and digestibility with good precision b y multiplying digestible protein, fat, crude fibre + Nfree extractives by 4.4, 9.4 and 4.1 or 1 8 . 4 1 , 3 9 . 3 3 and 17.15 respectively
198 and adding the results (Schiemann et al., 1971). For poultry these feed tables list the ME content for this animal species, so here no further calculations are needed. Also for protein it holds true that man can only utilize the digestible part of it. Here again for feedstuffs we cannot use Atwater's high digestibility of 92% b u t should use digestibilities from the above-mentioned feed tables for pigs to predict the c o n t e n t of digestible protein for man (DPh). For simplicity we shall neglect the differences in nutritive value between (digestible) proteins resulting from their amino acid composition. We can therefore assess the nutritive value for man of foods and feeds (MEh and DPh) with good precision from their c o n t e n t of ME and (apparent digestible protein for pigs and poultry. FOOD ACCEPTABILITY Bad taste or smell, high cellulose content, and contamination with dirt or chemicals are some of the reasons w h y man is rather particular as to which feeds and foods to eat. We have to account for these differences in acceptability. Two situations will be distinguished: (1) great food shortage; and (2) ample f o o d supply. It is assumed that man will eat all feedstuffs for farm animals except forages in the first situation, while in the other man is assumed to eat none of the forages, 50% of the concentrate mixtures for ruminants, 60% of those for pigs, 75% of those for poultry and all of the artificial milk mixtures for veal calves. For countries like The Netherlands where the concentrate mixtures contain a high percentage of by-products which are not very competitive with foods for man, man very probably will eat still less of these feeds than is assumed above. For countries where the concentrate mixture contains much grain, some of the figures are t o o low. In fact correct acceptability figures can only be given when the composition of the concentrate mixture is known. The figures for the second situation given above hold true for The Netherlands. Multiplication of the MEh and DPh contents of the feeds of our farm animals by the acceptability indicates the nutritive value of these feeds for man (MEh,a and DPh,a, respectively). With regard to animal products, their nutritive value for man should include only the edible part. Thus for meat only the MEh and DPh of the carcass should be used while calculating MEh,a and DPh, a. For all edible animal products it holds true that ME h and DPh are close to 90% of total energy and 95% of total protein, respectively. COMPARISON BETWEEN INPUT AND OUTPUT OF MEh,a AND DPh,a FOR VARIOUS ANIMAL SPECIES DURING THEIR LIVES When we k n o w the average intake for a c o u n t r y of the various feedstuffs by a given t y p e of farm animal and the animal's average production during its
199
0
°~
D-~°~
"~
0
~o
°~
LO
D'-
0 °~
P~
:o
0
Q;
,.~ 0 0
.o 0
~o
0
0
0
0
~
0
~
0
~
~w o
o
~o
°~
z~ t-i
~
200 whole life, we can calculate the output--input ratio for MEh, a and DPh,a calculated according to the rules given above. It should be stressed that in the input the feed needed for the parent animals and for rearing young animals has also been included. Table I (Van Es, 1975) gives a survey of all the results of output--input ratios obtained for The Netherlands. Columns are added which give the ratios for a situation of great food shortage, for example, during a war. It is clear that in monogastrics input of MEh,a and DPh,a considerably surpasses the output. Obviously there is a loss with regard to food supply if man prefers meat and eggs over the edible part of the feeds eaten by these animals. The higher the animal's rate of production, the smaller are the losses. Hence a further reduction of these losses could be achieved by using animals with a higher production level t h r o u g h o u t life. Another possibility is using more feeds which are less favoured by man. Table I also contains a column giving the ratio of edible energy o u t p u t versus total energy input for some farm animals as computed by Holmes (1970). Holmes' ratio figures are very low in comparison with the other ratios. They disregard the fact that only part of the input energy, especially in the case of ruminant feeds, can be of any nutritive use to man. For the older ruminants the ratios are above one, a result of the fact that these animals, with the aid of their rumen microbes, can utilize cellulose so well and can even convert non-protein nitrogen into protein. The high-yielding dairy cow has the highest ratio. However, owing to an intensification of dairy husbandry in The Netherlands during recent years the ratio has decreased somewhat (Table I). Because o f the low net profit per animal (product prices went up much more slowly than those of labour and feeds) the Dutch dairy farmer increased his herdsize. The required extra feed was derived mainly from concentrates, as pasture in The Netherlands is scarce. Thus, the ratio decreased but still remained well above one, because higher milk yields per animal were obtained and because the concentrate mixtures used contained more by-products. Research aimed at improving knowledge of the nutritive value of forages and by-products helped greatly in promoting their use. FURTHER IMPROVEMENT IN THE CONVERSION OF FEED INTO FOOD It follows from the last section that there are two main ways to improve the conversion of feed into food: (1) using in the feed more materials which are less acceptable and utilizable by man; and (2) using high production levels t h r o u g h o u t the animal's life. The first needs no comment. The second is a consequence of the fact t h a t high amounts of energy are needed to maintain the animal, i.e. to keep its body in a good state. For example, the dally a m o u n t of feed needed for maintenance is equal to the feed required (above the a m o u n t needed for maintenance) to produce 12 kg milk in a dairy cow, about 0.4 and 1 kg live weight gain in a pig of 90 kg and in a steer of 500 kg, respectively, and about 1.5 egg in a laying hen. Therefore, in the dairy cow,
201 a short rearing period, high production levels during lactation, short dry periods and a high number of lactations improve the conversion efficiency most. Similar reasoning holds true for the other types of animals.
Monogastrics High production levels can be reached only b y animals which eat a lot and which are genetically productive. However, when monogastrics are offered feeds which, due to their higher cellulose content, are not acceptable or utilizable by man, feed intake, and in particular, intake of metabolizable nutrients, may be low. As mentioned above, the cellulose and hemicelluloses cannot be digested by the animals' own enzymes. The indigestible part of the feed fills the digestive tract and this may influence feed intake negatively. Finer grinding of the feed might prevent this reduction of intake; it might also make the feed more suitable for enzymatic digestion. Perhaps in the future a pretreatment with (hemi)cellulases might help. Hydrolysis of the (hemi) celluloses by other means might be beneficial also. However, such methods will be costly. Another possibility is to make use of the (hemi)celluloses of the feed via microbial fermentation in the hindgut since the resulting fermentation products (such as volatile fatty acids, VFA) are easily absorbed into the blood. However, in poultry there is little fermentation in the hindgut. Henkel (1980) concluded from his own work that the pig does not benefit from hindgut fermentation. Just (1979) found a small benefit in pigs b u t it decreased at higher plant cell wall levels in the feed. At this institute, in energy balance experiments with rapidly growing boars fed rations with high levels of byproducts, hardly any increase in methane production was found in comparison with a diet based on cereals and soybean meal. This suggests a low level of fermentation at this high feeding level and also, therefore, little benefit to the animal. This may be due to a high rate of passage of the by-product feeds, leaving t o o little time for microbial fermentation in the hindgut (in man fibre has been found to stimulate the rate of passage of digesta). In these experiments the digestibility of the diets was measured as well as the utilization of the digested energy for maintenance and production. Fermentation of plant cell walls in the hindgut might be of some benefit to the energy supply to mature breeding pigs which are kept at lower feeding levels, giving more time for fermentation. It may be concluded that pigs and poultry obtain little benefit from fermentation of untreated (hemi)cellulose in the hindgut. Treatment of the (hemi)cellulose might help if its reaction products could be absorbed in the small intestine. The use of feeds with higher (hemi)cellulose content than preferred b y man should be stimulated to achieve maximum food production because besides (hemi)cellulose such feeds often contain considerable amounts of starch, protein and fat. The other alternative is to offer such feeds to ruminants which can utilize part of the (hemi)cellulose b u t are
202 a b o u t 10--20% less efficient than monogastrics in utilizing the feed's starch, sugar and protein. This lower efficiency of utilization by ruminants is also the reason why, for maximizing food production, low-cellulose materials rejected as food for man because of reduced palatability, bad smell, contamination etc., might better be fed to productive pigs or poultry than to ruminants. With increasing (hemi)cellulose content of feeds, the advantage of the better utilization of starch, sugar and protein b y the pig is soon outweighed by the utilization of (hemi)cellulose by the ruminant.
Feed analysis The evaluation of the nutritive value of feeds for monogastrics would benefit from an analytical m e t h o d which separates those parts of the diet which can be utilized only via microbial fermentation. Treatment of the feed with neutral detergent gives a fairly good estimate of that part b u t feeds rich in starch or pectins need pretreatments to facilitate filtration (EAAP, 1980). Another b u t more laborious way is to analyse for protein, fat, starch, sugar and ash and calculate cell wall by difference.
Ruminants Although ruminants, by their symbiosis with microbes, can benefit from feeds rich in (hemi)cellulose, the benefit may differ considerably from feed to feed. Roughly, plant cell wall can be said to contain a lignified and an unlignified component. The first c o m p o n e n t is partly, the other c o m p o n e n t is easily degradable by microbes. With advancing maturity the first c o m p o n e n t becomes more lignified while the hemicelluloses, lignin and some other chemical groups form a network around cellulose which becomes more and more dense. It blocks the cellulases from hydrolyzing the cellulose; moreover, the hemicelluloses of the network become less degradable (Morrison, 1979). For optimal microbial degradation the pH should be above 6.0 and there should be available ammonia, some peptides and probably also some soluble carbohydrate to overcome the initial period of energy shortage during which the microbes attach themselves to the cell wall particles. The degree of degradation under these optimal conditions differs with plant species, with plant tissue, with maturity and m a y b e even with the climate during the plant's growth due to changes in plant growth with age which differ between species and tissues. In the case of a slow cell wall degradation the breakdown of long forage particles will take much time. It is a long time until these particles are small enough to leave the forestomachs through the narrow reticulo-omasal orifice. While in the forestomachs, they further hamper feed uptake. As a result, total nutrient absorption will be low so that the production level will also be low. Grinding the long particles increases voluntary feed intake considerably (Van der Honing, 1975) but, due to the lower retention time in the forestomachs,
203 microbial breakdown of cell wall is less, resulting in a lower digestibility. However, the gain due to the higher voluntary intake far outweighs the loss due to the lower digestibility. Treatment of the feeds with alkali, discussed by others at this workshop, is more useful. The long feed particles will disintegrate sooner because the possibility of microbial attack of the plant cell walls has been increased and this allows higher feed intake. Due to an increased fermentation a greater quantity of absorbable nutrients will become available to the animal.
Feed analysis It is very useful to have laboratory techniques to predict the nutritive value of feeds for ruminants and to test the efficacy of procedures to improve their quality. It is clear that not only a m e t h o d is needed to partition the feed into a -- readily available -- cell c o n t e n t part and a -- less available -cell wall part, but also one which partitions the cell wall part into a degradable and an undegradable part. Treatment with neutral detergent does the first separation. In the case of feeds with higher starch contents, filtration may be difficult. In forages we found that the m e t h o d of drying the sample (by freeze-drying or by drying at 60--70 ° C) influenced the residue after neutral detergent considerably. Neither the determination of crude fibre nor of acid detergent fibre gives the required content of degradable cell wall as the separation techniques of these methods do n o t partition in terms of degradability. The best way would be to incubate neutral detergent fibre with rumen fluid or with a cellulolytic enzyme preparation. The first procedure necessitates the presence of donor animals. Cellulolytic enzyme preparations at present on the market contain other enzymes than rumen fluid; moreover, their strength is variable. The various difficulties mentioned probably explain why the determination of the in vitro digestibility using 48-h incubation with rumen fluid followed by one of similar length with pepsin of the ground feed sample is still the most popular for precise prediction of apparent digestibility. It has become clear that for the highest comparability with in vivo digestibilities, samples with known in vivo digestibilities, treated in the same way as the samples to be tested, should be used as standards together with the test samples in the same in vitro run (EAAP, 1980; IRDC, 1980) . The necessity of using such standards of similar type is because this in vitro procedure is still a poor imitation of what actually happens in the animal. In vivo the feed particles may stay shorter or longer than 48 h in the forestomachs, and the rate of microbial degradation in vitro of the ground sample m a y differ from that in the forestomachs. The use of standards makes it possible to correct the in vitro results toward the in vivo situation. Other prediction methods based on relationships between in vivo digestibility at about maintenance feeding level and e.g. crude fibre, acid detergent fibre, stage of maturity, etc. have a considerably lower precision. This is even more so for feeds treated mechanically or chemically.
204 REFERENCES Armstrong, D.G., 1969. Cell bioenergetics and energy metabolism. In: W. Lenkeit, K. Breirem and E. Crasemann (Editors), Handbuch der Tierern~'hrung I. Allgemeine Grundlagen. Paul Parey, Hamburg/Berlin, pp. 385--414. CVB, 1977. Veevoedertabel. Centraal Veevoederbureau in Nederland, Wageningen, The Netherlands, pp. B 1--B 17. DLG, 1970. Futterwerttabelle fiir Schweine. DLG-Verlag, Frankfurt am Main, 32 pp. EAAP, 1980. A.J.H. van Es and J. van der Meer (Editors), Workshop on Methodology of Analysis of Feedingstuffs for Ruminants. Preprint N 5.4 of 31. Annual EAAP Meeting, 106 pp. Henkel, H., 1980.34. Tagung Gesellschaft ffir Ern~/hrungsphysiologie der Haustiere, GSttingen, Z. Tierphysiol. Tierern~ihr. Futtermittelkd., 44: 34--36. Holmes, W., 1970. Animals for food. Proc. Nutr. Soc., 29: 237--244. IDRC/IUNS, 1980. W.J. Pigden, C.C. Balch and M. Graham (Editors), Workshop on Standardization of Analytical Methodology of Feeds. IDRC, Ottawa, Ont., 134e: 7--14, 49--78. Just, A., 1979. The influence of diet composition and site of absorption on the efficiency of utilization of ME in growing pigs. Proc. 8th Syrup. Energy Metab. Farm Animals, EAAP. In: L.E. Mount (Editor), Energy Metabolism. Butterworths, London/Boston, pp. 27--30. Morrison, I.M., 1979. Carbohydrate chemistry and rumen digestion. Proc. Nutr. Soc., 38: 269--274. Nehring, K. and Sehiemann, R., 1966. Die energetische Bewertung der Nahrungs- und Futterstoffe. In: A. Hock (Editor), Handbuch der vergleichenden Ernahrungslehre. VEB Fischer Verlag, Jena, pp. 581--633. Scheimann, R. et al., 1971. Energetische Futterbewertung und Energienormen. VEB Deutscher Landwirtschaftsverlag,Berlin, 344 pp. Van der Honing, Y., 1975. Intake and utilization of energy of rations with pelleted forages by dairy cows. Diss. Agric. Res. Rep. 836, Pudoc, Wageningen, The Netherlands, 156 pp. Van Es, A.J.H., 1975. Losses and gains of energy during production of food for human consumption in animal husbandry. Agricultura (Leuven, Belgium), 23: 359--374.