Value of grass silage, dried pelleted grass and malt distillers' grains as supplements to barley straw

Value of grass silage, dried pelleted grass and malt distillers' grains as supplements to barley straw

School of Agriculture, University of Aberdeen, 581 King Street, Aberdeen AB9 1UD, Scotland, UK

An experiment was carried out to determine intake and digestibility in cattle of long barley straw supplemented with grass silage (S), dried pelleted grass (G), malt distillers' grains (D) or barley and soya bean meal (B). The supplements were offered at two levels, low (L) and high (H). The low level was 2.5 kg and high level 3.25 kg DM per animal per day. Eight British Friesian steers initially weighing 378 kg liveweight were used in an incomplete block design derived from half of an 8x8 latin square.

All supplements depressed straw intake and total diet DM intake when compared with the basal diet in which barley and soya bean meal were used. Similar effects were noted on digestibility except for the silage supplement which did not effect digestibility. The most noticeable depressing effect was with the malt distiller's solubles.

It is suggested that the barley/soya bean was more effective than the other supplements in maintaining a high protein:energy ratio in the mixture of nutrients presented for metabolism, probably because the soya bean meal supplied more bypass protein than the other supplements. The fact that the distiller's grains had the most negative effect both on intake and digestibility (especially of the fibre) is probably because of its high lipid content and the known bacteriostatic effect of fat on cellulolytic microorganisms.

KEY WORDS: Intake, digestibility, straw, supplements, rumen function, bypass protein, malt distiller's grains, lipids, silage, dried grass

The first limiting factor to animal production in the tropics is nutrition. Yet in these regions there are vast feed resources for ruminant livestock that are unused or poorly utilised and could make a major impact on livestock production. Crop production gives rise to considerable amounts of by-products that ruminants can convert into highly nutritious animal products for people. Pressure on land, scarcity of concentrates, and frequent droughts are some of the factors that justify increased use of these non-conventional feed resources for ruminant animal feeding.

Nutrient analyses of cereal straws indicate that they fall within the definition of Balch (1977) of a low-quality roughage. They are low in protein (20 - 65 g CP/kg DM) and phosphorus, marginal in calcium and high in fibre and lignin. However, when the inherent nutrient deficiencies in these by-products are corrected by supplementation or manipulation of the diet or the rumen ecosystem, moderate levels of productivity can be achieved. The principal objective of this research was to determine the value of grass silage, dried grass (high-quality conserved forages) and malt distillers' grains (agro-industrial by-product) as supplements to cereal straw.

The primary limitations to ruminant productivity on crop residues are the unbalanced nature of the nutrients in the available forages and their low digestibility (Preston and Leng 1987). The purpose of supplementation is to increase voluntary intake and digestibility of the basal low-cost diet. Leng (1986) proposes two major strategies for improving ruminant production in animals fed crop residues or mature tropical pastures as (1) to supplement nutrients to ensure efficient rumen function, and (2) to provide by-pass supplements to balance those available from fermentative digestion thus ensuring maximum intake and efficiency of utilisation of the absorbed nutrients by the animal.

Supplementation of cereal straws with good quality conserved grass and agro-industrial by-products is an area in ruminant nutrition that has not been extensively investigated. However, available literature suggests that a potential exists in the use of high- quality forages to up-grade the nutritive value of the low-quality diets. Guttierez et al (1983), Leng (1982) and Preston and Para (1981) have noted the stimulatory effect of small amounts of fresh green forage on increasing dry matter (DM) intake and digestibility of low-quality forage. Rissanen and Kossila (1977) showed how increasing amounts of grass silage reduced the amount of concentrate required to supplement NH₃-treated straw. Terry et al (1975) appreciated the complementarity of grass silage and alkali treated straw in improving both intake and digestibility of the whole diet. Mbatya (1980) used good quality dried perennial rye- grass (162 g CP/kg DM) as supplement to straw treated with urea/molasses and found significant increase in both total DM intake and digestibility as the grass level increased in the diet. The by-products of brewing and distilling industries have been used quite extensively in the feeding of livestock, especially in the finishing of beef cattle and in dairy production. In Scotland, malt distillers' grains, known locally as draff is quantitatively the most important. The greatest limitation to the use of this by- product is its high lipid content (80 - 90 g/kg DM) which tends to depress both intake and digestibility of the roughage diet. However, provided malt distillers' grains is fed at a suitable level and is correctly supplemented with minerals it is a valuable source of energy as well as protein.

An experiment was laid out in an incomplete block design derived from half of an 8 x 8 latin square (Cochran and Cox 1960) using eight British Friesian steers aged about 18 months and averaging 378 kg liveweight to determine intake and digestibility in cattle fed on long barley straw as basal diet supplemented with grass silage (S), dried pelleted grass (G), malt distillers' grains (D), and barley/soya bean mixture (B). Both grass silage and dried pelleted grass were made from perennial rye grass. The experiment was divided into four periods involving change-over periods, and the animals were randomly allocated to treatments within each period. The supplements were offered at two levels, low (L) 2.50 kg DM/d, and high (H) 3.25 kg DM/d.

The barley/soya bean mixture (B) was used as a control. Table 1 shows the mean composition of the diet ingredients as determined by chemical analyses at the end of each period.

Table 1: Mean composition of diet ingredients over the four periods (g/kg DM)

Ingredient	DM (g/kg)	CP	MADF	EE	Ash	pH

Barley straw	888	37	490		36
Barley grain	827	124	66		23
Soya bean meal	880	439	109		62
Silage	271	189	276		89	4.2
Dried grass	900	140	334		91
Malt distillers' grains	282	228	46	128	38	4.5

Barley straw was offered ad libitum in all the treatments and 60 g per animal of a commercial mineral and vitamin supplement was given daily.

Intake and digestibility measurements were carried out in individual metabolism cages. Animals were fed straw in two feed (morning and afternoon). Supplements were offered once a day, and were given before straw. Water was available ad libitum in automatic water troughs.

Each of the four feeding periods was of three weeks duration, divided into a 14-day preliminary feeding period followed by a 7- day measurement period. Total collection was used for digestibility measurements. High moisture feeds (D and S) were analyzed daily for DM and feeds with less variable moisture content (G and B) were analyzed for DM weekly. Feed and faecal samples for each animal were taken daily and stored at -20°C. At the end of each measurement period the samples were pooled, thoroughly mixed, and sub-sampled for laboratory analysis for DM, crude protein (CP), modified acid detergent fibre (MADF), ash, ether extract (EE) and pH. The results of these analyses were used to calculate digestibility.

Since the animals did not receive equal treatments, the data for intake and digestibility were compared using least squares analysis of variance (Harvey 1977).

To compare the effects of the four supplements on intake, the mean of the low and high level for each supplement was used (Table 2). All supplements depressed straw intake and total diet DM intake when compared with the basal diet in which barley and soya bean meal were used. Similar effects were noted on digestibility except for the silage supplement which did not effect digestibility. The most noticeable depressing effect was with the malt distiller's solubles.

Table 2: The effects of supplements on intake (mean values for low and high levels of supplements used).

	S u p p l e m e n t
	B	D	G	S	SED	CV(%)

Intake of DM (kg/d)	6.9a	5.4c	6.1b	5.8b	0.189	5.7
Diet DM (g/kgw0.75/d)	76.5a	59.4c	67.9b	66.3b	2.20	3.9
Diet OM (kg/d)	6.7a	5.2c	5.8b	5.5bc	0.18	5.7
Straw DM (kg/d)	4.1a	2.6c	3.3b	2.7c	0.20	11.5
Straw DM (g/kgw0.75/d)	44.9a	28.6c	36.9b	30.6c	2.24	11.7

SED = Standard error of difference between least squares means.
CV = Coefficient of variation
abc: Similar superscripts for each variable denote no significant difference between least squares means.

Table 3: The effects of supplements on digestibility (mean values of low and high levels of supplements used).

	S u p p l e m e n t
	B	D	G	S	SED	CV(%)

Diet DM (%)	61.6a	47.8b	48.9b	61.4a	1.73	5.8
Diet OM	62.7a	49.7b	50.1b	62.2a	1.71	5.6
Diet MADF	53.1ab	16.7c	47.5b	60.9a	4.42	18.2

SED = Standard error of the difference between least squares means.
CV = Coefficient of variation.
abc: Similar superscripts denote no significant differences between the least squares means.

Table 4 shows the effects of low and high level of supplement on intake and digestibility. Increasing supplement level depressed straw intake but the degree of depression varied with supplement: on barley/soya the reduction was 7%, on distiller's grains 27%, on dried grass 3% and silage 10%. Effects on digestibility also varied according to supplement: barley/soya increased digestibility 8%, distiller's grains reduced it 5%, dried grass increased it 5% and silage increased it by 2%.

Table 4: Mean values of intake and digestibility for the low and high level of supplement.

	Bar/soy		Grains		Grass		Silage
	BL	BH	DL	DH	GL	GH	SL	SH

DM intake (kg/d)
Total	6.69	7.10	5.41	5.33	5.79	6.48	5.53	6.07
Straw	4.20	3.90	3.00	2.20	3.40	3.30	2.90	2.60
Digestibility (%)
Diet DM	59.3	63.8	49.0	46.6	47.7	50.0	60.9	61.9
Diet OM	60.2	65.2	51.0	48.4	48.8	51.4	61.6	62.7

The main findings were that none of the supplements was as effective as the control supplement of barley and soya bean meal in maintaining the intake of the basal diet of straw. This presumably can be related to the probability that the barley/soya bean was more effective than the other supplements in maintaining a high protein:energy ratio in the mixture of nutrients presented for metabolism. It appears likely that the soya bean meal supplied more bypass protein than the other supplements. The fact that the distiller's grains had the most negative effect both on intake and digestibility (especially of the fibre) is probably because of its high lipid content and the known bacteriostatic effect of fat on cellulolytic microorganisms.

The results of this study show clearly the interacting effects of different supplements on a basal diet of cereal straw according, it can be assumed (Preston and Leng 1987), some of the effects being mediated at the level of the rumen (eg: the oil in distiller's grains) or post-ruminally (increasing the level of soya bean meal).

When the treatments were formulated an attempt was made to have them comparable according to the recommendations of the ARC (19--) for metabolizable energy (ME) and rumen degradable protein; the fact that the results were quite different from what had been predicted demonstrates clearly that the ARC (19--) system does not predict responses when the basal diet is a crop residue which is highly sensitive to the balance of available nutrients both at the rumen and post-rumen level (Preston and Leng 1987).

We wish to thank the British Council for sponsoring the research which has led to the writing of this paper. Our thanks also go to Professor J F D Greenhalgh of the School of Agriculture, University of Aberdeen for his valuable advice and suggestions during the writing of this paper.

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