Strategies for feeding straw to small ruminants: Upgrading or generous feeding to allow selective feeding

E. Owen, R.A. Wahed¹, R. Alimon² and W. El-Naiem³ Department of Agriculture, University of Reading, Earley Gate, P.O. Box 236, Reading RG6 2AT, UK

¹ Permanent address: University of Mosul, Mosul, Iraq.
² Permanent address: University of Pertanian Malaysia, Serdang, Selangor, Malaysia.
³ Permanent address: Gezira Board, Barakat, Sudan.

Abstract
Introduction
Materials and methods
Results
Discussion
References

Abstract

Population growth will accelerate crop - animal integration, creating a need for stall-feeding systems for goats and sheep based on crop residues, especially on small farms. Five experiments with goats and two with sheep showed that allowing small ruminants to 'stall-graze' long barley straw will increase intake because of selective feeding. Allowing goats to reject 50% of the straw offered, instead of the conventional ad libitum refusal-rate of 10 to 20%, increased straw dry-matter intake by 30 to 47%, and that of the estimated digestible straw by 41 to 78%. In one experiment, goats fed rejected straw (50% rate) after treatment with ammonia, consumed as much digestible straw as when the original straw was fed generously. Botanical fractionation of offered straw and rejected straw showed goats to select leaf rather than stem when the amount offered was large enough to allow selective feeding. Similar experiments with tropical straws are required.

Introduction

The case for crop residues, small ruminants and stall-feeding

The future importance of agricultural by-products as feeds for livestock, particularly fibrous by-products for ruminants, is acknowledged (Owen, 1985). Identifying ways of overcoming constraints to their greater utilisation as feed is therefore an appropriate theme for the present workshop.

Sundstol and Owen (1984) pointed to the inevitability of straw being produced as a by-product of growing cereals for man. With world population predicted to double by 2025, cereal production and hence straw production, will obviously increase. This will be particularly so in the developing tropics where population is likely to treble by 2025. The change will reduce the area available for pastoral farming and accelerate the integration of crop and animal (mainly ruminant) agriculture (Gartner, 1984).

During the past decade the importance of small ruminants (especially goats) to the agriculture of developing countries has at last been recognised (Devendra and Burns, 1983; World Bank and Winrock International, 1983; Timon and Hanrahan, 1986). Less publicised is the fact that the small ruminants are mainly associated with small farmers; and it is the small farmers who will increasingly need to practise crop - animal integration. A major constraint to the latter is damage to food crops caused by indiscriminate grazing, especially by goats. For these reasons, we believe that there is need to research and develop stall-feeding systems for small ruminants based on crop by-products.

Background to experiments undertaken - the grazing animal

Methods of upgrading straws as feed are well documented (Sundstol and Owen, 1984) and guidelines on researching the subject are published (Preston et al, 1985). There has been more emphasis on upgrading straws for cattle than sheep (Greenhalgh, 1984) but goats have received little attention (Owen, 1981; Owen and Kategile, 1984). The expense of upgrading techniques and the technical expertise required are frequently cited as being inappropriate for developing countries. Greenhalgh (1984) concluded that in many situations chemical upgrading will be superceded by breeding more nutritious straws, improved harvesting methods and judicious supplementation. The research to be reported suggests another approach, namely to improve animal productivity from crop residues by using the selective eating behaviour of goats and sheep.

The literature on feeding straws to sheep and goats involves experiments where intake and digestibility have been measured under ad libitum feeding. Ad libitum is defined as offering sufficient (usually in chopped form) to ensure that 15 to 20% is left (refused) at the end of the feeding period (Blaxter et al, 1961). This approach is standard and has the advantage (for the experimenter, but not the animal) of minimising selective feeding. We would argue that the latter is a disadvantage. The capacity of sheep (Gibb and Treacher, 1976) and goats (McCammon-Feldman et al, 1981) to graze and browse selectively is recognised. Indeed experiments (e.g. Gibb and Treacher, 1976) indicate that maximum intake by grazing sheep is achieved only if the herbage allowed (g organic matter per kg liveweight day, g OM/kg w.d) exceeds intake by 400%. We therefore hypothesised that conventional ad libitum feeding of straw, allowing excesses of only 15 to 20%, would restrict intake (by reducing the opportunity to select).

This hypothesis was tested in the experiments to be reported.

The experiments are also aimed at helping us develop strategies for stall-feeding straw to goats and sheep.

Materials and methods

Seven experiments conducted at Reading University during the past five years are presented. All involved measuring straw intake (as affected by various parameters) and aimed to assess the degree of selective feeding, by careful sampling and analysis of feed offered and refused. Except in Experiment 1 (no concentrate fed), all involved feeding a concentrate supplement (sugar beet pulp, 600 g/kg; soya bean meal, 180 g/kg; fish meal, 180 g/kg; minerals and vitamins, 40 g/kg) at 15 g DM/kg W^0.75.d. to satisfy nitrogen, mineral and vitamin requirements (Agricultural Research Council, 1980) for maintenance and modest growth in sheep. Numbers of animals used, type and mean weight are shown in Tables 1 to 7. All experiments involved housed (16 hours light, 8 hours dark) individually penned castrated animals bedded on sawdust and provided with water. In Experiment 6 goats were in metabolism cages and faeces was collected over 9 days following a preliminary period of 14 days. In all experiments preliminary periods were of 14 to 21 days and experimental periods (except Experiment 6) lasted 21 days (42 days in Experiment 4). Feeds were offered twice daily and straw refusals carefully collected daily. Representative samples (based on aliquots) of straw offered and refused were taken daily and subsequently analysed, after pooling (Wahed and Owen, 1986), for dry-matter, ash and nitrogen (AOAC, 1975), acid detergent fibre (Goering and Van Soest, 1970) and in vitro digestibility (Tilley and Terry, 1963).

Results

Treatments applied in given experiments and results obtained are shown in Tables 1 - 7.

Experiment 1 (Table 1) was designed to examine whether any of the claimed superiority of goats over sheep, in regard to roughage intake and digestion, could be attributed to differences in food selected in a stall-feeding situation. The straw used was treated with aqueous ammonia using a stack method (Sundstol and Coxworth, 1984). The experiment showed goats to eat more than sheep, but there was no large difference between the quality of straw refused by the two species. What was clear, however, was that both species were feeding selectively. Refused straw was of lower nutritive value than that offered.

Table 1. Experiment 1. Intake and selection of NH₃-treated barley straw by sheep and goats.

		Suffolk cross Mule wethers	Saanen castrate goats	SED
Number of animals		8	8
Liveweight (kg)		57.9	50.7	9.0
Straw intake
Offered a (g DM/d)		1299	1477
Intake (g DM/d)		956	1117	152.4
(g DM/kg W.d)		16.4	21.6	1.5
Chemical composition	Straw offered SE			Straw refused
Nitrogen (g/kg DM)	17	0.5	11.6	12.2	0.6
Acid-detergent fibre (ADF) (g/kg DM)	567	5.7	612	600	6.4
In vitro digestibility (DOMD) b (g OM/kg DM)	607	6.0	544	566

^a To allow a refusal rate of 20 to 25% of amount offered.
^b Tilley and Terry (1963).
Source: Wahed and Owen (1986a).

Experiment 2 (Table 2) was the first trial to test the hypothesis outlined earlier. Allowing goats to refuse 50% of the straw offered clearly led to greater DM intake (by 31%) than allowing the more conventional 20% refusal rate. Refusal quality indicated goats allowed the higher refusal rate were selecting more nutritious straw. Thus the estimated intake of straw digestible OM (based on in vitro digestibility) was markedly higher (by 40%). The goats used (18% treatment) represented a wide range of liveweight (15 to 65 kg). Small goats tended to be more selective than large ones.

In Experiment 3 (Table 3) increasing the refusal rate allowance increased intake of both long and chopped straw. The trend (non-significant) was for greater intake of long straw. Straw length interacted significantly with refusal-rate for refusal digestibility, indicating easier selective feeding with long rather than chopped straw. All subsequent experiments were therefore carried out with long straw.

Experiment 4 (Table 4) simulated the 'grazing approach' (e.g. Gibb and Treacher, 1976) in that the amount of straw offered was based on goat weight so as to achieve a target rate of refusal. The results, however, corroborated those of Experiments 2 and 3. They also showed, as expected, that intake response increased with increasing allowance rate. This was particularly so for the estimated digestible OM intake. Experiment 5 (Table 5) with sheep showed similar results.

Table 2. Experiment 2. Effect of allowing two rates of refusal on intake and selection of barley straw by goats.

	Straw refusal allowance
	20%	50%	SED
Number of goats1	18	18
Straw intake2 (g DM/kg W.d)	14.4	18.9	0.70
Straw intake (g DM/kg W0.75.d)	33.1	43.7	1.60
Straw refused (% of offered)	20.5	48.3
Estimated intake of straw digestible
OM3 (g/kg W.d)	5.9	8.3
Chemical composition of straw	Straw offered		Straw refused
	Mean	SE
Nitrogen (g/kg DM)	5.1	0.02	4.5	4.6	0.13
ADF (g/kg DM)	552	7.0	612	596	4.8
In vitro DOMD4 (g OM/kg DM)	412	4.8	320	347	7.7

1. Mean liveweight (W) 32.6 kg.
2. Concentrate supplement also fed at 15 g DM/kg W^0.75.d
3. Calculated from in vitro digestibility of straw offered and refused.
4. Tilley and Terry (1963).
Source: Wahed and Owen (1986b).

Table 3. Experiment 3. Effect of chopping the straw on the response of goats to increasing refusal-rate allowance.

Treatments1	Refusal-rate main effect		Straw length main effect		SED Main effect	Refusal rate x straw length interaction
	20%	50%	Long	Chopped2
Number of goats 3	16	16	16	16
Straw intake (g DM/kg W.d)	13.1	18.0	16.5	14.7	1.71	NS
Straw refused (% of offered)	19.4	49.1	39.3	40.8
Composition of refused straw
Nitrogen (g/kg DM)	4.9	5.0	4.7	5.2	0.22	NS
ADF (g/kg DM)	583	582	608	557	5.64	NS
In vitro DOMD4 (g OM/kg DM)	343	371	326	388	1.20	p<0 055

1. Design: 2 x 2 factorial, 8 replicates.
2. Using a precision-chop stationary forage harvester.
3. Mean liveweight (W), 30.5 kg.
4. Tilley and Terry (1963).
5. Difference between long and chopped greater with 20% refusal-rate.
Source: Wahed (1987).

Table 4 .Experiment 4. Effect of amount offered on intake and selection of barley straw by goats.

	Straw offered (g DM/kg W.d)
			18	54	90	SED
Number of goats			12	12	12
Initial (day 1) liveweight (kg)			30.2	30.6	30.4	0.56
Final (day 42) liveweight (kg)			30.1	33.1	34.0	0.71
Straw intake 1 (g DM/kg W.d)			15.5	22.8	26.2	0.74
Straw intake (g DM/kg W0.75 .d)			36.0	54.2	62.3	1.73
Straw refused (% of offered)			12.5	56.6	70,3
Estimated intake of straw digestible OM2 (g/kg W.d)			7.2	12.8	14.5
Chemical composition of straw	Straw offered			Straw refused
	Mean	SE
Nitrogen (g/kg DM)	7.4	0.12	5.5	5.7	6.1	0.11
ADF (g/kg DM)	528	2.0	565	583	574	6.9
In vitro DOMD 3 (g OM/kg DM)	443	4.5	354	370	403	14.5

1. Concentrate supplement also fed at 15 g DM/kg W^0.75.d
2. Calculated from in vitro digestibility of straw offered and refused.
3. Tilley and Terry (1963).
Source: Wahed and Owen (1986b).

Table 5. Experiment 5. Effect of amount offered on intake and selection of barley straw by sheep.

	Straw offered (g DM/kg W.d)
			18	54	90	SED
Number of wethers 1			10	10	10
Straw intake (g DM/kg W.d)2			14.1	19.0	22.2	0.81
Estimated digestibility of straw consumed3 (g OM/kg DM)			467	562	572
Straw refused (% of offered)			20.8	64.7	75.1
Chemical composition of straw	Straw offered			Straw refused
	Mean	SE
Nitrogen (g/kg DM)	6.4	0.2	4.5	5.1	5.5	0.12
ADF (g/kg DM)	542	7.9	610	581	564	8.4
In vitro DOMD4 (9 OM/kg DM)	432	0.8	294	361	374	7.2

1. Mean liveweight (W) 52.8 kg.
2. Concentrate supplement also fed at 15 9 DM/kg W^0.75 .d
3. Calculated from in vitro digestibility of straw offered and refused.
4. Tilley and Terry (1963).
Source: Naate (1986).

Experiment 6 (Table 6) investigated the feasibility of refeeding 'stall-grazed straw', as such, or after treatment with ammonia (stack method-Sundstol and Coxworth, 1984). Intake of untreated 'stall-grazed straw' (straw-previously-refused) was significantly less than that of the original straw, but after treatment, the intake of digestible OM (measured in vivo) was the same as the original, untreated straw.

Experiment 7 (Table 7) was only recently completed and aimed to assess whether intake and selection response to increasing refusal allowance would be affected by whether or not the straw was treated with sodium hydroxide (dip method according to Sundstol et al, 1981). The preliminary results are somewhat surprising, indicating no apparent increase in straw DM intake due to increasing the refusal-allowance rate. There was a response to NaOH treatment. In this experiment samples of straw offered and refused were botanically fractionated (Ramazin et al, 1986). Interestingly, the results (Table 7) indicate that with generous feeding (allowing high refusals) intake of leaf plus sheath increased and stem decreased. As expected with barley straw (Ramazin et al, 1986), Table 8 shows leaf plus sheath to be of higher nutritive value than stem.

Discussion

The results obtained clearly support the hypothesis stated. Goats and sheep will consume more barley straw if they are permitted to reject 50% of that offered, rather than the conventional 10 to 20%. Furthermore, the improvement in consumption of digestible straw is even greater because generous feeding allows animals to select the more digestible fractions (leaf rather than stem).

There is need to corroborate the findings with direct measurements (in vivo) of digestible straw intake and also animal productivity measurement. The experiments reported

(Tables 1 - 8) are tedious to execute and offer much scope for arriving at erroneous conclusions. For example, incomplete collection of straw refusals would exaggerate treatment response, as unrecorded refusal would be deemed eaten. Grazing research techniques (e.g. Mayes et al, 1986) might have application for measuring quantity and quality of straw consumed .

The extent to which selective feeding of straw occurs with higher levels of concentrate supplementation, or when feeding other forages, needs investigating. Similarly the effect of straw allowance on consumption and selection of straws other than barley needs researching. The works of Capper et al (1986), Tuah et al (1986), Ramazin et al (1986) and Doyle et al (1986) stress the magnitude of differences between straws of a given type in terms of feeding value. Difference in leaf: stem ratios probably accounts for much of this. There are likely to be other factors e.g. content of soluble phenolics (Reed, 1986) contributing to nutritive value differences in some crop residues and other tropical forages. Interactions between straw allowance rate and straw type, as affecting intake and selectivity, are therefore likely. Zemmelink (1986) has clearly shown this to be so for tropical forages.

Table 6. Experiment 6. Digestible-straw intake by goats fed straw or straw-previously-refused, with or without ammonia treatment.

	Straw 1		Straw-previously-refused by goats
	Untreated	NH3 treated	Untreated	NH3 - treated	SED
Number of goats 3	6	6	6	6
Straw intake
g DM/kg W.d	22.8	24.5	15.8	19.4	1.98
DM/kg W0.75 .d	53.9	58.9	37.5	47.3	4.71
g digestible OM4/kg W.d	9.7	12.6	6.6	9.7	0.90
g digestible OM/kg W0.75.d	22.9	30.4	15.6	23.7	2.11

1. Barley straw fed in Experiments 2 and 4; fed to allow 50% rate of refusal. straw chopped.
Concentrate supplement also fed, at 15 g DM/kg W^0.75.d

2. Straw from 50% refusal rates in Experiments 2 and 4; straw chopped. Concentrate supplement also fed at 15 9 DM/kg W^0.75 .d

3. Mean liveweight (W) 36.0 kg.

4. In vivo digestibility measured; concentrate OMD assumed to be 80%.

Source: Wahed and Owen (1987).

Table 7. Experiment 7. Effect of refusal rate and NaOH-treatment of barley straw on intake and selection by goats.

	Untreated straw		NaOH - straw
Straw refusal allowance (% of offered)	20	50	20	50
Number of goats1	9	9	9	9
Straw offered2
Amount (g DM/d)	805	1398	1031	1807
Leaf plus leaf sheath (g/kg straw DM) 449	449	451	451
Stem (g/kg straw DM)	477	477	502	502
Straw (g/kg straw DM)
Amount (g DM/d)	166	758	197	933
Leaf plus leaf sheath(g/kg straw DM)	307	359	355	405
Stem (g/kg straw DM)	661	613	612	559
Straw consumed
Total (g DM/kg W.d)	16.5	16.7	19.8	20.0
Leaf plus leaf sheath (g DM/kg W.d)	6.6	10.6	7.1	11.2
Stem (g DM/kg W.d)	7.1	5.3	9.4	8.8

1. Saanen castrates, mean liveweight (W) 41.4 kg
2. Concentrate supplement also fe, at 18 g DM/kg W^0.75 .d
Source: Owen et al, Reading University, Reading, UK, unpublished data.

Table 8. Experiment 8. Composition of straw offered.

	Untreated straw		NaOH - straw
	Leaf + leaf sheath	Stem	Leaf + leaf sheath	Stem
Ash (g/kg DM)	28.0	22.0	72.0	44.8
NA (g/kg DM)	1.7	1.2	21.1	14.5
ADF (g/kg DM)	521	668	501	610
DOMD in vitro (g OM/kg DM)	515	262	664	367

1. Tilley and Terry (1963).
Source: Owen et al, Reading University, Reading, UK, unpublished data.

A feeding strategy allowing goats and sheep to reject 50% of the straw offered is clearly wasteful (Table 9) and can only be justified if the rejected straw can be used too. Experiment 6 (Table 6) demonstrated that rejected straw can be refed to achieve high levels of digestible straw intake provided it is treated with ammonia. Feeding untreated straw to allow 50% refusals and refeeding these after ammonia treatment would result in little wastage and would also achieve high intakes of digestible straw (Table 9). The economics of such a strategy need investigating as labour costs would be high. A simpler approach would be to graze the straw in the field. Ungrazed straw would then be collected and refed after upgrading.

Table 9. Strategies for feeding barley straw to goats.

	Feed to allow 20% refusal (conventional ad lib feeding)	Feed to allow 50% refusal(generous feeding)	Feed to allow 50% refusal, then reffeed refusal after NH3 treatment and allow 20% refusal	Treat with NH3 and feed to allow 20% refusal
Initial quantity of straw (kg DM)	1000	1000	1000	1000
Unused (refused) straw;
available for bedding or other purposes (kg DM)	200	500	100	200
Total straw fed (kg DM)	800	500	900	800
Total digestible straw fed (kg OM)	350a	244b	444c	430d
Daily intake of digestible straw (g OM/kg W.d)	6.2	9.5	9.5	9.5 e.

^a Assuming digestibility of consumed straw to be 438 g OM/kg DM.
^b. Assuming digestibility of consumed straw to be 488 g OM/kg DM.
^c. Assuming digestibility of treated refusals to be 500 g OM/kg DM.
^d. Assuming digestibility of treated straw consumed to be 538 g OM/kg DM.
^e. Assuming NH₃ treatment increases DM intake by 25%.

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