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Comparison of the nutritional value of low and medium tannin sorghum grains for pigs
Animal Feed Science and Technology, 8 (1983) 85--90 Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands
85
COMPARISON OF THE NUTRITIONAL VALUE OF LOW AND MEDIUM TANNIN SORGHUM GRAINS FOR PIGS
A.C. KONDOS Queensland Agricultural College, Lawes 4343 (Australia)
M.A. FOALE CSIRO, Division of Tropical Crops and Pastures, Cooper Laboratories, Lawes (Australia)
(Received 28 December 1981; accepted for publication 1 September 1982) ABSTRACT Kondos, A.C. and Foale, M.A., 1983. Comparison of the nutritional value of low and medium tannin sorghum grains for pigs. Anita. Feed Sci. TechnoI., 8: 85--90. The effect of sorghum grain of low (0.32%) and medium (0.94%) tannic acid content was studied in a growth experiment on Large White pigs. Medium tannin sorghum in pig diets had no significant adverse effect on growth, back fat thickness or feed conversion efficiency, but it was associated with a trend towards lower growth rate. All-sorghum diets, with either low or medium tannin, resulted in a significantly inferior feed conversion ratio compared to the wheat control. It was concluded that sorghum grain with up to about 0.94% tannic acid can be used as the main or sole grain in grower pig diets without the risk of any significant adverse effect on growth or back fat thickness. The production of medium tannin sorghum grain in bird-infested areas will depend on the extent to which pig producers would be prepared to use it in pig diets. INTRODUCTION S o r g h u m grain is w i d e l y used in a n i m a l p r o d u c t i o n , especially in t r o p i c a l a n d s u b t r o p i c a l areas w h e r e it is g r o w n as a m a j o r grain c r o p . A l t h o u g h t h e general n u t r i t i o n a l value o f s o r g h u m is w i d e l y r e c o g n i s e d , t h e r e are large v a r i a t i o n s in t h e n u t r i e n t c o m p o s i t i o n o f t h e grain w h i c h are k n o w n t o b e d u e t o d i f f e r e n c e s in cultivar, l o c a l i t y a n d p l a n t n u t r i e n t s u p p l y (Miller et al., 1 9 6 4 ; D e y o e a n d Shellenberger, 1 9 6 5 ; Harris et al., 1 9 7 4 ; N e u c e r e a n d Sumrell, 1 9 8 0 ) . Similar variations, b u t o f smaller m a g n i t u d e , also o c c u r in o t h e r cereal grains, (Hill, 1 9 6 4 ; H e p b u r n a n d B r a d l e y , 1 9 6 5 ; L a r s e n a n d Nielsen, 1 9 6 6 ) a n d this c r e a t e s p r o b l e m s in d i e t f o r m u l a t i o n w h e n c o m p l e t e a n a l y t i c a l i n f o r m a t i o n is n o t available. S o r g h u m grain also s h o w s m a r k e d d i f f e r e n c e s in t h e t a n n i n c o n c e n t r a t i o n o f t h e grain (Guirgossian et al., 1 9 7 8 ; R o t u n n o a n d Desiderio, 1 9 7 9 ; Sisodia et al., 1 9 7 9 ; R a d h a k r i s h n a n a n d Sivaprasad, 1 9 8 0 ) . H i g h - t a n n i n grain varieties h a v e b e e n u s e d b e c a u s e o f t h e i r r e s i s t a n c e t o bird d a m a g e , b u t a h i g h - t a n n i n c o n t e n t h a s b e e n f o u n d t o l o w e r t h e p r o t e i n digestibility o f t h e grain in pigs (Elkin et al., 1 9 7 8 ; A l m o n d et al., I 9 7 9 ; F o r d a n d H e w i t t , 1 9 7 9 , 1 9 8 0 ) . 0377-8401/83/0000--0000/$03.00
© 1983 Elsevier Scientific Publishing Company
86 In m a n y areas of the potential sorghum-growing regions of northeastern Australia, bird damage of low-tannin sorghum cultivars Can be responsible for severe economic loss and this has limited the expansion of sorghum production is these areas. The present study was designed to investigate, in pig growth experiments, the nutritional value of two sorghum cultivars; Texas 610 SR, containing 0.32%, and NK 300F, containing 0.94%, tannic acid equivalent, in order to establish the acceptability of sorghum of moderate tannin content. MATERIALS AND METHODS Animals Seven pure Large White pigs of mixed sexes and an average body weight of 38 kg were used in each of five treatments. They were all penned individually on slatted floors in a minimal disease piggery and fed ad libitum for a period of 50 days. Diets The treatments were based on the following five isonitrogenous diets: (1) control; no sorghum, all grain was wheat (2) all grain was m e d i u m tannin (MT) sorghum (MT -- 0.94% (3) all grain was 50/50 m e d i u m tannin sorghum/wheat (4) all grain was low tannin (LT) sorghum (LT -- 0.32%) (5) all grain was 50/50 low tannin sorghum/wheat The medium tannin sorghum was specially grown in northern Australia. The diets contained about 17.0% crude protein by analysis. Their composition is given in Table I and essential amino acid profiles in Table II. Observations Nitrogen was determined by the m e t h o d of McKenzie and Wallace (1954) and crude protein was calculated as N X 6.25. Total amino acid analysis was carried out using ion exchange chromatography according to Bidmead and Ley (1958). Tannins were measured as tannic acid equivalent according to Burns (1963). Back fat measurements were made at position P2 on the h o t carcass. The results were statistically analysed using one:way analysis of variance. All pigs were fasted for 24 h at the beginning and end of the experiment before b o d y weights were recorded. Water was always available. No signs of any illness were obvious at any stage of the experiment.
87 TABLE I I n g r e d i e n t s a n d n u t r i e n t c o m p o s i t i o n (%) o f t h e five e x p e r i m e n t a l d i e t s Diet 1
2
3
Wheat Sorghum (low tannin) Sorghum (medium tannin) Meat meal Soya bean meal C o t t o n seed m e a l Bran Mineral and vitamin premix a Salt L-lysine HC1 Limestone
78 --
--73 10 5 8 2.7 0.7 0.3 0.3
38 -38 9 3 8 2.8 0.7 0.3 0.2
Crude protein Calcium Tannin b
17.0 1.02 --
17.1 1.04 0.69
17.1 1.03 0.36
6 6 8 0.6 0.7 0.3 0.1 0.3
4
5
10 5 8 2.7 0.7 0.3 0.3 --
38 38 -9 3 8 2.8 0.7 0.3 0.2 --
17.0 1.05 0.23
17.1 1.03 0.12
73
a T h e m i n e r a l a n d v i t a m i n m i x t u r e Supplied t h e f o l l o w i n g n u t r i e n t s in m g k g - ' o f air-dry d i e t : zinc, 5 5 ; m a n g a n e s e , 2 4 ; c o p p e r , 7 ; iron, 8 8 ; c o b a l t , 0~7 ; s e l e n i u m , 0.2; v i t a m i n A, 1.2 ; D, 8.25 ; E, 0 . 3 3 ; K, 0.66 ; Niacin, 16; p a n t o t h e n i c acid, 2.5 ; r i b o f l a v i n , 3.3 ; p y r i d o x ine, 1.1; B,~, (}.019; b i o t i n , 0 . 0 6 ; choline-C1, 1 1 0 0 . b T a n n i c acid e q u i v a l e n t c o n t r i b u t e d t o t h e diet b y t h e s o r g h u m c o m p o n e n t .
T A B L E II E s s e n t i a l a m i n o acid c o n t e n t o f t h e diets (% o f d i e t ) A m i n o acid
Lysine Methionine + cystine Histidine Threonine Leucine Iso-leucine Phenylalanine Tryptophan Valine Arginine
Diet 1
2
3
4
5
0.80 0.64 0.42 0.56 1.20 0.64 0.92 0.46 0.70 1.04
0.74 0.66 0.42 0.58 1.14 0.64 0.86 0.42 0.76 1.11
0.76 0.64 0.46 0.61 1.16 0.64 0.80 0.39 0.73 1.13
0.76 0.61 0.48 0.57 1.20 0.60 0.89 0.44 0.73 1.06
0.74 0.60 0.41 0.59 1.06 0.63 0.86 0.40 0.79 1.06
88 RESULTS Table III shows the mean values and standard errors for all observations for the whole experimental period. The m ean growth rate of pigs in the control group was slightly, b u t n o t significantly, greater t han the o t h e r groups. There were no significant differences in back fat measurements and the lowest value of 17.86 m m was r e c or ded for the MT wheat t r e a t m e n t which also showed the lowest growth rate. The highest back fat m e a s u r e m e n t of 19.14 m m was recorded for the MT sorghum t reat m ent . The feed conversion ratio of the control group was significantly bet t er (P < 0.05) than for all o t h e r treatments, which did n o t differ significantly among themselves. TABLE III Mean growth per pig, back fat measurement and feed conversion ratio Diet
Total growth (kg/pig)
Back fat thickness (mm)
Feed conversion ratio (kg/kg)
1. Control 2. Medium tannin 3.50/50 MT/wheat 4. Low tannin 5.50/50 LT/wheat SE
38:68 36.86 36.08 38.08 37.37 1,81
19.00 19.14 17.86 18.14 18.29 0.90
2.86* 3.09 3.26 3.12 3.09 0.065
*Significantly different from other treatments (P < 0.05). DISCUSSION Pigs fed on diets containing m e d i u m (0,94%) or low (0,32%) tannin sorghum, representing all or half the a m o u n t of grain in t he diet (Table I), did n o t show any significant difference in their growth rate. The cont rol diet, which contained no sorghum, s u p p o r t e d only a slightly higher mean growth rate c o m p a r e d with ot her treatments, with a m a x i m u m difference of 2.6 kg above th e M T / w h e a t group b u t was no t significantly different. T here was a tr en d for th e t w o high-tannin diets t o support the lowest growth rate b u t neither o f t h e m was significantly below t he low-tannin sorghum diets. This indicates that, u n d e r t he conditions o f the experi m ent , sorghum with 0.94% tannic acid equivalent can be used with onl y a m i n o r risk of being nutritionally inferior to a low-tannin-containing sorghum. Despite this statem e nt, it should n o t be ignored t hat in sorghum, lysine is t he first limiting amino acid f o r pig growth (Axtel et al., 1974), b u t this was o v e r c o m e in this e x p e r i m e n t by supplementing all diets t o an appropriate level (Table I). The feed conversion efficiency figures are in a fairly similar order t o t h a t o f growth rate for each t r e a t m e n t with t he c ont rol showing the best, and
89 the MT/wheat treatment the worst, efficiency. It has been shown b y Armstrong et al. (1974), Elkin et al. (1978), and Ford and Hewitt (1979) that high-tannin sorghums have low protein digestibility and that methionine supplementation of diets containing such sorghums improves feed utilization in chickens. The most important aspect in the c o n t e x t of this experiment is that no significant differences were d e t e c t e d between any of the sorghumcontaining diets, despite the obvious trend towards a lower growth rate in pigs fed on the t w o diets with a medium tannin content. According to Axtell et al. (1974), the reduced protein availability of hightannin sorghums m a y be the result o f the binding effect of polyphenolic c o m p o u n d s (tannins) present with proteins in the testa layer of the grain. Almond et al. (1979) have also shown that the growth of pigs fed on lowtannin sorghum diets was faster than that on high-tannin diets and this was associated with marked differences in the energy and protein digestibility between the low and high tannin treatments. This information supports the possibility that the trend for the lower performance of pigs in the mediumtannin treatments could be due to some interference o f the polyphenolic c o m p o u n d s with the efficiency of feed utilization. Close examination of the amino acid profiles in all treatments shows only minor differences. It is possible that in addition to methionine, which has been found to b e c o m e limiting in high-tannin diets, the availability of lysine and other amino acids m a y also be involved as shown b y Rostagno et al. (1973). Itis, therefore, possible that in the present investigation, methionine and other essential amino acids could have become marginally limiting in the medium-tannin sorghum diets. This interesting point should be investigated further for verification of reports on the favourable effect of methionine and for testing whether the lower growth rate and feed conversion efficiency in these trials were due to a mild deficiency caused by the increased tannic acid level. Back fat measurements were in reverse order to growth rate with the least thickness of 17.86 m m in the MT/wheat treatment, which showed the slowest growth rate. This trend may indicate some effect on energy intake or utilization through lower feed consumption and/or digestibility, although the difference in feed efficiency was significant only between the control and the other treatments as a whole. It could be concluded that in grower diets in which medium-tannin sorghum (0.9%) m a y represent a b o u t 75% of the diet, pig growth and back fat measurements would not be expected to be significantly affected. From the feed conversion efficiency it appears that wheat diets can be nutritionally superior to diets in which all or half of the grain is sorghum. This could be due to the higher digestibility of wheat as compared to sorghum, especially if grinding is not complete. Medium-tannin sorghums are reported to show a certain degree of resistance to bird damage, which could be critical in farming districts in southern Queensland where sorghum fields are interspersed with undeveloped land which provides sanctuary to grain-feeding birds. If pig producers accept the indications of this study, a use should develop for medium-tannin sorghum grain.
90 REFERENCES Almond, M., Smith, W.C., Savage, G.P., Lawrence, T.L.J., 1979. A comparison of two contrasting types o f grain sorghum in the diet of the growing pig. Anim. Prod., 29 : 143--150. Armstrong, W.D., Rogler, J.C. and Featherston, W.R., 1974. Effect of tannin extraction on the performance of chicks fed bird resistant sorghum grains. Poult. Sci., 53: 714-720. Axtell, J.D., Ozwalt, D.L., Gertz, E.T., Pickett, R.C., Jambonathay, R. and Srinivasan, G., 1974. Components o f nutritional quality in grain sorghum. Paper No. 5175, Purdue Univ. Agric. Exp. Stn., pp. 374--386. Bidmead, D.S. and Ley, F.J., 1958. Amino acid analysis using ion exchange chromatraphy. Biochem. Biophys. Acta, 29: 562--569. Burns~ R.E., 1963. Methods of tannin analysis for forage crop evaluation. Bull. Agric. Exp. Stn., N.C. No. 32, p. 5, Deyoe, C.W. and Shellenberger, J.A., 1965. Amino acids and proteins in sorghum grain. J. Agric. F o o d Chem., 13: 446--450. Elkin, R.G., Rogler, J.C. and Featherston, W.R., 1978. Influence of sorghum grain tannins on methionine utilization in chicks. Poult. Sci., 57: 704--710. Ford, J.E. and Hewitt, D., 1979. Protein quality in cereals and pulses. No.3 Bioassays. Br. J. Nutr., 42: 325--340. Ford, J.E. and Hewitt, D., 1980. Vicia Faba, Feeding Value, Processing and Viruses. Martinus Nijhoff, The Hague, Netherlands, pp. 125--140. Guiragossian, V., Chibber, B.A.D. Van Scoyoc, S., Jambunathan, R., Mertz, E.T. and Axtell, J.D., 1978. Characteristics of proteins from normal, high lysine and high tannin sorghums. J. Agric. F o o d Chem., 26: 219--223. Harris, H.B., Massey, J.H., Cummins, D.C., Burns, R.E. and Yetlum, M.D., 1974. Effects of hybrids and nitrogen rates on grain sorghum yields and chemical characteristics. Publ. Am. Soc. Agron., No. 54 Madison U.S.A. Hepburn, F.N. and Bradley, W.B., 1965. The amino acid composition of hard wheat varieties as a function of nitrogen content. Cereal Chem., 42: 140--149. Hill, K.W., 1964. Protein content of wheat as affected by agronomic practices. Can. J. Plant Sci., 44: 115--122. Larsen, I. and Nielsen, I.D., 1966. The effect of varying nitrogen supplies on the content of amino acids in wheat grain. Plant Soil, 24: 299--308. McKenzie, H.A. and Wallace, M.S., 1954. The Kjeldahl determination of nitrogen: a critical study of digestion conditions - - temperature, catalyst and oxidizing agent. Aust. J. Chem., 7: 55--63. Miller, G.D., Deyoe, E.W., Walter, T.L. and Smith, F.W., 1964. Variations in protein levels in Kansas sorghum grain. Agron., J., 56: 302--304. Neucere, N.J. and Sumrell, G., 1980. Chemical composition of different varieties of grain sorghum. J. Agric. F o o d Chem., 28: 19--20. Radhakrishnan, M.R. and Sivaprasad, J., 1980. Tannin content of sorghum and its role in iron bioavailability. J. Agric. F o o d Chem., 28: 55--57. Rostagno, H.S., Rogler, J.C. and Featherston, W.R., 1973. Studies on the nutritional value of bird resistant sorghum grains for chicks. 2. Amino acid digestibility studies. Poult. Sci., 52: 772--778. Rotunno, M. and Desiderio, E., 1979. Tannin content of different grain sorghum hybrids. Sementi Elette, 25: 33--35. Sisodia, B.S., Apte, B.C. and Solankey, B.S., 1979. Note on the tannin, protein and starch contents of some sorghum varieties. Indian J. Agric. Sci., 49: 135--136.
85
COMPARISON OF THE NUTRITIONAL VALUE OF LOW AND MEDIUM TANNIN SORGHUM GRAINS FOR PIGS
A.C. KONDOS Queensland Agricultural College, Lawes 4343 (Australia)
M.A. FOALE CSIRO, Division of Tropical Crops and Pastures, Cooper Laboratories, Lawes (Australia)
(Received 28 December 1981; accepted for publication 1 September 1982) ABSTRACT Kondos, A.C. and Foale, M.A., 1983. Comparison of the nutritional value of low and medium tannin sorghum grains for pigs. Anita. Feed Sci. TechnoI., 8: 85--90. The effect of sorghum grain of low (0.32%) and medium (0.94%) tannic acid content was studied in a growth experiment on Large White pigs. Medium tannin sorghum in pig diets had no significant adverse effect on growth, back fat thickness or feed conversion efficiency, but it was associated with a trend towards lower growth rate. All-sorghum diets, with either low or medium tannin, resulted in a significantly inferior feed conversion ratio compared to the wheat control. It was concluded that sorghum grain with up to about 0.94% tannic acid can be used as the main or sole grain in grower pig diets without the risk of any significant adverse effect on growth or back fat thickness. The production of medium tannin sorghum grain in bird-infested areas will depend on the extent to which pig producers would be prepared to use it in pig diets. INTRODUCTION S o r g h u m grain is w i d e l y used in a n i m a l p r o d u c t i o n , especially in t r o p i c a l a n d s u b t r o p i c a l areas w h e r e it is g r o w n as a m a j o r grain c r o p . A l t h o u g h t h e general n u t r i t i o n a l value o f s o r g h u m is w i d e l y r e c o g n i s e d , t h e r e are large v a r i a t i o n s in t h e n u t r i e n t c o m p o s i t i o n o f t h e grain w h i c h are k n o w n t o b e d u e t o d i f f e r e n c e s in cultivar, l o c a l i t y a n d p l a n t n u t r i e n t s u p p l y (Miller et al., 1 9 6 4 ; D e y o e a n d Shellenberger, 1 9 6 5 ; Harris et al., 1 9 7 4 ; N e u c e r e a n d Sumrell, 1 9 8 0 ) . Similar variations, b u t o f smaller m a g n i t u d e , also o c c u r in o t h e r cereal grains, (Hill, 1 9 6 4 ; H e p b u r n a n d B r a d l e y , 1 9 6 5 ; L a r s e n a n d Nielsen, 1 9 6 6 ) a n d this c r e a t e s p r o b l e m s in d i e t f o r m u l a t i o n w h e n c o m p l e t e a n a l y t i c a l i n f o r m a t i o n is n o t available. S o r g h u m grain also s h o w s m a r k e d d i f f e r e n c e s in t h e t a n n i n c o n c e n t r a t i o n o f t h e grain (Guirgossian et al., 1 9 7 8 ; R o t u n n o a n d Desiderio, 1 9 7 9 ; Sisodia et al., 1 9 7 9 ; R a d h a k r i s h n a n a n d Sivaprasad, 1 9 8 0 ) . H i g h - t a n n i n grain varieties h a v e b e e n u s e d b e c a u s e o f t h e i r r e s i s t a n c e t o bird d a m a g e , b u t a h i g h - t a n n i n c o n t e n t h a s b e e n f o u n d t o l o w e r t h e p r o t e i n digestibility o f t h e grain in pigs (Elkin et al., 1 9 7 8 ; A l m o n d et al., I 9 7 9 ; F o r d a n d H e w i t t , 1 9 7 9 , 1 9 8 0 ) . 0377-8401/83/0000--0000/$03.00
© 1983 Elsevier Scientific Publishing Company
86 In m a n y areas of the potential sorghum-growing regions of northeastern Australia, bird damage of low-tannin sorghum cultivars Can be responsible for severe economic loss and this has limited the expansion of sorghum production is these areas. The present study was designed to investigate, in pig growth experiments, the nutritional value of two sorghum cultivars; Texas 610 SR, containing 0.32%, and NK 300F, containing 0.94%, tannic acid equivalent, in order to establish the acceptability of sorghum of moderate tannin content. MATERIALS AND METHODS Animals Seven pure Large White pigs of mixed sexes and an average body weight of 38 kg were used in each of five treatments. They were all penned individually on slatted floors in a minimal disease piggery and fed ad libitum for a period of 50 days. Diets The treatments were based on the following five isonitrogenous diets: (1) control; no sorghum, all grain was wheat (2) all grain was m e d i u m tannin (MT) sorghum (MT -- 0.94% (3) all grain was 50/50 m e d i u m tannin sorghum/wheat (4) all grain was low tannin (LT) sorghum (LT -- 0.32%) (5) all grain was 50/50 low tannin sorghum/wheat The medium tannin sorghum was specially grown in northern Australia. The diets contained about 17.0% crude protein by analysis. Their composition is given in Table I and essential amino acid profiles in Table II. Observations Nitrogen was determined by the m e t h o d of McKenzie and Wallace (1954) and crude protein was calculated as N X 6.25. Total amino acid analysis was carried out using ion exchange chromatography according to Bidmead and Ley (1958). Tannins were measured as tannic acid equivalent according to Burns (1963). Back fat measurements were made at position P2 on the h o t carcass. The results were statistically analysed using one:way analysis of variance. All pigs were fasted for 24 h at the beginning and end of the experiment before b o d y weights were recorded. Water was always available. No signs of any illness were obvious at any stage of the experiment.
87 TABLE I I n g r e d i e n t s a n d n u t r i e n t c o m p o s i t i o n (%) o f t h e five e x p e r i m e n t a l d i e t s Diet 1
2
3
Wheat Sorghum (low tannin) Sorghum (medium tannin) Meat meal Soya bean meal C o t t o n seed m e a l Bran Mineral and vitamin premix a Salt L-lysine HC1 Limestone
78 --
--73 10 5 8 2.7 0.7 0.3 0.3
38 -38 9 3 8 2.8 0.7 0.3 0.2
Crude protein Calcium Tannin b
17.0 1.02 --
17.1 1.04 0.69
17.1 1.03 0.36
6 6 8 0.6 0.7 0.3 0.1 0.3
4
5
10 5 8 2.7 0.7 0.3 0.3 --
38 38 -9 3 8 2.8 0.7 0.3 0.2 --
17.0 1.05 0.23
17.1 1.03 0.12
73
a T h e m i n e r a l a n d v i t a m i n m i x t u r e Supplied t h e f o l l o w i n g n u t r i e n t s in m g k g - ' o f air-dry d i e t : zinc, 5 5 ; m a n g a n e s e , 2 4 ; c o p p e r , 7 ; iron, 8 8 ; c o b a l t , 0~7 ; s e l e n i u m , 0.2; v i t a m i n A, 1.2 ; D, 8.25 ; E, 0 . 3 3 ; K, 0.66 ; Niacin, 16; p a n t o t h e n i c acid, 2.5 ; r i b o f l a v i n , 3.3 ; p y r i d o x ine, 1.1; B,~, (}.019; b i o t i n , 0 . 0 6 ; choline-C1, 1 1 0 0 . b T a n n i c acid e q u i v a l e n t c o n t r i b u t e d t o t h e diet b y t h e s o r g h u m c o m p o n e n t .
T A B L E II E s s e n t i a l a m i n o acid c o n t e n t o f t h e diets (% o f d i e t ) A m i n o acid
Lysine Methionine + cystine Histidine Threonine Leucine Iso-leucine Phenylalanine Tryptophan Valine Arginine
Diet 1
2
3
4
5
0.80 0.64 0.42 0.56 1.20 0.64 0.92 0.46 0.70 1.04
0.74 0.66 0.42 0.58 1.14 0.64 0.86 0.42 0.76 1.11
0.76 0.64 0.46 0.61 1.16 0.64 0.80 0.39 0.73 1.13
0.76 0.61 0.48 0.57 1.20 0.60 0.89 0.44 0.73 1.06
0.74 0.60 0.41 0.59 1.06 0.63 0.86 0.40 0.79 1.06
88 RESULTS Table III shows the mean values and standard errors for all observations for the whole experimental period. The m ean growth rate of pigs in the control group was slightly, b u t n o t significantly, greater t han the o t h e r groups. There were no significant differences in back fat measurements and the lowest value of 17.86 m m was r e c or ded for the MT wheat t r e a t m e n t which also showed the lowest growth rate. The highest back fat m e a s u r e m e n t of 19.14 m m was recorded for the MT sorghum t reat m ent . The feed conversion ratio of the control group was significantly bet t er (P < 0.05) than for all o t h e r treatments, which did n o t differ significantly among themselves. TABLE III Mean growth per pig, back fat measurement and feed conversion ratio Diet
Total growth (kg/pig)
Back fat thickness (mm)
Feed conversion ratio (kg/kg)
1. Control 2. Medium tannin 3.50/50 MT/wheat 4. Low tannin 5.50/50 LT/wheat SE
38:68 36.86 36.08 38.08 37.37 1,81
19.00 19.14 17.86 18.14 18.29 0.90
2.86* 3.09 3.26 3.12 3.09 0.065
*Significantly different from other treatments (P < 0.05). DISCUSSION Pigs fed on diets containing m e d i u m (0,94%) or low (0,32%) tannin sorghum, representing all or half the a m o u n t of grain in t he diet (Table I), did n o t show any significant difference in their growth rate. The cont rol diet, which contained no sorghum, s u p p o r t e d only a slightly higher mean growth rate c o m p a r e d with ot her treatments, with a m a x i m u m difference of 2.6 kg above th e M T / w h e a t group b u t was no t significantly different. T here was a tr en d for th e t w o high-tannin diets t o support the lowest growth rate b u t neither o f t h e m was significantly below t he low-tannin sorghum diets. This indicates that, u n d e r t he conditions o f the experi m ent , sorghum with 0.94% tannic acid equivalent can be used with onl y a m i n o r risk of being nutritionally inferior to a low-tannin-containing sorghum. Despite this statem e nt, it should n o t be ignored t hat in sorghum, lysine is t he first limiting amino acid f o r pig growth (Axtel et al., 1974), b u t this was o v e r c o m e in this e x p e r i m e n t by supplementing all diets t o an appropriate level (Table I). The feed conversion efficiency figures are in a fairly similar order t o t h a t o f growth rate for each t r e a t m e n t with t he c ont rol showing the best, and
89 the MT/wheat treatment the worst, efficiency. It has been shown b y Armstrong et al. (1974), Elkin et al. (1978), and Ford and Hewitt (1979) that high-tannin sorghums have low protein digestibility and that methionine supplementation of diets containing such sorghums improves feed utilization in chickens. The most important aspect in the c o n t e x t of this experiment is that no significant differences were d e t e c t e d between any of the sorghumcontaining diets, despite the obvious trend towards a lower growth rate in pigs fed on the t w o diets with a medium tannin content. According to Axtell et al. (1974), the reduced protein availability of hightannin sorghums m a y be the result o f the binding effect of polyphenolic c o m p o u n d s (tannins) present with proteins in the testa layer of the grain. Almond et al. (1979) have also shown that the growth of pigs fed on lowtannin sorghum diets was faster than that on high-tannin diets and this was associated with marked differences in the energy and protein digestibility between the low and high tannin treatments. This information supports the possibility that the trend for the lower performance of pigs in the mediumtannin treatments could be due to some interference o f the polyphenolic c o m p o u n d s with the efficiency of feed utilization. Close examination of the amino acid profiles in all treatments shows only minor differences. It is possible that in addition to methionine, which has been found to b e c o m e limiting in high-tannin diets, the availability of lysine and other amino acids m a y also be involved as shown b y Rostagno et al. (1973). Itis, therefore, possible that in the present investigation, methionine and other essential amino acids could have become marginally limiting in the medium-tannin sorghum diets. This interesting point should be investigated further for verification of reports on the favourable effect of methionine and for testing whether the lower growth rate and feed conversion efficiency in these trials were due to a mild deficiency caused by the increased tannic acid level. Back fat measurements were in reverse order to growth rate with the least thickness of 17.86 m m in the MT/wheat treatment, which showed the slowest growth rate. This trend may indicate some effect on energy intake or utilization through lower feed consumption and/or digestibility, although the difference in feed efficiency was significant only between the control and the other treatments as a whole. It could be concluded that in grower diets in which medium-tannin sorghum (0.9%) m a y represent a b o u t 75% of the diet, pig growth and back fat measurements would not be expected to be significantly affected. From the feed conversion efficiency it appears that wheat diets can be nutritionally superior to diets in which all or half of the grain is sorghum. This could be due to the higher digestibility of wheat as compared to sorghum, especially if grinding is not complete. Medium-tannin sorghums are reported to show a certain degree of resistance to bird damage, which could be critical in farming districts in southern Queensland where sorghum fields are interspersed with undeveloped land which provides sanctuary to grain-feeding birds. If pig producers accept the indications of this study, a use should develop for medium-tannin sorghum grain.
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