Napier or groundnut hay as supplements in diets of sheep consuming poor quality natural pasture-hay 2. Effect on intake and rumen digesta kinetics

Livestock Production Science 49 (1997) 43-52

Napier or groundnut hay as supplements in diets of sheep consuming poor quality natural pasture-hay 2. Effect on intake and rumen digesta kinetics B. Manyuchi a3*, F.D. Deb Hovel1 b, L.R. Ndlovu ‘, J.H. Topps ‘, A. Tigere d a Africa Uniuersity, P.B. 1320, Mutare, Zimbabwe b Rowett Research Institute, Aberdeen, Scotland AB2 9SB, UK ’ University of Zimbabwe, Harare, Zimbabwe ’ Grasslands Research Station, Marondera, Zimbabwe

Accepted 7 February 1997

Abstract Three experiments, each of a 5 X 5 Latin square design, were carried out using sheep to assess the effect of supplementing poor quality natural pasture (veld) hay with graded levels of napier or groundnut hay on feed intake, rumen fermentation, digesta pool size and passage rate. In experiment 1, napier hay was fed at 0 g day-‘, 100 g day-‘, 200 g day-‘, 300 g day-’ or ad libitum. In experiment 2 the same levels of feeding were used with groundnut hay, while in experiment 3, napier and groundnut hay were each fed at 0 g day-‘, 150 g day-’ or 300 g day-’ to enable a direct comparison of the two supplemental forages. All the forages used were chopped to a particle size of about 1 cm prior to feeding. The veld hay was supplemented with 1% urea and was always offered ad lib&urn. The forage supplements were fed separately. In all the experiments, the forage supplements increased total feed intake, digestibility and concentration of ammonia and volatile fatty acids in the rumen. Incubation of veld hay in the rumen of sheep given napier and groundnut hay as supplements or as sole feeds, showed no effect on degradation pattern including rate of degradation. Rumen digesta pool size was not altered, but fractional outflow rate was increased by supplementation. The increase in feed intake was therefore largely facilitated by an increase in digesta passage rate. 0 1997 Elsevier Science B.V. Keywords: Sheep; Intake; Forages; Supplements; Digesta kinetics

1. Introduction Research has shown that high quality forage supplements are effective in increasing feed intake when animals are given poor quality forages (Minson and Milford, 1967, Siebert and Kennedy, 1972, Smith et

* Corresponding author.

al., 1989). The increase

in feed intake is associated with an increase in digestibility of the total diet. Since poor quality forages are deficient in nitrogen, the response to high quality forage supplements has been attributed to an increased supply of dietary nitrogen, stimulating rumen digestion and hence intake. Works by Ndlovu and Buchanan-Smith (1985), Silva (19851, McMeniman et al. (1988) and Manyuchi et al. (1992), demonstrate improvements

0301-6226/97/$17.00 0 1997 Elsevier Science B.V. All rights reserved. PII SO301-6226(97)00042-O

44

B. Manyuchi et al. /Livestock Production Science 49 f 1997) 43-52

in microbial degradation of fibre in the reticulo-rumen, supporting the hypothesis that the observed increase in feed intake is due to an increase in fibre digestion. However, in most of the feeding experiments, it has not been possible to show this positive associative effect of high quality forage on the digestibility of the low quality forage.In recent studies (e.g., Mbatya et al., 1983; Silva, 1985; Manyuchi et al., 19921, similar responses to forage supplements have been observed when the basal diet was supplemented with nitrogen, suggesting that the response can not be entirely due to nitrogen. Studies by Ndlovu and Buchanan-Smith (1985) and Vanzant and Co&ran (1994) indicate that increased rate of passage of digesta following supplementation might be an important mechanism by which the increase in feed intake is effected. The mechanism by which forage supplements increase feed intake is therefore still poorly understood. The current study assessed changes in feed intake together with changes in rumen environment, digesta kinetics and pool size when poor quality natural pasture hay was supplemented with either groundnut or napier hay.

2. Materials and methods Three experiments, each of a 5 X 5 Latin square design, were carried out using sheep to assess the effect of supplementing poor quality natural pasture (veld) hay with graded levels of napier or groundnut hay on feed intake, rumen fermentation, digesta pool size and passage rate. In experiment 1, napier hay was fed at 0 g day- ‘, 100 g day-‘, 200 g day-‘, 300 g day-’ or ad libitum. In experiment 2, the same levels of feeding were used with groundnut hay, while in experiment 3, napier and groundnut hay were each fed at 0 g day-‘, 150 g day-’ or 300 g day-’ to enable a direct comparison of the two supplemental forages. All the forages used were chopped to a particle size of about 1 cm prior to feeding. The veld hay was supplemented with 1% urea and was always offered ad libitum. The forage supplements were fed separately. A full description of the diets and the management of animals are given in the first paper in this series.

During each period of the latin square, measurements were carried out in the following order after a minimum of 14 days adaptation to the diet and 2-3 days adaptation to the metabolism cages. 2.1. Days 1 to 5 2.1.1. Digesta kinetics Cr-mordanted hay (10 g) and Co-EDTA (2 g) were pulse dosed into the rumen at 06:OO h (2 h before feeding at 08:OOh) on day-one and sampling was carried out at time intervals of 0 h, 1 h, 2 h, 4 h, 6 h, 8 h, 10 h, 12 h, 14 h, 16 h, 26 h, 32 h, 40 h, 50 h, 74 h and 98 h after dosing. At each sampling, approximately 30 ml of composite digesta were collected from different sites in the rumen using a hand pump. The digesta samples were strained through two layers of muslin cloth. The solid fraction was oven dried at 60°C and ground through a 1 mm screen to await Cr analysis. The liquid fraction was centrifuged at 21000 rpm for 15 minutes and frozen pending analysis for Cobalt. Rumen liquid samples were collected up to 32 h after dosing, while rumen dry matter samples were collected throughout the sampling period. Chromium mordanted hay and Co-EDTA were prepared according to the method outlined by Uden et al. (1980). 2.1.2. Rumen fermentation Rumen fluid samples for the determination of pH, ammonia and volatile fatty acids (VFAs) were collected at 08:OOh immediately before feeding, and at intervals of 2 h, 4 h, 6 h, 8 h, 10 h, 12 h and 14 h after offering feed. About 30 ml of rumen fluid were collected at each sampling time and strained through two layers of muslin cloth. The pH of rumen fluid was determined immediately and aliquots were taken for ammonia and VFA determination. Rumen fluid for ammonia analysis was acidified by adding 1 ml of 20% acetic acid per 4 ml of strained rumen fluid. The acidified rumen fluid was centrifuged at 8000 rpm for 15 minutes and the supematant stored at 4°C pending ammonia analysis. Rumen fluid for VFA analysis was acidified using 1 ml of 20% sulphuric acid per 4 ml of strained rumen fluid and centrifuged at 8000 rpm for 15 minutes. Subsequently, 1 ml of 20% metaphosphoric

8. Manyuchi et al. / Liuestock Production Science 49 (1997) 43-52

acid containing 20 mM of 2-Ethyl Butyric Acid as internal standard were added to 4 ml of the supernatant. The samples were centrifuged at 20000 rpm for 15 minutes and stored at - 16°C pending VFA analysis. In experiment 3, rumen fluid samples were collected at the same intervals to measure osmotic pressure.

45

4. Results 4.1. Dry matter intake and digestibility In all the experiments, supplementation increased total feed intake and digestibility of the diet (P < 0.05; Tables l-3). These results are discussed in detail in the first paper in this series.

2.2. Day 6 to 10 4.2. Rumen pool size 2.2.1. In sacco DM degradability of ueld hay The nylon bag method (Orskov et al., 1980) was used. Veld hay was incubated in the rumen of each sheep for 6 h, 12 h, 24 h, 36 h, 48 h, 12 h and 96 h. Degradability data were fitted to the non-linear model: P = a + b(1 - e-“) (0rskov and McDonald, 1979). 2.3. Day 8 to 13 2.3.1. Digestibility and nitrogen retention Faeces and urine samples were collected over 5 consecutive days as described in the first paper in this series.

3. Analysis 3. I. Chemical analysis

The dry matter content of samples was determined in a forced draught oven at 60°C for 48 h, ash was obtained by incineration in a muffle furnace at 550°C for 12 h, nitrogen was analyzed using the Kjeldhal method and neutral and acid detergent fibres were analyzed using the method of Goering and Van Soest (1970). Cr and Co were analyzed by standard Atomic Absorption Spectrophotometer procedure.

4.2.1. Liquid

There was an indication that rumen liquid pool size with napier hay alone was slightly reduced (experiment 1, Table l), but this did not reach significance. In experiment 2 (Table 2), none of the small differences in liquid pool size were statistically significant, although the very small differences were in the same direction as those of experiment 1. There was no significant effect of supplementation on rumen liquid volume, although all ten values with the supplementation treatments (from the three experiments), when compared with the appropriate controls, were marginally larger than the veld hay control. 4.2.2. Dry matter (DM) In all the experiments, there was no significant effect of diet on rumen DM pool size (Tables l-3). Rumen DM pool size tended to reduce with napier supplementation and with napier alone. In experiments 2 and 3, seven of the eight supplemented diets (Tables 1 and 2) resulted in slightly larger DM pools than the Control, differences were all less than one standard error of difference. 4.3. Rumen outjlow rates 4.3.1. Liquid

3.2. Statistical analysis Data from each lat in square were analyzed using the analysis of variance procedure for a balanced lat in square design with 12 degrees of freedom in the residual. Differences between treatment means were tested using t-tests.

Both fractional and absolute liquid outflow rates were significantly (P < 0.05) increased with the napier hay in experiment I (Table I), and with groundnut and napier hays in experiment 3 (Table 3). However with the groundnut in experiment 2 (Table 2) there was no effect on either fractional or absolute outflow rates of liquid.

B. Manyuchi et al. /Liuestock Production Science 49 (1997) 43-52

46

Table 1 Experiment 1. Dry matter intake and rumen digesta kinetics measured in rumen fistulated sheep given veld hay with 1% urea alone (Cant), napier hay alone (Nap) or veld hay with 1% urea plus 100 g, 200 g or 300 g napier hay on fresh weight basis (100 Nap, 200 Nap or 300 Nap, respectively) (values are means of 5 observations) SED

Diet

DM INTAKE (g day- ’ ): Veld hay Napier Total Faecal DM excretion (g day DM Digestibility RUMEN LIQUID Volume (1) Out-flow rate Out-flow (1 day- ’ > RUMEN SOLIDS Pool size (kg) Out-flow rate (% h-’ ) d Out-flow rate (% h- ’ ) e

’)

Cont

100 Nap

200 Nap

300 Nap

Nap

812 0 812 a 440” 0.458 a

889 89 978 4,b 527 b 0.461 ’

846 178 1025 b 562 b 0.451 =

987 267 1254 = 593 b 0.527 ’

1194 1194 b,c 450 ash 0.623 ’

10.28 5.37 a 13.20 a 2.06 1.98 a 0.95

11.33 6.02 ’ 16.27 b 2.11 2.42 a.b 1.04

10.49 6.42 b 16.16 b 1.86 2.82 a*b 1.37

-

99.4 40.3 0.023

11.50 6.14 b 17.05 b

8.99 8.96 ’ 19.02 ’

0.88 0.36 1.57

1.68 3.42 b 1.46

1.54 3.12 b 1.49

0.26 0.49 0.27

ab*c Means in the same row with different superscripts are significantly different (P < 0.05). d Estimated using Cr. ’ Estimated as a percentage of faecal excretion (g hK’ ) divided by rumen solids pool size.

4.3.2. Dry mutter

In experiment 1, fractional outflow rates of DM showed progressive increases (P < 0.05) with napier

hay and napier hay supplementation, irrespective as to whether measured as chromium mordanted hay (Cr-hay) or as faecal output described as a fractional

Table 2 Experiment 2. Dry matter intake and rumen digesta kinetics measured in fistulated sheep given veld hay with 1% urea alone (Cant) or veld hay plus 1% urea plus 100 g, 200 g or 300 g of groundnut hay on fresh weight basis (100 Gnut, 200 Gnut or 300 Gnut, respectively) or groundnut hay alone (values are means of 5 observations) SED

Diet 100 Gnut

200 Gnut

300 Gnut

912 0

896 90

905 180

785 270

912 ’

986 a,b

1085 b

Cont DMINTAKE(gday-‘): Veld hay Gnut hay Total Faecal DM excretion (g day-’ DM digestibility RUMEN LIQUID Volume (1) Out-flow rate (% h-’ ) Daily out-flow (1) RUMEN SOLIDS Pool size (kg) Outflow rate (% h-’ ) d Outflow rate (% h- ’ ) e

)

471 a 0.458 * 12.5 6.53 19.55 2.04 2.59 = 1.06

47s a 0.517 a.b 13.1 6.49 19.92 1.76 2.90 a 1.20

504” 0.533 b

1055 b 480 a 0.546 b

12.6 6.45 19.51 2.32 3.14 b 0.99

qbqc Means in the same row with different superscripts are significantly different (P < 0.05). d Estimated using Cr. e Estimated as a percentage of faecal excretion (g h- ’ > divided by mmen solids pool size.

12.5 6.64 19.73 2.27 2.98 a 0.98

Gnut _ 1656 1656 ’ 604b 0.636 11.7 6.70 17.89 2.38 3.79 c 1.13

58.0 35.9 0.020 1.68 0.67 2.11 0.41 0.24 0.21

47

B. Manyuchi et al. /Livestock Production Science 49 (I 997) 43-52

Table 3 Experiment 3. Dry matter intake and rumen digesta kinetics measured in rumen fistulated sheep fed a basal diet of veld hay with 1% urea alone (Cant) or veld hay with 1% urea plus 150 g or 300 g of napier hay or groundnut hay (150 Nap, 150 Gnut, 300 Nap, 300 Gnut, respectively) (values are means of 5 observations) Diet

DM INTAKE (g day-’ ): Veld hay Supplement Total Faecal DM excretion (g dayDM Digestibility RUMEN LIQUID Volume (1) Out-flow rate (% hh’ ) Out-flow (1 day - ’ ) RUh4EN SOLIDS Pool size (kg) Out-flow rate (% hh ’ ) e Out-flow (% h-’ ) f

’)

SED

Cont

150 Nap

150 Gnut

300 Nap

300 Gnut

997 0 997 a 494 a 0.505 a

996 135 1131 a 549 a 0.515 a

956 135 1091= 546 a 0.500 a

944 270 1214 b 662 b 0.545 b

989 270 1259 b 666 b 0.529 a.b

11.2 5.32 a 14.4 a

12.7 6.37 b 19.4 bc

2.20 1.95 0.96

12.4 6.41 b 18.4 b

2.96 2.65 1.12

13.1 6.40 b 20.3 ’

2.11 2.66 1.21

71.5 _ 71.5 130.9 0.014

13.1 7.07 22.1 c

1.62 0.39 2.28

2.58 2.88 1.06

0.80 0.56 0.26

2.98 2.68 1.2

a,b,c,d Means in the same row with different superscripts are significantly different (P < 0.05). ’ Estimated using Cr. f Estimated as a percentage of faecal excretion (g hh ’ ) divided by rumen solids pool size.

to increase and was significantly greater (P < 0.05) with the 200 Gnut diet and when only groundnut was fed (Gnut). The absolute outflow rate of DM (i.e. faecal DM) was not significantly increased by supplementation. Therefore the fractional DM outflow

outflow rate based on the Cr-hay determined reticulorumen DM pool (Table 1). With the groundnut hay (experiment 2, Table 2) although the effect on DM outflow as indicated by Cr-hay fractional outflow rate was not so large, fractional outflow rates tended

Table 4 Experiment 3. Dry matter degradation (%) of veld hay incubated in the rumen of sheep fed a basal diet of veld hay with 1% urea alone (Cant) or veld hay with 1% urea plus 150 g or 300 g of napier hay or groundnut hay (150 Nap, 150 Gnut, 300 Nap, 300 Gnut, respectively) (values are means of 5 observations) Incubation

6 12 24 36 48 72 96 Constants a b a-t-b C

a Constants

time (h)

Diet

SED

Cont

150 Nap

150 Gnut

300 Nap

300 Gnut

8.5 18.3 27.4 35.1 36.7 46.0 50.2

11.9 20.0 27.4 36.3 40.4 47.3 51.8

11.1 19.9 25.5 31.5 39.0 47.3 53.3

7.4 18.1 27.8 30.6 39.4 45.1 49.6

10.5 19.9 27.9 34.1 41.4 45.9 51.7

1.69 1.81 2.08 3.48 2.78 1.57 2.22

0.4 52.3 52.7 0.0330

4.7 51.8 55.5 0.0263

6.8 57.4 64.2 0.0196

0.9 53.2 54.1 0.0289

4.6 52.1 56.7 0.0258

2.66 3.40 4.28 oOO5 1

a

in the model, P = a + b(l - eKC’) (0rskov

and McDonald,

1979).

48

B. Manyuchi et al. /Livestock

Production Science 49 (1997143-52

Table 5 Experiment 2. Dry matter degradation (o/o) of veld hay incubated in the rumen of fistulated sheep given veld hay with 1% urea alone (Control) or veld hay with 1% urea plus 100 g, 200 g or 300 g of groundnut hay on fresh weight basis (100 Gnut, 200 Gnut or 300 Gnut, respectively) or groundnut hay alone (values are means of 5 observations) Incubation

time (h)

6 12 24 36 48 72 96 Constants a b a+b c

Diet

SED

Control

100 Gnut

200 Gnut

300 Gnut

Gnut

7.5 11.3 a 19.1 29.3 37.0 43.3 48.9

7.9 14.1 b 22.5 31.0 42.5 45.7 53.1

8.4 14.7 a 22.1 31.6 38.8 48.0 51.8

8.2 15.3 a 24.0 35.1 39.6 47.9 54.8

7.5 13.3 a 25.1 32.5 37.8 46.8 52.5

1.33 1.21 3.11 2.87 1.92 2.39 2.31

0.0 66.4 66.4 0.0192

0.1 61.8 61.9 0.0214

1.5 62.8 64.3 0.0187

0.7 64.7 65.4 0.0223

2.0 63.1 65.1 0.0216

2.63 4.50 4.44 0.0037

’

a*b Means in the same row with different superscripts are significantly different (P < 0.05). ’ Constants in the model, P = a + b(1 - eeC’) (@rskov and McDonald, 1979).

based on faecal production and rumen DM volume showed no effects since the small increases in faecal output were cancelled out by the small (non significant) increases in rumen volume. In experiment 3 (Table 3), faecal DM excretion was increased significantly (P < 0.001) at the 300 g day-’ level of supplementation, and approached significance (P < 0.10) at the 150 g day-’ level. Fractional outflow

rate as measured by chromium was increased by supplementation from 1.95% h-’ on the veld hay Control to a highest value of 2.88% h-’ on the 300 Gnut diet. Differences between treatments were however not statistically significant. Fractional outflow rate as measured by faecal output was not increased by supplementation. There was no obvious difference between napier and groundnut supplementation.

Table 6 Experiment 1. Dry matter degradation (%) of veld hay in the mmen of fistulated sheep given veld hay with 1% urea alone (Cant), napier hay alone (Nap) or veld hay with 1% urea supplemented plus 100 g, 200 g or 300 g napier hay on fresh weight basis (100 Nap, 200 Nap or 300 Nap, respectively) (values are means of 5 observations) Incubation

time (h)

6 12 24 36 48 72 96 Constants 6 a+b C

SED

Diet Cont

100 Nap

200 Nap

300 Nap

Nap

7.7 15.8 19.9 29.1 32.9 41.0 a 49.4

11.8 16.6 26.6 31.1 38.4 46.4 b 50.7

9.7 14.8 24.2 30.5 37.3 45.4 b 49.7

10.5 16.1 22.4 28.0 37.9 43.8 a,b 49.1

9.8 15.8 22.4 25.3 32.5 43.7 a,b 48.2

1.24 1.44 2.51 2.68 2.83 1.25 2.51

61.9 4.3 66.2 0.0209

63.6 1.0 64.6 0.0167

51.1 5.6 56.6 0.0232

59.8 1.9 61.6 0.0188

54.6 2.7 57.3 0.0210

6.29 3.23 4.69 0.0047

’

kb Means in the same row with different superscripts are significantly different (P < 0.051. ’ Constants in the model, P = a + b(1 - e-“1 (Grskov and McDonald, 1979).

49

B. Manyuchi et at. / Livestock Production Science 49 (1997) 43-52

Table 7 Experiment 1. Concentration of ammonia and VFA in rumen fluid and pH in fistulated sheep given veld hay with 1% mea alone (Con& napier hay alone (Nap) or veld hay with 1% urea plus 100 g, 200 g or 300 g napier hay on fresh weight basis (100 Nap, 200 Nap or 300 Nap, respectively) (values are means of 5 observations) Diet

Ammonia (mg 1- ’ ) Total VFAs (mmol I-’ ) Molar proportion (%): Acetic Propionate Butyric Others

SED

Cont

100 Nap

200 Nap

300 Nap

Nap

106 ’ 53.9 a

90 * 70.7 b

112 a 76.7 h

114 a 82.6 ’

225 b 144.1 c

76.1 16.9 5.4 1.6

74.0 17.8 5.9 2.3

13.6 18.2 6.6 1.6

73.4 17.7 1.3 1.6

65.8 24.0 8.3 1.9

Rumen pH

6.36

6.57

“b’c Means in the same row with different superscripts

6.63

are significantly

4.4. Rumen environment

6.64

6.55

19.6 5.89 _ _ 0.070

different (P < 0.05).

1 (Table 6) were greater with supplementation and approached significance (P < 0.05) but in no systematic way.

4.4.1. Degradation of veld hay There were no great differences between the degradation curves of veld hay when incubated in any of the rumen environments (P > 0.05). In all cases, differences in the rate constant ‘c’ and potential degradation (a + b) were small and non-significant. The rate constant was somewhat reduced with the 150 Gnut diet (experiment 3; Table 4). In experiment 2 (Table 51, the degradation of veld hay was consistently lower on the Control compared to the Gnut and the supplemented diets, but this difference did not reach statistical significance. Some of the 72 h degradabilities with the napier diets in experiment

4.4.2. Rumen ammonia In experiments 1 and 2, the forage supplements did not increase rumen ammonia concentrations above the levels achieved with the veld hay Control except where the forages were fed ad libitum. (Tables 7 and 8). When experiments 1 and 2 are compared, groundnut hay supplements tended to give lower rumen ammonia levels than napier. This difference between groundnut and napier was not very distinct in experiment 3 (Table 9).

Table 8 Experiment 2. Concentration of ammonia and VFA in rumen fluid and pH in fistulated sheep given veld hay with 1% urea alone (Cant) or veld hay with 1% urea plus 100 g, 200 g or 300 g of groundnut hay on fresh weight basis (100 Gnut, 200 Gnut or 300 Gnut, respectively) or groundnut hay alone (values are means of 5 observations) Diet

Ammonia (mg 1- ’ ) Total VFAs (mm01 1- ’ ) Molar proportion (o/o): Acetic Propionate Butyrate Others Rumen pH

SED

Cont

100 Gnut

200 Gnut

300 Gnut

Gnut

84 a 59.1 a

92 a 71.5 b

98 a 77.8 b*c

101 a 82.1 ’

140 b 158.2 d

75.9 16.9 5.4 1.8

14.7 17.9 5.9 1.5

13.4 11.7 7.3 1.6

65.3 24.2 8.3

6.83 ’

a.’ Means in the same row with different

6.58 ’ superscripts

are significantly

74.1 18.1 6.6 1.2 6.65 ’

6.67 b

different (P < 0.05).

2.2 6.29 a

14.7 5.24 _ _ 0.068

50

B. Manyuchi et al. /Livestock

Production Science 49 (1997) 43-52

Table 9 Experiment 3. Rumen fluid ammonia and VFA, pH and osmotic pressure in fistulated sheep fed a basal diet of veld hay with 1% urea alone (Cant) or veld hay with 1% urea plus 150 g or 300 g of napier hay or groundnut hay (150 Nap, 150 Gnut, 300 Nap, 300 Gnut, respectively) (values are means of 5 observations) Diet Cont Ammonia (mg 1-l ) Total VFAs (mm01 1- ’ ) Molar proportions (o/o): Acetic Propionate Butyric Others Rumen pH Osmotic pressure (mosmols 1-l 1

SED 150 Nap

150 Gnut

300 Nap

300 Gnut

99 81.9

91 93.4

113 99.7

114 91.4

109 93.9

77.8 16.2

79.9 13.2

78.1 14.7

7.9 14.7

16.7 14.9

-

5.7 3.0

5.5 1.4

5.5 1.7

5.8 1.6

6.8 1.6

-

6.63 228

6.61 235

6.46 233

6.46 240

6.54 242

8.9 4.85

0.047 5.5

qb Means in the same row with different superscripts are significantly different (P < 0.05).

4.5. Rumen VFA

In experiments 1 and 2, the total and concentration of individual volatile fatty acids were increased by feeding the forage supplements and were highest when the supplements were fed on their own (Tables 8 and 9). The molar proportion of acetic acid decreased while molar proportions of propionic acid, butyric acid and valeric acid were increased by supplementation. In experiment 3, total volatile fatty acid concentrations were increased by supplementation (P > 0.05; Table 9) and there were no consistent changes in the molar proportions of the individual acids. 4.5.1. Rumen pH In experiment 1, there were no differences between diets in rumen pH (Table 7; P > 0.05). In experiment 2, pH was lowest on the Gnut diet (P < 0.05) and highest on the Control diet, while the supplemented diets gave intermediate values (Table 8). In experiment 3, differences between diets were very small and not significant (Table 9). In experiment 3, the groundnut hay supplemented diets maintained a lower pH than the napier supplemented diets though the differences were not significant (P > 0.09. 4.5.2. Rumen osmotic pressure

Differences between diets were not statistically significant. Osmotic pressure however tended to be

higher on the supplemented diets compared to the unsupplemented Control. 5. Discussion When ruminants are consuming poor quality forages, feed intake is limited by gut-fill due to the slow rate and low extent of digestion of poor quality forages in the reticula-rumen. Factors relating to the creation of space in the reticula-rumen such as increased rate of digestion, increased digesta passage rate and increased rumen fill are associated with an increase in intake of poor quality forage. When food intake increases, it may be due to a change in one or more of these factors. Studies by Silva (1985) confirm that rumen environment limits fibre digestion when animals are consuming poor quality forages and that forage supplements will increase the population of cellulolytic microbes in the rumen. Studies by McMeniman et al. (1988) and Manyuchi et al. (1992) show increased rate of degradation of straw in the rumen as one of the benefits associated with forage supplementation. The response to forage supplementation has largely been attributed to improved dietary nitrogen intake stimulating rumen microbial growth. In recent studies, similar responses to forage supplementation have been observed when dietary nitrogen was not limiting rumen cellulolysis, and it has been hypothesized that the response is also due to the supply of easily degradable fibre.

B. Manyuchi et al. /Livestock Production Science 49 (1997) 43-52

In the current study, there was no significant effect of forage supplementation on in sacco degradation of veld hay. It was anticipated that rumen environment was improved by increased ammonia levels, fermentable substrate and supply of micronutrients by the forage supplements. However it is concievable that the degradation of poor quality forage can not be entirely a function of rumen microbial population. Poor quality forages have limited sites for microbial colonization due to lignification and the presence of a cuticle layer (Akin, 1989). It is also not clear by how much the microbial population needs to increase to effect a unit increase in degradability of the poor quality forage. The degradation of poor quality forages can be enhanced by alkali treatment, because alkali treatment reduces the interference of lignin with cell wall degradation (Wood and Saddler, 1988). In all the three experiments, digesta outflow from the rumen was increased by feeding small amounts of napier or groundnut hay supplements, while digesta pool size was not altered. Work by Ndlovu and Buchanan-Smith (1985) and Vanzant and Cochran (1994) also showed that the rate of passage of liquid and particulate digesta was increased following alfalfa supplementation of poor quality forage. In the current study, ruminal digesta passage was predicted to increase, because faecal excretion increased as feed intake increased (Tables l-3). The cause for increased digesta outflow when small amounts of high quality forage supplements are fed is not easily discernible. It is well established that the volume and outflow rate of liquid from the rumen is controlled mainly by factors related to osmotic pressure of rumen fluid (Carter and Grovum, 1990) and that saliva contributes most of the liquid present in the rumen (Kay, 1966). Particulate passage from the rumen is facilitated by attainment of critical particle size and functional density (Letchner-Doll et al., 1991). Nengomasha (1992) observed that supplementation of a straw diet with small amounts of starch increased straw intake, despite the fact that the rumen degradation of straw was decreased by the starch supplements. Outflow rate of liquid and particulate digesta from the rumen was increased by the starch supplements, providing a possible explanation for the increase in feed intake. It was hypothesized that the increase in osmotic

51

pressure of rumen fluid due to improved fermentation, stimulated saliva secretion and hence outflow rate of liquid. Since particulate digesta exits from the rumen while suspended in the liquid, it would be expected that increased rate of liquid outflow will be associated with increased rate of particulate digesta passage rate (e.g., r = 0.76; Faichney and White, 1988). In this study, rumen liquid outflow was correlated with faecal dry matter excretion (experiments 1 and 2, r2 = 0.46; P < 0.05), confirming the hypothesis that liquid outflow was correlated with outflow of particulate digesta.

6. Conclusion The current study demonstrated that forage supplements increased feed intake without increasing degradation of the poor quality forage in the rumen. The increase in feed intake was largely facilitated by an increase in digesta outflow, while digesta pool size was not altered.

Acknowledgements

This study was sponsored by the Overseas Development and Natural Resources Institute (UK) and the Ministry of Lands, Agriculture and Water Development (Zimbabwe). We grateful to staff at the Rowett Research Institute (Scotland, UK) and Grasslands Research Station (Zimbabwe) for their assistance.

References Akin, D.E., 1989. Histological and physical factors affecting digestibility of forages. Agronomy Journal 81, 17-25. Carter, R.R., Grovum, W.L., 1990. A review of the physiological significance of hypertonic body fluids on feed intake and ruminal function, Salivation, motility and mmen microbes. J. Animal Sci. 68, 281 l-2832. Faichney, G.J., White, G.A., 1988. Rates of passage of solutes, microbes, and particulate matter through the gastro-intestinal tract of ewes fed at a constant rate throughout gestation. Aust. J. Agric. Res. 39, 481-492. Goering, H.K., Van Soest, P.J., 1970. Forage Fibre Analyses. United States Department of Agriculture Hand Book Number 379.

52

B. Manyuchi et al. / Livestock Production Science 49 (1997) 43-52

Kay, R.N.B., 1966. The influence of saliva on digestion in ruminants. World Review of Nutrition and Dietetics 6, 292325. Letchner-Doll, M., Kaske, M., Engelhardt, W., 1991. Factors affecting the mean retention time of particle in the fore stomach of ruminants. In: Tsuda, T., Sasaki, Y., Kawashima, R. (Eds.), Physiological Aspects of Digestion and Metabolism in the Ruminants. Academic Press, 455-482. McMeniman, N.P., Elliott, R., Ash, A.J., 1988. Supplementation with crop by-products of rice straw 1. Legume straw supplementation. Animal Feed Science and Technology 19, 43-53. Manyucbi, B., 0rskov, E.R., Kay, R.N.B., 1992. Effects of feeding small amounts of ammonia treated straw on degradation rate and intake of untreated straw. Animal Feed Science and Technology 38, 293-304. Mbatya, P.B.A., Kay, M., Smart, R.I., 1983. Methods of improving the utilization of cereal straw by ruminants 1. Supplements of urea, molasses and dried grass and treatment with sodium hydroxide. Animal Feed Science and Technology 8, 221-227. Minson, D.J., Milford, R., 1967. The voluntary intake and digestibility of diets containing different proportions of legume and mature pangola grass (Digitaria decumbens). Aust. J. Exp. Agric. Animal Husbandry 7, 545-551. Ndlovu, L.R., Buchanan-Smith, J.G., 1985. Utilization of poor quality roughage by sheep: Effects of alfalfa supplementation on ruminal parameters, fibre digestion and rate of passage from the rumen. Can. J. Animal Sci. 65, 693-703.

Nengomasha, E.M., 1992. The effect of rapidly fermentable carbohydrates on the voluntary intake of roughages, Rumen parameters. MSc. Thesis. University of Aberdeen. 0rskov, E.R., McDonald, I., 1979. Evaluation of protein degradability from incubation measurements weighted according to rate of passage. J. Agric. Sci. Cambridge 92, 499-503. Siebert, B.D., Kennedy, P.M., 1972. The utilization of spear grass (H eteropogon c 0 n to rtu s) 1. Factors limiting intake and utilization by sheep. Aust. J. Agric. Res. 23, 35-44. Silva, A.T., 1985. Assessment and control of condition in the rumen to increase utilization of fibrous roughages in ruminant. Ph.D thesis University of Aberdeen. Smith, T., Manyuchi, B., Mikayiri, S., 1989. Legume supplementation of maize stover. In: Dzowela, B.H., Said, A.N., Wendem-Agenehu, A., Kategile, J.A. (Eds.), Utilization of Research Results on Forage and Agricultural By-Product Materials as Animal Feed Resources in Africa. ILCA, Addis Ababa, Ethiopia. pp. 303-320. Uden, P., Colucci, P.E., Van Soest, P.J., 1980. Investigations of chromium, cerium and cobalt as markers of digesta rate of passage studies. J. Sci. Food Agric. 31, 625-632. Vanzant, E.S., Cochran, R.C., 1994. Performance and forage utilization by beef cattle receiving increasing amounts of alfalfa hay as supplement to low quality, tall grass-praire forage. J. Animal Sci. 72, 1059-1067. Wood, T.M., Saddler, J.N., 1988. Methods of increasing the availability of cellulose in biomass materials. Methods Enzymol. 160, 3-11.