Chemical composition, in vitro digestibility and kinetics of fermentation of whole-crop forage from 18 different varieties of oat (Avena sativa L.)

Abstract

The aim of this study was to compare the yield of forage and the yield of digestible organic matter as well as chemical composition and kinetics of fermentation of forage produced from 18 different varieties of oat (Avena Sativa L.) grown under the same agronomic condition. Dry matter and organic matter digestibility were determined using in vitro two-stage method and gas production technique was used to measure the kinetics of fermentation of forage from different varieties. Yield of forages varied significantly from 15.6 to 26.3 ton ha⁻¹ DM. LGorskij 1026 variety had the highest forage yield. The content of crude protein, neutral detergent fibre (NDF), acid detergent fibre (ADF) and acid detergent lignin (ADL) were significantly (P<0.01) different among varieties of oat used in this study. There were significant (P<0.01) differences among varieties in terms of in vitro dry matter digestibility (IVDMD) and organic matter digestibility (IVOMD) and metabolisable energy (ME) from 18 varieties of oat forage. Possum and LGorskij 1026 varieties had the highest and lowest IVOMD, respectively (67/4 vs. 54/9%). The ME varied from 5.76 to 7.13 MJ kg⁻¹ DM. The mean value of digestible organic matter (DOM) yield was 10.8 ton ha⁻¹. LGorskij 1026 variety had the highest DOM (13.3 ton ha⁻¹). Significant difference was observed in the potential gas production (A), constant rate (c) and lag time among different varieties. The fractional rate of gas production (c, h⁻¹) ranged from 0.029 to 0.040. The result emphasised that in any evaluation of oat varieties, not only yield and digestibility but also kinetics of digestion or fermentation should be taken into consideration.

Keywords:

• oat varieties
• chemical composition
• in vitro digestibility
• kinetics of fermentation

Introduction

Modern nutrition which is the basis of improving ruminant production particularly in developing countries has been devoted to nutritional characterisation of new vegetal species (Argüello 2011). Cereal forages of small grain are a major crop in many parts of the world, and their importance is increasing because of their significant economic and environmental benefits (Braunwart et al. 2001). Small grain cereals are used for forage at all stages of growth, from the grazing of small plants early in the growing season, to harvest of mature plants for hay at the end of the season (Bagley et al. 1987). Oats (Avena sativa L.) have historically been a widely used food and forage crop all over the world (Moreira 1989; Zhang et al. 1998; Wu 2007). Compared to other cereal crops, oat is reputed to be better suited for production under marginal environments, such as soils with low fertility (Hoffmann 1995). Chaudhary (1983) and Chaudhary et al. (1985) have evaluated the suitability of oats as a forage crop under different agro-climatic conditions. Oat as green forage, hay and silage is highly palatable and a very good forage to ruminants (Moreira 1989; Zhang et al. 1998; Wu 2007). Oat forage, for maximum nutritive value, should be harvested at early-dough stage as it may lose feeding value with advancing maturity (Christensen 1993). Other than stage of maturity, many other factors such as forage cultivar or variety (Ball et al. 2000) influences forage quality. Higher dry matter (DM) yield in forage production (Carr et al. 2001; Lithourgidis et al. 2006), higher intake by steers when fed as silage, similar digestibility and higher total digestible nutrients (TDN) for oat forages compared to barley and wheat forages were reported by Mtimuni (1976). Crude protein (CP) content of forage usually was considered as one of the most important criteria for forage quality evaluation (Caballero et al. 1995). CP content of oat forage reported in literatures ranges from 52 (Rodrigues et al. 2002) to 148 g kg⁻¹ DM (Lafore et al. 1999). Assefa and Ledin (2001) showed in vitro organic matter digestibility in three different varieties of oat was from 540 to 568 g kg⁻¹ DM. Higher dry matter digestibility (DMD) has been reported (59.0–65.3%) for oat hay in in vivo studies with sheep (Sileshi et al. 1998; Mazumder et al. 2004; Doran et al. 2007). Information on the quality and yield potential of small grain forages especially oat is lacking in Iran. Chemical composition, in combination with in vitro digestibility, is a useful indicator for preliminary evaluation of the likely nutritive value of feedstuffs. The objective of this study was to evaluate forage DM yield, in vitro digestibility and kinetics of fermentation of 18 different varieties of oat forage.

Materials and methods

Forage sample collection and preparation

Forages from 18 different varieties (Table 1) of oat were used in the study. They were grown under the same agronomic condition in three replicates in a randomised complete block design at Research Farm of School of Agriculture, Razi University, Kermanshah, Iran. The plots size was 1×2 m with line spacing of 15 cm. Forages were harvested from 20 June to 5 July at early dough stage (GS 83, based on zadoks cereal growth stage). Fresh forage yield was determined by harvesting each plot, approximately 4 cm above the soil surface and then weighted. A sub-sample of approximately 450 g was taken randomly from the harvested portion of each plot and dried for 48 h at 60°C for further analysis and at 100°C for 48 h to determine DM yield per hectare.

Table 1. Name and origin of different varieties of oat used in the experiment.

Variety	Origin
94046-57	SARDI (SA, Australia)
Dalyup	Wisconsin (USA)
Echidna	SARDI (SA, Australia)
Euro	SARDI (Australia)
GAM itchell	Georgia (USA)
Grise Dhiver	France
IL92-6745	Illinois (USA)
LGorskij 1026	USSR
Mortlock	WADA (Australia)
NZ2742	New Zealand
OH 1022	Ohio (USA)
Possum	SARDI (SA, Australia)
Potoroo	SARDI (SA, Australia)
Quall	SA (Australia)
UPF775456	Brazil
Wallaroo	SA (Australia)
Wandering	WADA (Australia)
Wintaroo	SARDI (SA, Australia)

Chemical composition

The dried forages were ground using a lab mill to pass a 1-mm screen. Standard methods as described in AOAC (1990) were used for determination of DM, ash and CP. The neutral detergent fibre (NDF), acid detergent fibre (ADF) and acid detergent lignin (ADL) were determined according to Van Soest et al. (1991).

In vitro digestibility

Rumen liquor was collected from three rumen cannulated sheep before the morning feeding. The liquor from three sheep was mixed on a volume basis, and filtered through four layers of cheesecloth. The incubation inoculum was prepared by diluting the rumen liquor with a buffer solution (Tilley and Terry 1963) in a 1:4 (v/v) ratio. Mixed inoculum was stirred in a water bath at 39°C with purging CO₂ until its use (10–15 min later). About 250 mg (1 mm ground) of each sample was placed into 50-ml sterile tubes, and 20 ml of the incubation inoculum was added. The tube was stoppered with Bunsen valve and incubated for 48 h at 39°C. Tubes were gently swirled by hand every 8 h. Each sample was incubated in three replicates. At the end of the 48 h of the incubation period, tube contents were acidified using 6 M HCL to reach a final pH of 1.3–1.5. After a few seconds, when the foam subsided, pepsin powder (EC 3.4.23.1) was added to a final concentration of 0.2% (w/v). The tubes were re-incubated for an additional 48 h period. The tubes were centrifuged at 2500 rpm for 15 min, and the supernatant was discarded. The pellets were dried in a forced air oven at 60°C for 48 h to determine the residual DM weights. Then, to determine ash content, the residues were kept at 550°C for 8 h to estimate organic matter (OM). In vitro DM and organic matter digestibility were calculated respectively as the DM and OM which disappeared from the initial weight inserted into the tube. Calculations were as follows:

In vitro gas production

The method used for gas production measurements was as described by Theodorou et al. (1994). All samples were ground to pass a 1 mm screen. About 125 mg of each sample was weighed into tubes kept at approximately 39°C and flushed with CO₂ before use. Each sample was incubated in three replicates. Fifteen millilitre of buffered rumen fluid (20% rumen fluid+80% buffer solution) prepared (as described in in vitro digestibility section) and were anaerobically dispensed in each tube at 39°C. All the tubes were crimped, placed in an incubator at 39°C, and shaken at regular times. The pressure of gas produced in each tube was recorded using a pressure transducer (Manometer Digital testo 512) at 2, 4, 6, 8, 12, 18, 24, 48, 72, 96 and 120 h after the start of the incubation. To estimate the kinetics of gas production, data on cumulative gas volume produced were fitted using the generalised Mitscherlich model proposed by France et al. (1993):

Where GP (ml) denotes cumulative gas production at time t, A (ml) is asymptotic gas production, c (h⁻¹) and d (h^−½) are rate constants and L (h) is lag time. The half-life (t _½, h) of the fermentable fraction of each substrate was calculated as the time taken for gas accumulation to reach 50% of its asymptotic value. All gas volumes were adjusted to a common sample weight of 200 mg DM (Lopez et al. 2007).

The volume of gas produced (GP) (ml 200 mg⁻¹ DM) after 24 h of incubation was used with CP content to estimate metabolisable energy (ME) concentration (MJ kg⁻¹ DM) based on the following equation reported by Menke and Steingass (1988) for roughage feeds:

Where:

ME=metabolisable energy (MJ kg⁻¹ DM); GP=gas production after 24 h (ml 200 mg⁻¹ DM); XP=crude protein (%).

In vitro gas production at 24 h

A shorter gas production test was done to determine in vitro true DM degradability (IVTDMD) and NDF degradability. Rumen digesta collection, sample and buffer preparations and incubation techniques were as described above. The incubation was terminated at 24 h after recording the gas volume and the entire residue in the incubation tubes was drained into 600 ml spotless beakers and refluxed with neutral detergent solution to determine IVTDMD and NDFD. This determination is the Goering and Van Soest (1970) modification of Tilley and Terry (1963) as described by Blümmel and Becker (1997). The partitioning factor (PF) was calculated as the ratio of milligram substrate truly degraded per millilitre gas produced by it according to Blümmel et al. (1997).

Statistical analysis

Data on yield performance, gas production parameters and DM digestibility from different oat varieties were subjected to one way analysis of variance using the SAS (2002) and significance between individual means was identified using Duncan multiple-range test (Duncan 1955).

Results and discussion

DM yield

DM yield of forages varied significantly (P<0.01) from 15.6 ton ha⁻¹ DM to 26.3 ton ha⁻¹ DM in different varieties used in the experiment (Table 2). DM yield of forage of different oat varieties reported in literatures vary from 5 (Zaman et al. 2006) to 35 ton ha⁻¹DM (Amanullah et al. 2004).

Table 2. Yield (ton ha⁻¹ DM) and chemical composition (% DM) of forage from 18 different varieties of oat.

Variety	Yield	DM	OM	CP	NDF	ADF	ADL
94046-57	15.6^k	24.6^def	91.4^de	11.0^b	54.9^efg	32.2^hi	3.76^fg
Dalyup	17.4^ij	23.2^f	91.1^e	7.61^ef	55.3^defg	36.8^bcde	5.24^abcd
Echidna	18.6^ghi	24.2^ef	91.6^bcde	6.11^ghi	55.9^cdef	33.5^gh	5.17^abcde
Euro	19.0^fgh	24.8^def	91.8^abcde	5.89^hi	59.1^ab	36.6^cde	5.62^ab
GAM itchell	19.5^efg	23.6^f	91.3^de	11.1^b	58.2^abc	38.1^abcd	4.45^ef
Grise Dhiver	20.3^cdef	24.7^def	91.2^de	5.97^hi	58.4^ab	38.8^abc	5.15^abcde
IL92-6745	21.8^c	27.9^bc	92.2^abcd	5.96^hi	59.7^a	38.9^ab	4.87^bcde
LGorskij 1026	26.3^a	26.1^cd	91.5^cde	7.29^fg	58.7^ab	39.9^a	5.41^abc
Mortlock	19.9^efg	27.5^bc	92.8^a	5.22^i	50.0^h	35.0^efg	4.52^de
NZ2742	23.6^b	29.4^ab	92.5^abc	10.3^bc	58.7^ab	38.1^abcd	5.12^abcde
OH 1022	21.6^cd	28.2^b	91.7^bcde	9.50^cd	57.1^bcde	35.9^def	4.45^ef
Possum	16.3^jk	25.7^de	91.8^abcde	12.2^a	53.5^fg	30.6^i	4.72^cde
Potoroo	19.7^efg	30.7a	92.6^abc	5.88^hi	53.1^g	33.9^fgh	4.81^cde
Quall	17.9^hi	23.6^f	92.2^abcde	9.08^d	57.3^abcd	32.5^hi	3.31^g
UPF775456	24.9^b	25.7^de	92.2^abcde	8.55^de	57.5^abcd	36.2^de	4.59^de
Wallaroo	19.3^fgh	27.6^bc	92.1^abcde	7.15^fgh	54.5^fg	35.4^efg	5.80^a
Wandering	21.1^cde	28.2^b	91.6^cde	7.15^fgh	57.3^abcd	34.8^efg	5.64^ab
Wintaroo	20.2^def	28.9^b	92.7^ab	5.55^i	54.7^fg	35.6^efg	4.90^bcde
Mean	20.2	26.4	91.9	7.86	56.3	35.7	4.86
P-value	<0.001	<0. 001	0.009	<0.001	<0. 001	<0.001	<0.001
SEM	0.383	3.19	0.320	3.03	3.71	3.66	0.961
Note: Values in the same columns without common letters are significantly different (p<0.05).
DM, dry matter; OM, organic matter; CP, crude protein; NDF, neutral detergent fibre;
ADF, acid detergent fibre; ADL, acid detergent lignin; SEM, standard error of the means.

Chemical composition

The chemical composition of different varieties of oat is presented in (Table 2). There were significant (P<0.01) differences between varieties in terms of CP, NDF, ADF and ADL contents. CP content (ranging from 52 to 122 g kg⁻¹ DM) of different oat varieties used in the present experiment was similar to those reported by Assefa and Ledin (2001), Calabro et al. (2005) and Doran et al. (2007). Possum variety had the highest CP content.

Other important quality characteristics for forages are the concentrations of NDF and ADF (Caballero et al. 1995; Assefa and Ledin 2001). The mean NDF, ADF and ADL content of oat forages were 563.3, 357.2 and 48.6 g kg⁻¹ DM, respectively. The greatest difference between the highest and the lowest value in cell wall fractions was observed in lignin contents (58 vs. 33 g kg⁻¹ DM), followed by hemicellulose (248.5 vs. 149.8 g kg⁻¹ DM) and cellulose (345.3 vs. 258.8 g kg⁻¹ DM). The NDF levels of oat varieties in the present study were lower than the values reported by Rodrigues et al. (2002) and Doran et al. (2007), but the obtained ADF and ADL contents were consistent with their values indicating lower hemicellulose content in the varieties under present study.

In vitro digestibility and metabolisable energy

The result of in vitro digestibility (Table 3) showed that there were significant (P<0.01) differences between DM and OM digestibility of forages from different oat varieties. The IVOMD ranged from 553 to 674 g kg⁻¹ DM. About 20% difference was observed between the highest and the lowest IVDMD and IVOMD among different varieties (Table 3).

Table 3. In vitro digestibility of DM and OM (IVDMD and IVOMD, % DM) and digestible DM and OM yield (DDM and DOM, ton ha⁻¹) and metabolisable energy (ME, MJ kg⁻¹ DM) of forage from 18 different varieties of oat.

Variety	IVDMD	IVOMD	DDM	DOM	ME
94046-57	64.6^b	65.3^b	10.10^hi	9.33^g	7.13^a
Dalyup	56.4^fg	58.3^d	9.83^i	9.26^g	6.17^ef
Echidna	61.6^cd	62.2^c	11.46^defg	10.61^def	6.24^de
Euro	56.5^efg	56.0^ef	10.74^fghi	9.77^fg	5.96^h
GAM itchell	54.7^ghi	55.3^f	10.69^ghi	9.86^fg	6.38^cd
Grise Dhiver	54.8^ghi	55.3^f	11.12^defgh	10.17^defg	5.80^hi
IL92-6745	56.0^g	56.3^ef	12.19^cd	11.32^cd	5.81^hi
LGorskij 1026	53.5^i	54.9^f	14.07^a	13.24^a	5.76^i
Mortlock	55.8^gh	55.6^f	11.09^defgh	10.26^defg	6.02^fg
NZ2742	54.7^ghi	55.6^f	12.91^bc	12.15^bc	6.36^cd
OH 1022	55.7^gh	55.5^f	12.04^cd	11.00^de	6.29^cde
Possum	66.7^a	67.4^a	10.85^efghi	10.05^efg	7.04^a
Potoroo	60.8^d	62.0^c	12.00^cde	11.31^cd	6.22^de
Quall	63.1^bc	63.8^bc	11.27^defg	10.52^def	6.59^b
UPF775456	53.9^hi	55.6^f	13.04^ab	12.72^ab	6.34^cde
Wallaroo	54.6^ghi	55.7^f	10.51^ghi	9.87^fg	6.22^de
Wandering	57.8^ef	57.9^de	12.20^cd	11.19^cd	6.46^bc
Wintaroo	58.2^e	58.9^d	11.78^cdef	11.03^de	6.00^g
Mean	57.7	58.4	11.57	10.76	6.27
P-value	<0.001	<0.001	<0.001	<0.001	<0.001
SEM	5.47	5.47	0.165	0.161	0.052
Note: Values in the same columns without common letters are significantly different (P<0.05).
SEM, standard error of the means.

Table 4. Correlations (r) between in vitro organic matter digestibility (IVOMD), gas production components, metabolisable energy (ME, MJ kg⁻¹ DM) and chemical composition (%).

	NDF	ADF	ADL	CP	A (ml)
IVOMD	−0.35*	−0.79**	−0.39**	−0.35*	0.71**
A (ml)	−0.48**	−0.83**	−0.23 ns	0.14 ns	–
c (h^−1)	−0.38**	−0.67**	−0.42**	0.65**	0.55**
ME	−0.26 ns	−0.72**	−0.44**	0.73**	0.60**
A (ml), asymptotic gas production; c (h^−1), fractional rate of gas production.
*P<0.05; **P<0.01; ns, non-significance.

LGorskij 1026 variety with the highest ADF had the lowest IVOMD while Possum variety with the lowest ADF had the highest IVOMD. It is well accepted that the chemical linkages between lignin and hemicellulose are the principal plant factors restricting digestion of the plant cell wall by ruminants (Howe et al. 1988). Therefore, the cell wall fractions may have a negative influence on forage digestibility as generally described in conventional feedstuffs (Van Soest 1994). A strong negative correlation was found between ADF content with IVOMD (r=−0.79, P<0.01) across all varieties (Table 4). Differences in the digestibility of forages from different varieties may be due, not only to the chemical composition (Dias da-silva and Guedes 1990) but also to different stem, leaves and seed ratios (Bhargava et al. 1988). Assefa and Ledin (2001) showed that the mean in vitro organic matter digestibility of 3 oat varieties was 554 g kg⁻¹ DM. Higher in vivo OM digestibility (626 g kg⁻¹ DM) has been reported by Doran et al. (2007). The predicted metabolisable energy (MJ kg⁻¹ DM) of forage from different varieties was calculated from the amount of gas produced at 24 h incubation with the supplementary analysis of CP. There were significant (P<0.01) differences between ME of forages from different varieties. The metabolisable energy ranged from 5.8 to 7.1 MJ kg⁻¹ DM. Possum variety and LGorskij 1026 variety had the highest and lowest ME, respectively. Metabolisable energy was negatively correlated with ADF concentrations (r=−0.72, P<0.01) and positively correlated with the CP content (r=0.73, P<0.01). The mean value of digestible OM yield (DOM) was 10.8 ton ha⁻¹. LGorskij 1026 variety had the highest DOM (13.3 ton ha⁻¹) while Dalyup variety produced the lowest DOM (9.3 ton ha⁻¹). Other factors known to affect the composition and digestibility of forage are varieties and cultivars (Ørskov et al. 1990; Emile Mile et al. 2007; Bartl et al. 2009), environmental and seasonal effects (Mathison et al. 1999; Jacobsen et al. 2005), proportion of different morphological fractions of forage (Agbagla-Dohnani et al. 2001; Fulkerson et al. 2008), stage of maturity at harvested (Meyer et al. 1957; Ammar et al. 2008). In addition, the effect of additives such as enzymes (Gómez-Vázquez et al. 2011) on digestibility has also been studied.

Table 5. Cumulative gas production and kinetic parameters model for different varieties of oat forages incubated with rumen fluid in vitro.

	Gas parameters
Variety	A (ml 200 mg^−1)	c (h^−1)	L (h)	t ½ (h)
94046-57	62.2^ab	0.038^b	0.630^e	22.1^k
Dalyup	59.9^i	0.032^f	0.707^cd	24.8^f
Echidna	61.2^d	0.032^f	0.577^f	23.7^i
Euro	62.0^b	0.029^i	0.507^g	25.7^cd
GAM itchell	59.5^j	0.031^g	0.737^bc	25.8^c
Grise Dhiver	59.0^l	0.031^g	0.853^a	27.2^a
IL92-6745	59.2^k	0.030^h	0.737^bc	26.2^b
LGorskij 1026	58.9^l	0.029^i	0.870^a	27.2^a
Mortlock	61.2^d	0.034^cd	0.843^a	25.4^de
NZ2742	59.4^j	0.034^cd	0.767^b	25.5^cd
OH 1022	60.3^g	0.033^e	0.853^a	25.4^de
Possum	62.3^a	0.040^a	0.63^e	22.0^k
Potoroo	61.6^c	0.031^g	0.560^f	24.1^gh
Quall	62.1^b	0.033^d	0.627^e	24.2^g
UPF775456	60.1^h	0.034^c	0.703^cd	23.8^hi
Wallaroo	61.0^e	0.031^g	0.517^g	23.9^ghi
Wandering	61.5^c	0.034^c	0.560^f	22.6^j
Wintaroo	60.8^f	0.031^g	0.683^d	25.2^e
Mean	60.7	0.033	0.702	24.7
P-value	<0.001	<0.001	<0.001	<0.001
SEM	0.160	0.000	0.017	0.207
Note: Values in the same columns without common letters are significantly different (P<0.05).
A, potential gas production; L, lag time; t ½, time to half asymptote; (c), fractional rate of gas production; SEM, standard error of the means.

In vitro gas production

Gas production kinetic parameters of forage from oat varieties are presented in (Table 5). There were significant (P<0.01) differences among varieties in asymptotic gas production. Potential gas production (A) ranged from 58.9 to 62.6 (ml 200 mg⁻¹ DM). Significant difference was observed in constant rate (c) and lag time among different varieties. The c values ranged from 0.029 to 0.040 (h⁻¹) and the lowest rate of gas production was observed in LGorskij 1026 variety.

Table 6. In vitro true dry matter degradability (IVTDMD,%), NDF degradability (NDFD,%) at 24 h incubation and the partitioning factor PF (IVTDMD: 24 h gas production, mg ml⁻¹).

Variety	NDFD	IVTDMD	PF
94046-57	44.5^a	65.5^a	2.83^cd
Dalyup	41.9^abc	63.9^ab	3.16^ab
Echidna	36.1^cde	60.3^cdef	2.93^bcd
Euro	38.3^bcd	59.54^cdef	2.87^bcd
GAM itchell	36.4^cde	59.0^def	2.76^d
Grise Dhiver	40.1^abc	61.0^bcd	2.85^bcd
IL92-6745	39.5^abcd	59.8^cdef	2.75^d
LGorskij 1026	31.6^e	55.9^g	2.88^bcd
Mortlock	31.5^e	61.7^bcd	2.92^bcd
NZ2742	30.8^e	55.4^g	2.86^bcd
OH 1022	34.0^de	57.6^fg	2.88^bcd
Possum	43.8^ab	65.9^a	3.30^a
Potoroo	36.4^cde	57.9^efg	2.92^bcd
Quall	38.8^bcd	60.9^cde	3.09^abc
UPF775456	41.5^abc	62.4^bc	2.75^d
Wallaroo	30.8^e	55.4^g	2.86^bcd
Wandering	37.4^cd	60.1^cdef	2.81^cd
Wintaroo	41.5^abc	64.0^ab	2.91^bcd
Mean	37.5	60.4	2.91
P-value	<0.001	<0.001	0.013
SEM	0.682	2.35	0.026
Note: Values in the same columns without common letters are significantly different (P<0.05).
SEM, standard error of the means.

Ranges in gas production characteristics may partly be due to differences in CP, NDF and ADF contents. Nsahlai et al. (1994) and Larbi et al. (1998) reported that there were a positive correlation between CP and the rate of gas production and negative correlations between NDF and ADF, and the rate and extent of gas production. Asymptotic gas production (A) was lowest in LGorskij 1026 and highest in Possum varieties. These findings were in line with the in vitro digestibility results in which LGorskij 1026 and Possum varieties showed the lowest and the highest IVOMD, respectively. The high negative correlation found between ADF and either asymptotic gas production (A) (r=−0.83, P<0.01) or c (r=−0.69, P<0.01) is consistent with the results of Haddi et al. (2003) who reported that there were significant negative correlation between NDF and ADF, and the rate and extent of gas production. The negative effect of cell wall content on gas production could be due to the reduction of the microbial activity through increasing the adverse environmental conditions as incubation time progresses. The values of potential gas production and fractional rate of gas production (c) of experimental varieties of oat were higher or comparable to those (45.6 ml 200 mg⁻¹ DM and 0.015 h⁻¹, respectively), reported by Sallam et al. (2008) for alfalfa hay as commonly used forage for livestock.

NDF degradability (NDFD) at 24 h and the partitioning factor (PF)

Degradability of NDF is an important component of forage quality. Increased NDFD may result in reduced physical fill in the rumen over time and allow greater voluntary feed intake (Dado and Allen 1995). Oba and Allen (1999) reported that a one-unit increase in forage NDFD in vitro or in situ was associated with a 0.17 kg increase in DM intake (DMI). In vitro true DM degradability (IVTDMD) and NDF degradability (NDFD) were determined after terminating the incubation at 24 h and were calculated from the truly undegraded substrate. IVTDMD varied from 55.4 to 65.9% while NDFD varied from 30.83 to 44.52% (Table 6). The partitioning factor (PF), an index of the substrate dependant variation in the ratio of substrate degraded to gas volume produced by it at 24 h of incubation, is reported to be valuable in improving accuracy of voluntary DMI prediction of temperate, tropical crop residues and Mediterranean hays and forages with high PF result in high DMI (Blümmel et al. 1997). Blümmel et al. (1997) noted that in forage fermentation, PF values between 2.75 and 4.41 mg ml⁻¹ do correspond to ATP yield (YATP) from 10 to 32 mg, and a YATP of 32 mg is considered to be maximum microbial efficiency. Calculated PF from different varieties in the present experiment ranged from 2.75 to 3.30. Blümmel et al. (2005) reported 48.8% and 2.81 mg ml⁻¹ for IVTDMD and PF in oat hay, respectively.

Conclusion

Cell wall fraction, digestibility, potential and kinetics of gas production from forages varied among different varieties grown under similar agronomic condition. Great variation was also observed in the yield of digestible organic matter. The high significant and negative correlation observed between each of IVOMD, potential gas production (A, ml) and rate of fermentation (c, h⁻¹) with ADF indicates that in any evaluation of oat forage varieties, not only yield and digestibility but also potential gas production and the rate at which the gas is produced should be taken into consideration.