Use of the nylon-bag technique in determining the complementarity of feedstuffs for dairy cattle rations

A Abate ¹ and B Kiflewahid ²

¹ Department of Animal Production
University of Nairobi
PO Box 29053, Nairobi, Kenya

² International Development Research Centre (IDRC)
PO Box 62084, Nairobi, Kenya

ABSTRACT

Degradation of various feedstuffs (protein sources, energy concentrates and roughages) eaten by dairy cattle in the high potential areas of Kenya was studied by incubating samples in nylon bags in the rumens of steers. For each feedstuff, degradation characteristics were estimated from a hand-drawn degradation curve

Average dry-matter solubility was about 25.3, 18.0 and 24.8% for protein feeds, energy concentrates and roughages, respectively. Maize germ flakes had the highest degradable fraction (65.0%) and fish meal the lowest (38.5%). Degradation rate constants averaged 0.05% per hour for protein feeds, 0.06% per hour for energy concentrates and 0.03% per hour for roughages.

The use of degradation characteristics in evaluating livestock feedstuffs and in formulating ration combinations for dairy cattle is discussed. The nylon-bag technique is a useful tool for improving tropical animal production systems because it is reliable, cheap and easy to perform.

RESUME

Etude comparée de la dégradabilité de divers aliments bétail par la technique d'incubation des sacs en nylon dans le rumen

Le rythme de dégradation de différents aliments (sources de protéines, aliments énergétiques et fourrages grossiers) consommés par les bovins de race laitière dans les zones à forte potentialité du Kenya a été étudié par incubation de sacs en nylon dans le rumen des animaux. Une courbe a été tracée pour chaque aliment en vue de déterminer ses paramètres de dégradabilité.

Il ressort des résultats enregistrés que la solubilité moyenne de la matière sèche était d'environ 25,3; 18,0; et 24,8% respectivement pour les sources de protéines, les aliments énergétiques et les fourrages grossiers. Les taux de dégradation variaient de la valeur maximum de 65,0% pour la farine de germe de maïs au chiffre minimum de 38,5% pour la farine de poisson. Quant à la constante horaire de dégradation, elle s'élevait à 0,05%; 0,06%; et 0,03% respectivement pour les sources de protéines, les aliments énergétiques et les fourrages grossiers.

L'évaluation des aliments du bétail et la formulation des rations à partir des études de dégradabilité ont été discutées. La technique du sac en nylon constitue un instrument utile d'amélioration des systèmes d'élevage dans les régions tropicales dans la mesure où elle est fiable, peu chère et d'utilisation facile.

INTRODUCTION

As a tool for studying rumen digestion, the artificial-bag technique is not new (see, for example, Quin et al, 1938; Fina et al, 1958). In recent years the nylon-bag technique has become more widely used, and it is now being recommended as a means for evaluating tropical livestock feeds. However, little work has been done on the practical application of the technique to the evaluation of feeding systems. This study aimed to use degradation characteristics as a means of formulating ration combinations for dairy cattle.

MATERIALS AND METHODS

Samples of various feedstuffs, used for feeding dairy cattle on small-scale farms in different parts of Kenya, were studied. The feedstuffs included byproducts of the food industry, pasture forage, crop residues and non-conventional feeds. Samples were degraded in nylon bags in the rumens of three crossbred steers (average weight 590 kg) fitted with permanent cannulae. The daily diet of the animals comprised Napier grass and concentrate.

Feedstuffs for analysis were oven-dried at 70°C and then ground to pass through a 2-mm screen. Samples (2-5 g) were measured into nylon bags (approximately 14 x 8 cm) which were securely tied with a knot at the end of a 24-cm long string. The bag strings were permanently tied in groups of 20 to a main string about 60 cm long, and the whole assembly was inserted into the ventral sac of the rumen of one of the steers. All samples were incubated in duplicate per steer and replicated in the three steers. Withdrawal of the bags was timed to give incubation times of 1, 6, 12, 18, 24 and 36 hours for protein feeds and energy concentrates and 1, 12, 24, 36, 48 and 72 hours for forages. Zero-hour incubation was obtained by extrapolation of the degradation curve to the y-axis. Retrieved bags were washed under running water until the water was clear. Dry-matter degradation was determined from dry-matter determinations on original and degraded samples.

For each feedstuff, degree of dry-matter degradation was plotted against incubation time and the curve drawn by hand (Figure 1). The values of a (% solubility) and b (potentially degradable fraction) were determined from the graph. The steepest section of the curve (where the change dy/dt was most rapid, indicating maximum rate of dry-matter degradation) was identified and the percentage degradation (p) and incubation time (t) corresponding to the midpoint of this section were read off; this enabled the degradation rate constant (c) to be calculated from the following exponential equation (Ørskov et al, 1980):

p = a + b(1 - e^-ct)

where:

p = dry-matter degradation (%)
a = dry-matter solubility (%)
b = potentially degradable fraction (%)
t = time of maximum rate of dry-matter degradation (hours)
c = degradation rate constant

Figure 1. Hand-drawn curve for estimating % degradation (p) at time t

The constants estimated from the hand-drawn curve were compared to those derived using a computer program.

RESULTS AND DISCUSSION

The contents of major nutrients in the feeds (Table 1) were within the published range (Said, 1971; Abate, 1980; Kamande, 1988) except that the level of insoluble material in the fish meal was very high, probably because of a high proportion of scale and contamination in the sample analysed.

The degradation characteristics (Table 2) are generally comparable to those obtained for similar feeds by other authors (Ørskov et al, 1980; Kamande, 1988). Previous studies used sinkers of sand or water (Kamande, 1988) or nylon tubes, or weights to position bags in the rumen (Ørskov et al, 1980), but the present study indicates that reliable, accurate, results can be obtained without these aids. There is, however, some variation in estimates of solubility. For example, compared with the values found in this study, Kamande (1988) reported a higher solubility value for wheat bran and a lower solubility for lucerne. This may suggest that solubility is unreliable as an indicator of feeding value.

Table 1. Dry-matter, ash, crude-protein and fibre contents of feedstuffs

Peed	Content (% DM)
	DM (%)	Ash	Crude protein	Crude fibre	Neutral detergent fibre
Protein sources
Fish meal	96.4	15.6	35.0	nd	nd
Soybeans	90.9	4.9	40.8	9.1	nd
Cottonseed cake	97.3	5.9	31.1	20.7	nd
Sunflowerseed cake	96.5	5.7	33.6	28.8	nd
Lucerne, flowering	91.8	10.4	22.7	nd	77.8
Energy concentrates
Maize germ flakes	87.8	6.0	16.5	11.4	nd
Wheat bran	91.2	7.9	17.2	13.4	nd
Cornflakes waste	91.4	2.7	8.1	9.3	nd
Green banana fruit	92.5	6.1	5.0	6.4	nd
Roughages
Bean stalks and pods	90.7	8.7	6.3	nd	70.8
Coffee pulp	89.9	10.7	13.6	nd	51.5
Hay (mixed species)	92.0	8.7	8.8	nd	76.6
Kikuyu grass	93.5	12.6	19.0	nd	64.8

nd = value not determined

Table 2. Degradation characteristics of feedstuffs obtained from a hand-drawn curve and by computer

Feed	a		b		c
	Hand	Computer	Hand	Computer	Hand	Computer
Protein sources
Fish meal	15.3	13.9	38.5	43.1	0.054	0.066
Soybeans	34.9	34.8	62.6	66.5	0.053	0.078
Cottonseed cake	21.8	14.6	45.6	47.8	0.057	0.220
Sunflowerseed cake	22.5	21.7	41.4	46.3	0.054	0.069
Lucerne, flowering	32.1	30.3	47.6	48.0	0.050	0.074
Energy concentrates
Maize germ flakes	10.2	11.9	65.0	61.8	0.047	0.086
Wheat bran	14.4	12.7	55.2	56.6	0.072	0.128
Cornflakes waste	28.8	30.3	51.4	56.5	0.063	0.067
Green banana fruit	18.4	17.5	53.6	169.4	0.044	0.012
Roughages
Bean stalks and pods	15.0	12.6	45.9	52.7	0.033	0.041
Coffee pulp	42.8	41.3	48.4	54.9	0.035	0.037
Hay (mixed species)	16.4	15.3	41.8	48.2	0.029	0.033
Kikuyu grass	25.2	22.9	55.3	62.2	0.037	0.043

p = a + b(1 - e^-ct)

where:

p = dry-matter degradation (%)
a = dry-matter solubility (%)
b = potentially degradable fraction (%)
t = time of maximum rate of dry-matter degradation (hours)
c = degradation rate constant

Degradation characteristics derived by computer are also given in Table 2. There was a high correlation (r = 0.97) between the solubility estimates obtained by the two methods, indicating that both methods are equally applicable for determining this parameter. However, there was little or poor agreement between the methods in the estimation of b (r = 0.32) and c (r = 0.60); on average the b and c values were higher than those obtained from the hand-drawn curve. It is therefore difficult to judge the reliability of either method, but comparison of results from this study with those reported in the literature suggests that the estimates made from the hand-drawn curve are more accurate.

All the protein sources showed similar rates of degradation. However, this similarity can be misleading. The relatively low total degradability (potentially degradable fraction) of the fish meal implies that this feed could be a source of undegradable protein and therefore would be useful for cows that are high producers of milk. In contrast soybean was so highly degraded that more of its protein would be exposed to hydrolysis, and so diets based on this feedstuff would need to be supplemented by a source of rumen degradable energy, such as cornflakes waste or possibly wheat bran (but not coffee pulp which, despite a high total degradability, has a low degradation rate).

The low solubility of the bean stalks and pods and hay is in agreement with similar values demonstrated for Kenyan pasture grasses and fodder legumes (Kamande 1988) and low quality range forage (Rutagwenda, 1989). More recently Abate (1990) has shown that maize forage harvested and ensiled at different stages of growth differed in solubility and degradability. Fibrous materials are less soluble because of the dominance of structural over soluble carbohydrates in the cell wall. Also, Van Soest (1982) has demonstrated that degree of signification has a negative effect on cell wall digestion in forages. The low solubility of fish meal, compared with cottonseed and sunflowerseed cakes, may be a result of the composition of the meal or of contamination with indigestible matter. Low and slow degradability, as found in the hay, can limit voluntary intake of feed. It is hence desirable that roughages such as hay or crop residues be combined with highly soluble feedstuffs; such a combination would stimulate a more active rumen microbial population which would degrade the fibre faster and hence increase digesta passage rate and DM intake.

CONCLUSIONS

Knowledge of dry-matter degradation characteristics of feedstuffs can be used as a basis for formulating ration combinations for dairy cattle in tropical animal production systems. The nylon-bag technique offers a cheap, reliable and easy to perform method of obtaining this information; analysis can be done by hand without the need for expensive computer equipment and software. However, assessment of protein degradability of feedstuffs is also desirable, to ensure that protein requirements are met.

ACKNOWLEDGEMENTS

We are grateful to the Head and staff of the Animal Production Research Programme at the Kenya Agricultural Research Institute (KARI), Muguga, where this study was carried out.

REFERENCES

Abate A. 1980. In vitro estimation of the energy content of some feeds commonly fed to livestock in Kenya. East African Agricultural and Forestry Journal 45:255-260.

Abate AN. 1990. Nutritive value of maize forage silage fed to dairy cows for an integrated system of grain and milk production in Kenya. PhD Thesis. University of Aberdeen, Aberdeen, UK 199 pp.

Fina L R. Teresa G W and Bartley E E. 1958. An artificial rumen technique for studying rumen digestion in vivo. Journal of Animal Science 17:667.

Kamande G M. 1988. Forage and concentrate protein utilization by dairy cattle. MSc Thesis. University of British Columbia, Vancouver, BC, Canada. 113 pp.

Ørskov E R. Hovell F D DeB and Mould F. 1980. The use of the nylon bag technique for the evaluation of feedstuffs. Tropical Animal Production 5: 195-213.

Quin J I, Van Der Wath J G and Hyburth S. 1938. Studies on the alimentary canal of Merino sheep in South Africa. 4. Description of experimental technique. Onderstepoort Journal of Veterinary Science and Animal Industry 11:341.

Rutagwenda T. 1989. Adaptation of sheep and goats to seasonal changes of forage on a semi-arid thornbush savannah pasture in northern Kenya. PhD Dissertation. Tierärztliche Hochschukle, Hanover, FR Germany. 131 pp.

Said A N. 1971. In vivo digestibility and nutritive value of Kikuyu grass (P. clandestinum) with a tentative assessment of its yield of nutrients. East African Agricultural and Forestry Journal 37:15.

Van Soest P J. 1982. Nutritional ecology of the ruminant. O & B Books, Corvallis, Oregon, USA. 374 pp.